Java基础-集合框架-ArrayList源码分析

一、JDK中ArrayList是如何实现的

1、先看下ArrayList从上而下的层次图：

 说明：

　　从图中可以看出，ArrayList只是最下层的实现类，集合的规则和扩展都是AbstractList、List、Collection等上层的接口所设定的，而ArrayList实现或继承了上层的规则，然后重新或扩展来处理集合中的数据。

2、看看接口：Iterable<E>中都定义了那些规则？

JDK1.8中的源码：

package java.lang;

import java.util.Iterator;
import java.util.Objects;
import java.util.Spliterator;
import java.util.Spliterators;
import java.util.function.Consumer;
public interface Iterable<T> {//实现这个接口允许对象成为 "foreach" 语句的目标。 
    Iterator<T> iterator();
    default void forEach(Consumer<? super T> action) {
        Objects.requireNonNull(action);
        for (T t : this) {
            action.accept(t);
        }
    }
    default Spliterator<T> spliterator() {
        return Spliterators.spliteratorUnknownSize(iterator(), 0);
    }
}

 这里说明一下：

（1）Consumer<? super T> action  可以理解为：实现了Consumer接口的实现类对象。
（2）accept(T t) T 是每一次forEach方法处理的数据类型，t是数据
（3）spliterator()：Spliterator（splitable iterator可分割迭代器）接口是Java为了并行遍历数据源中的元素而设计的迭代器。也就是收JDK1.8版本已经支持多核并发处理集合中的数据了，你只需要告诉JDK你要做什么并行任务，关注业务本身，至于如何并行，怎么并行效率最高，就交给JDK自己去思考和优化速度了。

简单举个forEach的例子：

package com.xfwl.test;

import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
@SuppressWarnings("unchecked")
public class Test2 {
    public static void main(String[] args) {
        List alist=new ArrayList();
        alist.add("1");
        alist.forEach(new T());
    }
    /**
     * Consumer接口测试实现类T
     * @param <E>
     */
    static  class T implements Consumer{
        public T(){}
        public void accept(Object e) {
            System.out.println("当前数据："+e);
        }
        public Consumer andThen(Consumer after) {
            // TODO Auto-generated method stub
            return null;
        }
    }
}

 运行结果：

当前数据：1

总结一下：

　　（1）接口Iterable，定义了遍历集合中的数据的方法：forEach()，这个方法需要一个实现了Consumer接口的参数；

　　 （2）接口Iterable，定义了一个可以实现并发的迭代器，具体如何实现和优化交给jdk自己处理，我们只负责使用集合即可。

3、看看接口：Collection<E>中定义了那些规则？

JDK1.8中的源码： 

package java.util;
import java.util.function.Predicate;
import java.util.stream.Stream;
import java.util.stream.StreamSupport;

public interface Collection<E> extends Iterable<E> {
    int size();                                    //获取集合元素总数
    boolean isEmpty();                            //判断集合是否非空
    boolean contains(Object o);                    //判断集合是否包含指定元素
    Iterator<E> iterator();                        //获取迭代器
    Object[] toArray();                            //把集合转化成Object类型数组对象
    <T> T[] toArray(T[] a);                        //把集合转化成指定T类型数组对象
    boolean add(E e);                            //添加数据到集合
    boolean remove(Object o);                    //从集合中移除数据
    boolean containsAll(Collection<?> c);        //集合是否包含另一个集合
    boolean addAll(Collection<? extends E> c);    //集合添加另外一个集合
    boolean removeAll(Collection<?> c);            //集合中移除另外一个集合中的内容
    default boolean removeIf(Predicate<? super E> filter) {//JDK1.8开始加入的方法，按指定条件移除集合中的信息
        Objects.requireNonNull(filter);
        boolean removed = false;
        final Iterator<E> each = iterator();
        while (each.hasNext()) {
            if (filter.test(each.next())) {
                each.remove();
                removed = true;
            }
        }
        return removed;
    }
    boolean retainAll(Collection<?> c);            //移除此 collection 中未包含在指定 collection 中的所有元素
    void clear();                                //清除集合中的信息
    boolean equals(Object o);                    //比较此 collection 与指定对象是否相等。
    int hashCode();                                //返回此 collection 的哈希码值。
    @Override
    default Spliterator<E> spliterator() {        //JDK1.8开始新加入的方法，返回当前集合的并发迭代器
        return Spliterators.spliterator(this, 0);
    }
    default Stream<E> stream() {                //JDK1.8开始新加入的方法，通过传入并发迭代器，来使用stream过滤集合数据
        return StreamSupport.stream(spliterator(), false);//返回的流是顺序的
    }
    default Stream<E> parallelStream() {        //JDK1.8开始新加入的方法，通过传入并发迭代器，来使用stream过滤集合数据
        return StreamSupport.stream(spliterator(), true);//返回的流是并行的
    }
}

 这里说明一下：

（1）JDK8版本中，接口中可以定义方法的实现了，即方法可以定义方法体了。

（2）Collection中定义了一些针对集合数据的添加，删除，判空，是否存在，整块集合的包含+移除等操作，以及JDK1.8开始新增的方法：removeIf()和spliterator()，stream()以及parallelStream()方法。

简单举个stream()的例子：

package com.xfwl.test;

import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.stream.Collectors;
@SuppressWarnings("unchecked")
public class Test2 {
    public static void main(String[] args) {
        List<Integer> alist=Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        alist=alist.stream().filter(s -> (s>5)).collect(Collectors.toList());//lumbda表达式

for(int i:alist){ 13  System.out.println(i); 14  } 15 16  } 17 }

 运行结果：

6
7
8
9
10

继续修改代码，举个removeIf()的例子：

package com.xfwl.test;

import java.util.Arrays;
import java.util.List;
import java.util.function.Consumer;
import java.util.stream.Collectors;
@SuppressWarnings("unchecked")
public class Test2 {
    public static void main(String[] args) {
        List<Integer> alist=Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        alist=alist.stream().filter(s -> (s>5)).collect(Collectors.toList());//lumbda表达式
        alist.removeIf(s->s==10);
        for(int i:alist){
            System.out.println(i);
        }
    }
}

 运行结果：

6
7
8
9

4、看看抽象类：AbstractCollection<E>中定义了那些规则？

JDK1.8中的源码 

package java.util;
public abstract class AbstractCollection<E> implements Collection<E> {
    protected AbstractCollection() {
    }
    public abstract Iterator<E> iterator();

    public abstract int size();

    public boolean isEmpty() {
        return size() == 0;
    }
    public boolean contains(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return true;
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return true;
        }
        return false;
    }
    public Object[] toArray() {
        // Estimate size of array; be prepared to see more or fewer elements
        Object[] r = new Object[size()];
        Iterator<E> it = iterator();
        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) // fewer elements than expected
                return Arrays.copyOf(r, i);
            r[i] = it.next();
        }
        return it.hasNext() ? finishToArray(r, it) : r;
    }
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        // Estimate size of array; be prepared to see more or fewer elements
        int size = size();
        T[] r = a.length >= size ? a :
                  (T[])java.lang.reflect.Array
                  .newInstance(a.getClass().getComponentType(), size);
        Iterator<E> it = iterator();

        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) { // fewer elements than expected
                if (a == r) {
                    r[i] = null; // null-terminate
                } else if (a.length < i) {
                    return Arrays.copyOf(r, i);
                } else {
                    System.arraycopy(r, 0, a, 0, i);
                    if (a.length > i) {
                        a[i] = null;
                    }
                }
                return a;
            }
            r[i] = (T)it.next();
        }
        // more elements than expected
        return it.hasNext() ? finishToArray(r, it) : r;
    }

    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    @SuppressWarnings("unchecked")
    private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
        int i = r.length;
        while (it.hasNext()) {
            int cap = r.length;
            if (i == cap) {
                int newCap = cap + (cap >> 1) + 1;
                // overflow-conscious code
                if (newCap - MAX_ARRAY_SIZE > 0)
                    newCap = hugeCapacity(cap + 1);
                r = Arrays.copyOf(r, newCap);
            }
            r[i++] = (T)it.next();
        }
        // trim if overallocated
        return (i == r.length) ? r : Arrays.copyOf(r, i);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError
                ("Required array size too large");
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    public boolean add(E e) {
        throw new UnsupportedOperationException();
    }
    public boolean remove(Object o) {
        Iterator<E> it = iterator();
        if (o==null) {
            while (it.hasNext()) {
                if (it.next()==null) {
                    it.remove();
                    return true;
                }
            }
        } else {
            while (it.hasNext()) {
                if (o.equals(it.next())) {
                    it.remove();
                    return true;
                }
            }
        }
        return false;
    }
    public boolean containsAll(Collection<?> c) {
        for (Object e : c)
            if (!contains(e))
                return false;
        return true;
    }
    public boolean addAll(Collection<? extends E> c) {
        boolean modified = false;
        for (E e : c)
            if (add(e))
                modified = true;
        return modified;
    }
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<?> it = iterator();
        while (it.hasNext()) {
            if (c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        boolean modified = false;
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            if (!c.contains(it.next())) {
                it.remove();
                modified = true;
            }
        }
        return modified;
    }
    public void clear() {
        Iterator<E> it = iterator();
        while (it.hasNext()) {
            it.next();
            it.remove();
        }
    }
    public String toString() {
        Iterator<E> it = iterator();
        if (! it.hasNext())
            return "[]";

        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (;;) {
            E e = it.next();
            sb.append(e == this ? "(this Collection)" : e);
            if (! it.hasNext())
                return sb.append(']').toString();
            sb.append(',').append(' ');
        }
    }

}

 说明一下：

（1）整体上来看，抽象类：AbstractCollection<E>实现了Collection接口，并且定义了接口中没有实现的方法体。

 （2）其中的： public abstract Iterator<E> iterator();  依然是沿用之前jdk版本的使用方式，当前抽象类没有给出具体的实现，在另外一个抽象类AbstractList中给出了具体实现：

 public Iterator<E> iterator() {
        return new Itr();
    }

 那么其中：new Itr()拿到的到底是什么呢？

继续贴出jdk中的源码：

 private class Itr implements Iterator<E> {
        int cursor = 0;                    //下一个元素的索引下标
        int lastRet = -1;                //上一个被调用的元素下标
        int expectedModCount = modCount;
        public boolean hasNext() {
            return cursor != size();
        }

        public E next() {                //返回下一个元素
            checkForComodification();
            try {
                int i = cursor;
                E next = get(i);
                lastRet = i;
                cursor = i + 1;
                return next;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public void remove() {        //移除元素
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.remove(lastRet);
                if (lastRet < cursor)
                    cursor--;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

 总结一下：

　　到目前为止，我们知道，如果只是想简单地获取集合的迭代器（非并行），可直接调用集合的iterator()方法即可，但是如果想要调用几何的并发的迭代器，则需要换一个JDK1.8新增进去的方法：

default Stream<E> stream() {                //JDK1.8开始新加入的方法，通过传入并发迭代器，来使用stream过滤集合数据
        return StreamSupport.stream(spliterator(), false);//返回的流是顺序的
    }
    default Stream<E> parallelStream() {        //JDK1.8开始新加入的方法，通过传入并发迭代器，来使用stream过滤集合数据
        return StreamSupport.stream(spliterator(), true);//返回的流是并行的
    }

 5、看看抽象类：AbstractList<E>中定义了那些规则？

贴上JDK1.8中的源码：

package java.util;
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
   
    protected AbstractList() {
    }
    public boolean add(E e) {
        add(size(), e);
        return true;
    }

    abstract public E get(int index);

   
    public E set(int index, E element) {
        throw new UnsupportedOperationException();
    }
    public void add(int index, E element) {
        throw new UnsupportedOperationException();
    }

    public E remove(int index) {
        throw new UnsupportedOperationException();
    }
    public int indexOf(Object o) {
        ListIterator<E> it = listIterator();
        if (o==null) {
            while (it.hasNext())
                if (it.next()==null)
                    return it.previousIndex();
        } else {
            while (it.hasNext())
                if (o.equals(it.next()))
                    return it.previousIndex();
        }
        return -1;
    }
    public int lastIndexOf(Object o) {
        ListIterator<E> it = listIterator(size());
        if (o==null) {
            while (it.hasPrevious())
                if (it.previous()==null)
                    return it.nextIndex();
        } else {
            while (it.hasPrevious())
                if (o.equals(it.previous()))
                    return it.nextIndex();
        }
        return -1;
    }

    public void clear() {
        removeRange(0, size());
    }
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        boolean modified = false;
        for (E e : c) {
            add(index++, e);
            modified = true;
        }
        return modified;
    }
    public Iterator<E> iterator() {
        return new Itr();
    }
    public ListIterator<E> listIterator() {
        return listIterator(0);
    }
    public ListIterator<E> listIterator(final int index) {
        rangeCheckForAdd(index);

        return new ListItr(index);
    }

    private class Itr implements Iterator<E> {
      
        int cursor = 0;

        
        int lastRet = -1;

       
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor != size();
        }

        public E next() {
            checkForComodification();
            try {
                int i = cursor;
                E next = get(i);
                lastRet = i;
                cursor = i + 1;
                return next;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.remove(lastRet);
                if (lastRet < cursor)
                    cursor--;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException e) {
                throw new ConcurrentModificationException();
            }
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public E previous() {
            checkForComodification();
            try {
                int i = cursor - 1;
                E previous = get(i);
                lastRet = cursor = i;
                return previous;
            } catch (IndexOutOfBoundsException e) {
                checkForComodification();
                throw new NoSuchElementException();
            }
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor-1;
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                AbstractList.this.set(lastRet, e);
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                AbstractList.this.add(i, e);
                lastRet = -1;
                cursor = i + 1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }
    public List<E> subList(int fromIndex, int toIndex) {
        return (this instanceof RandomAccess ?
                new RandomAccessSubList<>(this, fromIndex, toIndex) :
                new SubList<>(this, fromIndex, toIndex));
    }
    public boolean equals(Object o) {
        if (o == this)
            return true;
        if (!(o instanceof List))
            return false;

        ListIterator<E> e1 = listIterator();
        ListIterator<?> e2 = ((List<?>) o).listIterator();
        while (e1.hasNext() && e2.hasNext()) {
            E o1 = e1.next();
            Object o2 = e2.next();
            if (!(o1==null ? o2==null : o1.equals(o2)))
                return false;
        }
        return !(e1.hasNext() || e2.hasNext());
    }
    public int hashCode() {
        int hashCode = 1;
        for (E e : this)
            hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
        return hashCode;
    }
    protected void removeRange(int fromIndex, int toIndex) {
        ListIterator<E> it = listIterator(fromIndex);
        for (int i=0, n=toIndex-fromIndex; i<n; i++) {
            it.next();
            it.remove();
        }
    }
    protected transient int modCount = 0;

    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size())
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size();
    }
}

class SubList<E> extends AbstractList<E> {
    private final AbstractList<E> l;
    private final int offset;
    private int size;

    SubList(AbstractList<E> list, int fromIndex, int toIndex) {
        if (fromIndex < 0)
            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
        if (toIndex > list.size())
            throw new IndexOutOfBoundsException("toIndex = " + toIndex);
        if (fromIndex > toIndex)
            throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")");
        l = list;
        offset = fromIndex;
        size = toIndex - fromIndex;
        this.modCount = l.modCount;
    }

    public E set(int index, E element) {
        rangeCheck(index);
        checkForComodification();
        return l.set(index+offset, element);
    }

    public E get(int index) {
        rangeCheck(index);
        checkForComodification();
        return l.get(index+offset);
    }

    public int size() {
        checkForComodification();
        return size;
    }

    public void add(int index, E element) {
        rangeCheckForAdd(index);
        checkForComodification();
        l.add(index+offset, element);
        this.modCount = l.modCount;
        size++;
    }

    public E remove(int index) {
        rangeCheck(index);
        checkForComodification();
        E result = l.remove(index+offset);
        this.modCount = l.modCount;
        size--;
        return result;
    }

    protected void removeRange(int fromIndex, int toIndex) {
        checkForComodification();
        l.removeRange(fromIndex+offset, toIndex+offset);
        this.modCount = l.modCount;
        size -= (toIndex-fromIndex);
    }

    public boolean addAll(Collection<? extends E> c) {
        return addAll(size, c);
    }

    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        int cSize = c.size();
        if (cSize==0)
            return false;

        checkForComodification();
        l.addAll(offset+index, c);
        this.modCount = l.modCount;
        size += cSize;
        return true;
    }

    public Iterator<E> iterator() {
        return listIterator();
    }

    public ListIterator<E> listIterator(final int index) {
        checkForComodification();
        rangeCheckForAdd(index);

        return new ListIterator<E>() {
            private final ListIterator<E> i = l.listIterator(index+offset);

            public boolean hasNext() {
                return nextIndex() < size;
            }

            public E next() {
                if (hasNext())
                    return i.next();
                else
                    throw new NoSuchElementException();
            }

            public boolean hasPrevious() {
                return previousIndex() >= 0;
            }

            public E previous() {
                if (hasPrevious())
                    return i.previous();
                else
                    throw new NoSuchElementException();
            }

            public int nextIndex() {
                return i.nextIndex() - offset;
            }

            public int previousIndex() {
                return i.previousIndex() - offset;
            }

            public void remove() {
                i.remove();
                SubList.this.modCount = l.modCount;
                size--;
            }

            public void set(E e) {
                i.set(e);
            }

            public void add(E e) {
                i.add(e);
                SubList.this.modCount = l.modCount;
                size++;
            }
        };
    }

    public List<E> subList(int fromIndex, int toIndex) {
        return new SubList<>(this, fromIndex, toIndex);
    }

    private void rangeCheck(int index) {
        if (index < 0 || index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private void rangeCheckForAdd(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    private void checkForComodification() {
        if (this.modCount != l.modCount)
            throw new ConcurrentModificationException();
    }
}

class RandomAccessSubList<E> extends SubList<E> implements RandomAccess {
    RandomAccessSubList(AbstractList<E> list, int fromIndex, int toIndex) {
        super(list, fromIndex, toIndex);
    }

    public List<E> subList(int fromIndex, int toIndex) {
        return new RandomAccessSubList<>(this, fromIndex, toIndex);
    }
}

 

简单看一下抽象类的结构图：

 说明一下：

这个抽象类中存在：抽象方法的声明，一些具体方法的实现，也内置了一些内部类：

　　private class Itr implements Iterator<E> {}

　　private class ListItr extends Itr implements ListIterator<E> {}

以上2两种迭代器丰富了AbstractList这个抽象类的迭代方式，以后继承这个抽象类的子类也可以重写会直接复用，为子类提供了多样化的功能。

一直在说：以上的接口或者抽象类只是在定义集合的规则，有些人就会有疑惑了，明明这个接口或者抽象类中也有一些实现了功能的方法啊。

其实对于一个集合来说，最重要的是什么？是如何把数据存进去，存在哪里，如何存放，如何取出来，如何存放到一个迭代器中（我们都知道是以链表的形式存放），那么从上面的接口或者抽象类中，我并没有发现：

（1）数据的存放的位置（数组，list集合中维护的是数组）。

（2）数据如何存放在迭代器中的链表。

以上这2中核心实现的代码，并不存在于上述的接口和抽象类中，所以我们接着往下看，继续去寻找答案！！！

6、看看接口：List<E>中定义了那些规则？

贴上JDK1.8中的源码：

package java.util;
import java.util.function.UnaryOperator;
public interface List<E> extends Collection<E> {
    // Query Operations

    /**
     * Returns the number of elements in this list.  If this list contains
     * more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of elements in this list
     */
    int size();

    /**
     * Returns <tt>true</tt> if this list contains no elements.
     *
     * @return <tt>true</tt> if this list contains no elements
     */
    boolean isEmpty();

    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return <tt>true</tt> if this list contains the specified element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    boolean contains(Object o);

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    Iterator<E> iterator();

    /**
     * Returns an array containing all of the elements in this list in proper
     * sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must
     * allocate a new array even if this list is backed by an array).
     * The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list in proper
     *         sequence
     * @see Arrays#asList(Object[])
     */
    Object[] toArray();

    /**
     * Returns an array containing all of the elements in this list in
     * proper sequence (from first to last element); the runtime type of
     * the returned array is that of the specified array.  If the list fits
     * in the specified array, it is returned therein.  Otherwise, a new
     * array is allocated with the runtime type of the specified array and
     * the size of this list.
     *
     * <p>If the list fits in the specified array with room to spare (i.e.,
     * the array has more elements than the list), the element in the array
     * immediately following the end of the list is set to <tt>null</tt>.
     * (This is useful in determining the length of the list <i>only</i> if
     * the caller knows that the list does not contain any null elements.)
     *
     * <p>Like the {@link #toArray()} method, this method acts as bridge between
     * array-based and collection-based APIs.  Further, this method allows
     * precise control over the runtime type of the output array, and may,
     * under certain circumstances, be used to save allocation costs.
     *
     * <p>Suppose <tt>x</tt> is a list known to contain only strings.
     * The following code can be used to dump the list into a newly
     * allocated array of <tt>String</tt>:
     *
     * <pre>{@code
     *     String[] y = x.toArray(new String[0]);
     * }</pre>
     *
     * Note that <tt>toArray(new Object[0])</tt> is identical in function to
     * <tt>toArray()</tt>.
     *
     * @param a the array into which the elements of this list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of this list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    <T> T[] toArray(T[] a);


    // Modification Operations

    /**
     * Appends the specified element to the end of this list (optional
     * operation).
     *
     * <p>Lists that support this operation may place limitations on what
     * elements may be added to this list.  In particular, some
     * lists will refuse to add null elements, and others will impose
     * restrictions on the type of elements that may be added.  List
     * classes should clearly specify in their documentation any restrictions
     * on what elements may be added.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     * @throws UnsupportedOperationException if the <tt>add</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this list
     */
    boolean add(E e);

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present (optional operation).  If this list does not contain
     * the element, it is unchanged.  More formally, removes the element with
     * the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list changed
     * as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws UnsupportedOperationException if the <tt>remove</tt> operation
     *         is not supported by this list
     */
    boolean remove(Object o);


    // Bulk Modification Operations

    /**
     * Returns <tt>true</tt> if this list contains all of the elements of the
     * specified collection.
     *
     * @param  c collection to be checked for containment in this list
     * @return <tt>true</tt> if this list contains all of the elements of the
     *         specified collection
     * @throws ClassCastException if the types of one or more elements
     *         in the specified collection are incompatible with this
     *         list
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #contains(Object)
     */
    boolean containsAll(Collection<?> c);

    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the specified
     * collection's iterator (optional operation).  The behavior of this
     * operation is undefined if the specified collection is modified while
     * the operation is in progress.  (Note that this will occur if the
     * specified collection is this list, and it's nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this list
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements, or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this list
     * @see #add(Object)
     */
    boolean addAll(Collection<? extends E> c);

    /**
     * Inserts all of the elements in the specified collection into this
     * list at the specified position (optional operation).  Shifts the
     * element currently at that position (if any) and any subsequent
     * elements to the right (increases their indices).  The new elements
     * will appear in this list in the order that they are returned by the
     * specified collection's iterator.  The behavior of this operation is
     * undefined if the specified collection is modified while the
     * operation is in progress.  (Note that this will occur if the specified
     * collection is this list, and it's nonempty.)
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>addAll</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of the specified
     *         collection prevents it from being added to this list
     * @throws NullPointerException if the specified collection contains one
     *         or more null elements and this list does not permit null
     *         elements, or if the specified collection is null
     * @throws IllegalArgumentException if some property of an element of the
     *         specified collection prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt; size()</tt>)
     */
    boolean addAll(int index, Collection<? extends E> c);

    /**
     * Removes from this list all of its elements that are contained in the
     * specified collection (optional operation).
     *
     * @param c collection containing elements to be removed from this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>removeAll</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean removeAll(Collection<?> c);

    /**
     * Retains only the elements in this list that are contained in the
     * specified collection (optional operation).  In other words, removes
     * from this list all of its elements that are not contained in the
     * specified collection.
     *
     * @param c collection containing elements to be retained in this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see #remove(Object)
     * @see #contains(Object)
     */
    boolean retainAll(Collection<?> c);

    /**
     * Replaces each element of this list with the result of applying the
     * operator to that element.  Errors or runtime exceptions thrown by
     * the operator are relayed to the caller.
     *
     * @implSpec
     * The default implementation is equivalent to, for this {@code list}:
     * <pre>{@code
     *     final ListIterator<E> li = list.listIterator();
     *     while (li.hasNext()) {
     *         li.set(operator.apply(li.next()));
     *     }
     * }</pre>
     *
     * If the list's list-iterator does not support the {@code set} operation
     * then an {@code UnsupportedOperationException} will be thrown when
     * replacing the first element.
     *
     * @param operator the operator to apply to each element
     * @throws UnsupportedOperationException if this list is unmodifiable.
     *         Implementations may throw this exception if an element
     *         cannot be replaced or if, in general, modification is not
     *         supported
     * @throws NullPointerException if the specified operator is null or
     *         if the operator result is a null value and this list does
     *         not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        final ListIterator<E> li = this.listIterator();
        while (li.hasNext()) {
            li.set(operator.apply(li.next()));
        }
    }

    /**
     * Sorts this list according to the order induced by the specified
     * {@link Comparator}.
     *
     * <p>All elements in this list must be <i>mutually comparable</i> using the
     * specified comparator (that is, {@code c.compare(e1, e2)} must not throw
     * a {@code ClassCastException} for any elements {@code e1} and {@code e2}
     * in the list).
     *
     * <p>If the specified comparator is {@code null} then all elements in this
     * list must implement the {@link Comparable} interface and the elements'
     * {@linkplain Comparable natural ordering} should be used.
     *
     * <p>This list must be modifiable, but need not be resizable.
     *
     * @implSpec
     * The default implementation obtains an array containing all elements in
     * this list, sorts the array, and iterates over this list resetting each
     * element from the corresponding position in the array. (This avoids the
     * n<sup>2</sup> log(n) performance that would result from attempting
     * to sort a linked list in place.)
     *
     * @implNote
     * This implementation is a stable, adaptive, iterative mergesort that
     * requires far fewer than n lg(n) comparisons when the input array is
     * partially sorted, while offering the performance of a traditional
     * mergesort when the input array is randomly ordered.  If the input array
     * is nearly sorted, the implementation requires approximately n
     * comparisons.  Temporary storage requirements vary from a small constant
     * for nearly sorted input arrays to n/2 object references for randomly
     * ordered input arrays.
     *
     * <p>The implementation takes equal advantage of ascending and
     * descending order in its input array, and can take advantage of
     * ascending and descending order in different parts of the same
     * input array.  It is well-suited to merging two or more sorted arrays:
     * simply concatenate the arrays and sort the resulting array.
     *
     * <p>The implementation was adapted from Tim Peters's list sort for Python
     * (<a href="http://svn.python.org/projects/python/trunk/Objects/listsort.txt">
     * TimSort</a>).  It uses techniques from Peter McIlroy's "Optimistic
     * Sorting and Information Theoretic Complexity", in Proceedings of the
     * Fourth Annual ACM-SIAM Symposium on Discrete Algorithms, pp 467-474,
     * January 1993.
     *
     * @param c the {@code Comparator} used to compare list elements.
     *          A {@code null} value indicates that the elements'
     *          {@linkplain Comparable natural ordering} should be used
     * @throws ClassCastException if the list contains elements that are not
     *         <i>mutually comparable</i> using the specified comparator
     * @throws UnsupportedOperationException if the list's list-iterator does
     *         not support the {@code set} operation
     * @throws IllegalArgumentException
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     *         if the comparator is found to violate the {@link Comparator}
     *         contract
     * @since 1.8
     */
    @SuppressWarnings({"unchecked", "rawtypes"})
    default void sort(Comparator<? super E> c) {
        Object[] a = this.toArray();
        Arrays.sort(a, (Comparator) c);
        ListIterator<E> i = this.listIterator();
        for (Object e : a) {
            i.next();
            i.set((E) e);
        }
    }

    /**
     * Removes all of the elements from this list (optional operation).
     * The list will be empty after this call returns.
     *
     * @throws UnsupportedOperationException if the <tt>clear</tt> operation
     *         is not supported by this list
     */
    void clear();


    // Comparison and hashing

    /**
     * Compares the specified object with this list for equality.  Returns
     * <tt>true</tt> if and only if the specified object is also a list, both
     * lists have the same size, and all corresponding pairs of elements in
     * the two lists are <i>equal</i>.  (Two elements <tt>e1</tt> and
     * <tt>e2</tt> are <i>equal</i> if <tt>(e1==null ? e2==null :
     * e1.equals(e2))</tt>.)  In other words, two lists are defined to be
     * equal if they contain the same elements in the same order.  This
     * definition ensures that the equals method works properly across
     * different implementations of the <tt>List</tt> interface.
     *
     * @param o the object to be compared for equality with this list
     * @return <tt>true</tt> if the specified object is equal to this list
     */
    boolean equals(Object o);

    /**
     * Returns the hash code value for this list.  The hash code of a list
     * is defined to be the result of the following calculation:
     * <pre>{@code
     *     int hashCode = 1;
     *     for (E e : list)
     *         hashCode = 31*hashCode + (e==null ? 0 : e.hashCode());
     * }</pre>
     * This ensures that <tt>list1.equals(list2)</tt> implies that
     * <tt>list1.hashCode()==list2.hashCode()</tt> for any two lists,
     * <tt>list1</tt> and <tt>list2</tt>, as required by the general
     * contract of {@link Object#hashCode}.
     *
     * @return the hash code value for this list
     * @see Object#equals(Object)
     * @see #equals(Object)
     */
    int hashCode();


    // Positional Access Operations

    /**
     * Returns the element at the specified position in this list.
     *
     * @param index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt;= size()</tt>)
     */
    E get(int index);

    /**
     * Replaces the element at the specified position in this list with the
     * specified element (optional operation).
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws UnsupportedOperationException if the <tt>set</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and
     *         this list does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt;= size()</tt>)
     */
    E set(int index, E element);

    /**
     * Inserts the specified element at the specified position in this list
     * (optional operation).  Shifts the element currently at that position
     * (if any) and any subsequent elements to the right (adds one to their
     * indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws UnsupportedOperationException if the <tt>add</tt> operation
     *         is not supported by this list
     * @throws ClassCastException if the class of the specified element
     *         prevents it from being added to this list
     * @throws NullPointerException if the specified element is null and
     *         this list does not permit null elements
     * @throws IllegalArgumentException if some property of the specified
     *         element prevents it from being added to this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt; size()</tt>)
     */
    void add(int index, E element);

    /**
     * Removes the element at the specified position in this list (optional
     * operation).  Shifts any subsequent elements to the left (subtracts one
     * from their indices).  Returns the element that was removed from the
     * list.
     *
     * @param index the index of the element to be removed
     * @return the element previously at the specified position
     * @throws UnsupportedOperationException if the <tt>remove</tt> operation
     *         is not supported by this list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         (<tt>index &lt; 0 || index &gt;= size()</tt>)
     */
    E remove(int index);


    // Search Operations

    /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the first occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    int indexOf(Object o);

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     *
     * @param o element to search for
     * @return the index of the last occurrence of the specified element in
     *         this list, or -1 if this list does not contain the element
     * @throws ClassCastException if the type of the specified element
     *         is incompatible with this list
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified element is null and this
     *         list does not permit null elements
     *         (<a href="Collection.html#optional-restrictions">optional</a>)
     */
    int lastIndexOf(Object o);


    // List Iterators

    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence).
     *
     * @return a list iterator over the elements in this list (in proper
     *         sequence)
     */
    ListIterator<E> listIterator();

    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * The specified index indicates the first element that would be
     * returned by an initial call to {@link ListIterator#next next}.
     * An initial call to {@link ListIterator#previous previous} would
     * return the element with the specified index minus one.
     *
     * @param index index of the first element to be returned from the
     *        list iterator (by a call to {@link ListIterator#next next})
     * @return a list iterator over the elements in this list (in proper
     *         sequence), starting at the specified position in the list
     * @throws IndexOutOfBoundsException if the index is out of range
     *         ({@code index < 0 || index > size()})
     */
    ListIterator<E> listIterator(int index);

    // View

    /**
     * Returns a view of the portion of this list between the specified
     * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.  (If
     * <tt>fromIndex</tt> and <tt>toIndex</tt> are equal, the returned list is
     * empty.)  The returned list is backed by this list, so non-structural
     * changes in the returned list are reflected in this list, and vice-versa.
     * The returned list supports all of the optional list operations supported
     * by this list.<p>
     *
     * This method eliminates the need for explicit range operations (of
     * the sort that commonly exist for arrays).  Any operation that expects
     * a list can be used as a range operation by passing a subList view
     * instead of a whole list.  For example, the following idiom
     * removes a range of elements from a list:
     * <pre>{@code
     *      list.subList(from, to).clear();
     * }</pre>
     * Similar idioms may be constructed for <tt>indexOf</tt> and
     * <tt>lastIndexOf</tt>, and all of the algorithms in the
     * <tt>Collections</tt> class can be applied to a subList.<p>
     *
     * The semantics of the list returned by this method become undefined if
     * the backing list (i.e., this list) is <i>structurally modified</i> in
     * any way other than via the returned list.  (Structural modifications are
     * those that change the size of this list, or otherwise perturb it in such
     * a fashion that iterations in progress may yield incorrect results.)
     *
     * @param fromIndex low endpoint (inclusive) of the subList
     * @param toIndex high endpoint (exclusive) of the subList
     * @return a view of the specified range within this list
     * @throws IndexOutOfBoundsException for an illegal endpoint index value
     *         (<tt>fromIndex &lt; 0 || toIndex &gt; size ||
     *         fromIndex &gt; toIndex</tt>)
     */
    List<E> subList(int fromIndex, int toIndex);

    /**
     * Creates a {@link Spliterator} over the elements in this list.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED} and
     * {@link Spliterator#ORDERED}.  Implementations should document the
     * reporting of additional characteristic values.
     *
     * @implSpec
     * The default implementation creates a
     * <em><a href="Spliterator.html#binding">late-binding</a></em> spliterator
     * from the list's {@code Iterator}.  The spliterator inherits the
     * <em>fail-fast</em> properties of the list's iterator.
     *
     * @implNote
     * The created {@code Spliterator} additionally reports
     * {@link Spliterator#SUBSIZED}.
     *
     * @return a {@code Spliterator} over the elements in this list
     * @since 1.8
     */
    @Override
    default Spliterator<E> spliterator() {
        return Spliterators.spliterator(this, Spliterator.ORDERED);
    }
}

 

 简单看一下抽象类的结构图：

说明一下：

从源码和抽象类的结构图中，同样可以看到：

（1）List接口中没有说明集合数据是存放在哪里的?只是声明了一些集合操作的方法。、

（2）List接口中没有看到迭代器是如何获取集合数据并存放到链表中的：如下：

ListIterator<E> listIterator();
ListIterator<E> listIterator(int index);

 

所以想要探明究竟，就只能继续往下看，层层往下，直到ArrayList实现子类。

 7、看看实现类：ArrayList<E>是如何实现集合功能的？

贴上JDK1.8中的源码：（内容有点多，会在下面捡重要的说明一下）

/*
 * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */

package java.util;

import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

/**
 * Resizable-array implementation of the <tt>List</tt> interface.  Implements
 * all optional list operations, and permits all elements, including
 * <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface,
 * this class provides methods to manipulate the size of the array that is
 * used internally to store the list.  (This class is roughly equivalent to
 * <tt>Vector</tt>, except that it is unsynchronized.)
 *
 * <p>The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,
 * <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant
 * time.  The <tt>add</tt> operation runs in <i>amortized constant time</i>,
 * that is, adding n elements requires O(n) time.  All of the other operations
 * run in linear time (roughly speaking).  The constant factor is low compared
 * to that for the <tt>LinkedList</tt> implementation.
 *
 * <p>Each <tt>ArrayList</tt> instance has a <i>capacity</i>.  The capacity is
 * the size of the array used to store the elements in the list.  It is always
 * at least as large as the list size.  As elements are added to an ArrayList,
 * its capacity grows automatically.  The details of the growth policy are not
 * specified beyond the fact that adding an element has constant amortized
 * time cost.
 *
 * <p>An application can increase the capacity of an <tt>ArrayList</tt> instance
 * before adding a large number of elements using the <tt>ensureCapacity</tt>
 * operation.  This may reduce the amount of incremental reallocation.
 *
 * <p><strong>Note that this implementation is not synchronized.</strong>
 * If multiple threads access an <tt>ArrayList</tt> instance concurrently,
 * and at least one of the threads modifies the list structurally, it
 * <i>must</i> be synchronized externally.  (A structural modification is
 * any operation that adds or deletes one or more elements, or explicitly
 * resizes the backing array; merely setting the value of an element is not
 * a structural modification.)  This is typically accomplished by
 * synchronizing on some object that naturally encapsulates the list.
 *
 * If no such object exists, the list should be "wrapped" using the
 * {@link Collections#synchronizedList Collections.synchronizedList}
 * method.  This is best done at creation time, to prevent accidental
 * unsynchronized access to the list:<pre>
 *   List list = Collections.synchronizedList(new ArrayList(...));</pre>
 *
 * <p><a name="fail-fast">
 * The iterators returned by this class's {@link #iterator() iterator} and
 * {@link #listIterator(int) listIterator} methods are <em>fail-fast</em>:</a>
 * if the list is structurally modified at any time after the iterator is
 * created, in any way except through the iterator's own
 * {@link ListIterator#remove() remove} or
 * {@link ListIterator#add(Object) add} methods, the iterator will throw a
 * {@link ConcurrentModificationException}.  Thus, in the face of
 * concurrent modification, the iterator fails quickly and cleanly, rather
 * than risking arbitrary, non-deterministic behavior at an undetermined
 * time in the future.
 *
 * <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
 * as it is, generally speaking, impossible to make any hard guarantees in the
 * presence of unsynchronized concurrent modification.  Fail-fast iterators
 * throw {@code ConcurrentModificationException} on a best-effort basis.
 * Therefore, it would be wrong to write a program that depended on this
 * exception for its correctness:  <i>the fail-fast behavior of iterators
 * should be used only to detect bugs.</i>
 *
 * <p>This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @see     Collection
 * @see     List
 * @see     LinkedList
 * @see     Vector
 * @since   1.2
 */

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * Default initial capacity.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer. Any
     * empty ArrayList with elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA
     * will be expanded to DEFAULT_CAPACITY when the first element is added.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    /**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    /**
     * Increases the capacity of this <tt>ArrayList</tt> instance, if
     * necessary, to ensure that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     *
     * @param   minCapacity   the desired minimum capacity
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
            // any size if not default element table
            ? 0
            // larger than default for default empty table. It's already
            // supposed to be at default size.
            : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * The maximum size of array to allocate.
     * Some VMs reserve some header words in an array.
     * Attempts to allocate larger arrays may result in
     * OutOfMemoryError: Requested array size exceeds VM limit
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * Increases the capacity to ensure that it can hold at least the
     * number of elements specified by the minimum capacity argument.
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    /**
     * Returns the number of elements in this list.
     *
     * @return the number of elements in this list
     */
    public int size() {
        return size;
    }

    /**
     * Returns <tt>true</tt> if this list contains no elements.
     *
     * @return <tt>true</tt> if this list contains no elements
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * Returns <tt>true</tt> if this list contains the specified element.
     * More formally, returns <tt>true</tt> if and only if this list contains
     * at least one element <tt>e</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;e==null&nbsp;:&nbsp;o.equals(e))</tt>.
     *
     * @param o element whose presence in this list is to be tested
     * @return <tt>true</tt> if this list contains the specified element
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * Returns the index of the first occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the lowest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    public int indexOf(Object o) {
        if (o == null) {
            for (int i = 0; i < size; i++)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = 0; i < size; i++)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    /**
     * Returns the index of the last occurrence of the specified element
     * in this list, or -1 if this list does not contain the element.
     * More formally, returns the highest index <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>,
     * or -1 if there is no such index.
     */
    public int lastIndexOf(Object o) {
        if (o == null) {
            for (int i = size-1; i >= 0; i--)
                if (elementData[i]==null)
                    return i;
        } else {
            for (int i = size-1; i >= 0; i--)
                if (o.equals(elementData[i]))
                    return i;
        }
        return -1;
    }

    /**
     * Returns a shallow copy of this <tt>ArrayList</tt> instance.  (The
     * elements themselves are not copied.)
     *
     * @return a clone of this <tt>ArrayList</tt> instance
     */
    public Object clone() {
        try {
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    /**
     * Returns an array containing all of the elements in this list
     * in proper sequence (from first to last element).
     *
     * <p>The returned array will be "safe" in that no references to it are
     * maintained by this list.  (In other words, this method must allocate
     * a new array).  The caller is thus free to modify the returned array.
     *
     * <p>This method acts as bridge between array-based and collection-based
     * APIs.
     *
     * @return an array containing all of the elements in this list in
     *         proper sequence
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * Returns an array containing all of the elements in this list in proper
     * sequence (from first to last element); the runtime type of the returned
     * array is that of the specified array.  If the list fits in the
     * specified array, it is returned therein.  Otherwise, a new array is
     * allocated with the runtime type of the specified array and the size of
     * this list.
     *
     * <p>If the list fits in the specified array with room to spare
     * (i.e., the array has more elements than the list), the element in
     * the array immediately following the end of the collection is set to
     * <tt>null</tt>.  (This is useful in determining the length of the
     * list <i>only</i> if the caller knows that the list does not contain
     * any null elements.)
     *
     * @param a the array into which the elements of the list are to
     *          be stored, if it is big enough; otherwise, a new array of the
     *          same runtime type is allocated for this purpose.
     * @return an array containing the elements of the list
     * @throws ArrayStoreException if the runtime type of the specified array
     *         is not a supertype of the runtime type of every element in
     *         this list
     * @throws NullPointerException if the specified array is null
     */
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        if (a.length < size)
            // Make a new array of a's runtime type, but my contents:
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        System.arraycopy(elementData, 0, a, 0, size);
        if (a.length > size)
            a[size] = null;
        return a;
    }

    // Positional Access Operations

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /**
     * Returns the element at the specified position in this list.
     *
     * @param  index index of the element to return
     * @return the element at the specified position in this list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /**
     * Replaces the element at the specified position in this list with
     * the specified element.
     *
     * @param index index of the element to replace
     * @param element element to be stored at the specified position
     * @return the element previously at the specified position
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    /**
     * Appends the specified element to the end of this list.
     *
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * Inserts the specified element at the specified position in this
     * list. Shifts the element currently at that position (if any) and
     * any subsequent elements to the right (adds one to their indices).
     *
     * @param index index at which the specified element is to be inserted
     * @param element element to be inserted
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

    /**
     * Removes the element at the specified position in this list.
     * Shifts any subsequent elements to the left (subtracts one from their
     * indices).
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public E remove(int index) {
        rangeCheck(index);

        modCount++;
        E oldValue = elementData(index);

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

    /**
     * Removes the first occurrence of the specified element from this list,
     * if it is present.  If the list does not contain the element, it is
     * unchanged.  More formally, removes the element with the lowest index
     * <tt>i</tt> such that
     * <tt>(o==null&nbsp;?&nbsp;get(i)==null&nbsp;:&nbsp;o.equals(get(i)))</tt>
     * (if such an element exists).  Returns <tt>true</tt> if this list
     * contained the specified element (or equivalently, if this list
     * changed as a result of the call).
     *
     * @param o element to be removed from this list, if present
     * @return <tt>true</tt> if this list contained the specified element
     */
    public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

    /**
     * Removes all of the elements from this list.  The list will
     * be empty after this call returns.
     */
    public void clear() {
        modCount++;

        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;

        size = 0;
    }

    /**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the
     * specified collection's Iterator.  The behavior of this operation is
     * undefined if the specified collection is modified while the operation
     * is in progress.  (This implies that the behavior of this call is
     * undefined if the specified collection is this list, and this
     * list is nonempty.)
     *
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * Inserts all of the elements in the specified collection into this
     * list, starting at the specified position.  Shifts the element
     * currently at that position (if any) and any subsequent elements to
     * the right (increases their indices).  The new elements will appear
     * in the list in the order that they are returned by the
     * specified collection's iterator.
     *
     * @param index index at which to insert the first element from the
     *              specified collection
     * @param c collection containing elements to be added to this list
     * @return <tt>true</tt> if this list changed as a result of the call
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws NullPointerException if the specified collection is null
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  // Increments modCount

        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * Removes from this list all of the elements whose index is between
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
     * Shifts any succeeding elements to the left (reduces their index).
     * This call shortens the list by {@code (toIndex - fromIndex)} elements.
     * (If {@code toIndex==fromIndex}, this operation has no effect.)
     *
     * @throws IndexOutOfBoundsException if {@code fromIndex} or
     *         {@code toIndex} is out of range
     *         ({@code fromIndex < 0 ||
     *          fromIndex >= size() ||
     *          toIndex > size() ||
     *          toIndex < fromIndex})
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    /**
     * Checks if the given index is in range.  If not, throws an appropriate
     * runtime exception.  This method does *not* check if the index is
     * negative: It is always used immediately prior to an array access,
     * which throws an ArrayIndexOutOfBoundsException if index is negative.
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * A version of rangeCheck used by add and addAll.
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * Constructs an IndexOutOfBoundsException detail message.
     * Of the many possible refactorings of the error handling code,
     * this "outlining" performs best with both server and client VMs.
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
     * Removes from this list all of its elements that are contained in the
     * specified collection.
     *
     * @param c collection containing elements to be removed from this list
     * @return {@code true} if this list changed as a result of the call
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see Collection#contains(Object)
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    /**
     * Retains only the elements in this list that are contained in the
     * specified collection.  In other words, removes from this list all
     * of its elements that are not contained in the specified collection.
     *
     * @param c collection containing elements to be retained in this list
     * @return {@code true} if this list changed as a result of the call
     * @throws ClassCastException if the class of an element of this list
     *         is incompatible with the specified collection
     * (<a href="Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if this list contains a null element and the
     *         specified collection does not permit null elements
     * (<a href="Collection.html#optional-restrictions">optional</a>),
     *         or if the specified collection is null
     * @see Collection#contains(Object)
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }

    /**
     * Save the state of the <tt>ArrayList</tt> instance to a stream (that
     * is, serialize it).
     *
     * @serialData The length of the array backing the <tt>ArrayList</tt>
     *             instance is emitted (int), followed by all of its elements
     *             (each an <tt>Object</tt>) in the proper order.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);

        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    /**
     * Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
     * deserialize it).
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

        // Read in size, and any hidden stuff
        s.defaultReadObject();

        // Read in capacity
        s.readInt(); // ignored

        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            ensureCapacityInternal(size);

            Object[] a = elementData;
            // Read in all elements in the proper order.
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }

    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence), starting at the specified position in the list.
     * The specified index indicates the first element that would be
     * returned by an initial call to {@link ListIterator#next next}.
     * An initial call to {@link ListIterator#previous previous} would
     * return the element with the specified index minus one.
     *
     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence).
     *
     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @see #listIterator(int)
     */
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * An optimized version of AbstractList.Itr
     */
    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    /**
     * An optimized version of AbstractList.ListItr
     */
    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor - 1;
        }

        @SuppressWarnings("unchecked")
        public E previous() {
            checkForComodification();
            int i = cursor - 1;
            if (i < 0)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i;
            return (E) elementData[lastRet = i];
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.set(lastRet, e);
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                ArrayList.this.add(i, e);
                cursor = i + 1;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

    /**
     * Returns a view of the portion of this list between the specified
     * {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.  (If
     * {@code fromIndex} and {@code toIndex} are equal, the returned list is
     * empty.)  The returned list is backed by this list, so non-structural
     * changes in the returned list are reflected in this list, and vice-versa.
     * The returned list supports all of the optional list operations.
     *
     * <p>This method eliminates the need for explicit range operations (of
     * the sort that commonly exist for arrays).  Any operation that expects
     * a list can be used as a range operation by passing a subList view
     * instead of a whole list.  For example, the following idiom
     * removes a range of elements from a list:
     * <pre>
     *      list.subList(from, to).clear();
     * </pre>
     * Similar idioms may be constructed for {@link #indexOf(Object)} and
     * {@link #lastIndexOf(Object)}, and all of the algorithms in the
     * {@link Collections} class can be applied to a subList.
     *
     * <p>The semantics of the list returned by this method become undefined if
     * the backing list (i.e., this list) is <i>structurally modified</i> in
     * any way other than via the returned list.  (Structural modifications are
     * those that change the size of this list, or otherwise perturb it in such
     * a fashion that iterations in progress may yield incorrect results.)
     *
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @throws IllegalArgumentException {@inheritDoc}
     */
    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }

    static void subListRangeCheck(int fromIndex, int toIndex, int size) {
        if (fromIndex < 0)
            throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
        if (toIndex > size)
            throw new IndexOutOfBoundsException("toIndex = " + toIndex);
        if (fromIndex > toIndex)
            throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                               ") > toIndex(" + toIndex + ")");
    }

    private class SubList extends AbstractList<E> implements RandomAccess {
        private final AbstractList<E> parent;
        private final int parentOffset;
        private final int offset;
        int size;

        SubList(AbstractList<E> parent,
                int offset, int fromIndex, int toIndex) {
            this.parent = parent;
            this.parentOffset = fromIndex;
            this.offset = offset + fromIndex;
            this.size = toIndex - fromIndex;
            this.modCount = ArrayList.this.modCount;
        }

        public E set(int index, E e) {
            rangeCheck(index);
            checkForComodification();
            E oldValue = ArrayList.this.elementData(offset + index);
            ArrayList.this.elementData[offset + index] = e;
            return oldValue;
        }

        public E get(int index) {
            rangeCheck(index);
            checkForComodification();
            return ArrayList.this.elementData(offset + index);
        }

        public int size() {
            checkForComodification();
            return this.size;
        }

        public void add(int index, E e) {
            rangeCheckForAdd(index);
            checkForComodification();
            parent.add(parentOffset + index, e);
            this.modCount = parent.modCount;
            this.size++;
        }

        public E remove(int index) {
            rangeCheck(index);
            checkForComodification();
            E result = parent.remove(parentOffset + index);
            this.modCount = parent.modCount;
            this.size--;
            return result;
        }

        protected void removeRange(int fromIndex, int toIndex) {
            checkForComodification();
            parent.removeRange(parentOffset + fromIndex,
                               parentOffset + toIndex);
            this.modCount = parent.modCount;
            this.size -= toIndex - fromIndex;
        }

        public boolean addAll(Collection<? extends E> c) {
            return addAll(this.size, c);
        }

        public boolean addAll(int index, Collection<? extends E> c) {
            rangeCheckForAdd(index);
            int cSize = c.size();
            if (cSize==0)
                return false;

            checkForComodification();
            parent.addAll(parentOffset + index, c);
            this.modCount = parent.modCount;
            this.size += cSize;
            return true;
        }

        public Iterator<E> iterator() {
            return listIterator();
        }

        public ListIterator<E> listIterator(final int index) {
            checkForComodification();
            rangeCheckForAdd(index);
            final int offset = this.offset;

            return new ListIterator<E>() {
                int cursor = index;
                int lastRet = -1;
                int expectedModCount = ArrayList.this.modCount;

                public boolean hasNext() {
                    return cursor != SubList.this.size;
                }

                @SuppressWarnings("unchecked")
                public E next() {
                    checkForComodification();
                    int i = cursor;
                    if (i >= SubList.this.size)
                        throw new NoSuchElementException();
                    Object[] elementData = ArrayList.this.elementData;
                    if (offset + i >= elementData.length)
                        throw new ConcurrentModificationException();
                    cursor = i + 1;
                    return (E) elementData[offset + (lastRet = i)];
                }

                public boolean hasPrevious() {
                    return cursor != 0;
                }

                @SuppressWarnings("unchecked")
                public E previous() {
                    checkForComodification();
                    int i = cursor - 1;
                    if (i < 0)
                        throw new NoSuchElementException();
                    Object[] elementData = ArrayList.this.elementData;
                    if (offset + i >= elementData.length)
                        throw new ConcurrentModificationException();
                    cursor = i;
                    return (E) elementData[offset + (lastRet = i)];
                }

                @SuppressWarnings("unchecked")
                public void forEachRemaining(Consumer<? super E> consumer) {
                    Objects.requireNonNull(consumer);
                    final int size = SubList.this.size;
                    int i = cursor;
                    if (i >= size) {
                        return;
                    }
                    final Object[] elementData = ArrayList.this.elementData;
                    if (offset + i >= elementData.length) {
                        throw new ConcurrentModificationException();
                    }
                    while (i != size && modCount == expectedModCount) {
                        consumer.accept((E) elementData[offset + (i++)]);
                    }
                    // update once at end of iteration to reduce heap write traffic
                    lastRet = cursor = i;
                    checkForComodification();
                }

                public int nextIndex() {
                    return cursor;
                }

                public int previousIndex() {
                    return cursor - 1;
                }

                public void remove() {
                    if (lastRet < 0)
                        throw new IllegalStateException();
                    checkForComodification();

                    try {
                        SubList.this.remove(lastRet);
                        cursor = lastRet;
                        lastRet = -1;
                        expectedModCount = ArrayList.this.modCount;
                    } catch (IndexOutOfBoundsException ex) {
                        throw new ConcurrentModificationException();
                    }
                }

                public void set(E e) {
                    if (lastRet < 0)
                        throw new IllegalStateException();
                    checkForComodification();

                    try {
                        ArrayList.this.set(offset + lastRet, e);
                    } catch (IndexOutOfBoundsException ex) {
                        throw new ConcurrentModificationException();
                    }
                }

                public void add(E e) {
                    checkForComodification();

                    try {
                        int i = cursor;
                        SubList.this.add(i, e);
                        cursor = i + 1;
                        lastRet = -1;
                        expectedModCount = ArrayList.this.modCount;
                    } catch (IndexOutOfBoundsException ex) {
                        throw new ConcurrentModificationException();
                    }
                }

                final void checkForComodification() {
                    if (expectedModCount != ArrayList.this.modCount)
                        throw new ConcurrentModificationException();
                }
            };
        }

        public List<E> subList(int fromIndex, int toIndex) {
            subListRangeCheck(fromIndex, toIndex, size);
            return new SubList(this, offset, fromIndex, toIndex);
        }

        private void rangeCheck(int index) {
            if (index < 0 || index >= this.size)
                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }

        private void rangeCheckForAdd(int index) {
            if (index < 0 || index > this.size)
                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }

        private String outOfBoundsMsg(int index) {
            return "Index: "+index+", Size: "+this.size;
        }

        private void checkForComodification() {
            if (ArrayList.this.modCount != this.modCount)
                throw new ConcurrentModificationException();
        }

        public Spliterator<E> spliterator() {
            checkForComodification();
            return new ArrayListSpliterator<E>(ArrayList.this, offset,
                                               offset + this.size, this.modCount);
        }
    }

    @Override
    public void forEach(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        final int expectedModCount = modCount;
        @SuppressWarnings("unchecked")
        final E[] elementData = (E[]) this.elementData;
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            action.accept(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    /**
     * Creates a <em><a href="Spliterator.html#binding">late-binding</a></em>
     * and <em>fail-fast</em> {@link Spliterator} over the elements in this
     * list.
     *
     * <p>The {@code Spliterator} reports {@link Spliterator#SIZED},
     * {@link Spliterator#SUBSIZED}, and {@link Spliterator#ORDERED}.
     * Overriding implementations should document the reporting of additional
     * characteristic values.
     *
     * @return a {@code Spliterator} over the elements in this list
     * @since 1.8
     */
    @Override
    public Spliterator<E> spliterator() {
        return new ArrayListSpliterator<>(this, 0, -1, 0);
    }

    /** Index-based split-by-two, lazily initialized Spliterator */
    static final class ArrayListSpliterator<E> implements Spliterator<E> {

        /*
         * If ArrayLists were immutable, or structurally immutable (no
         * adds, removes, etc), we could implement their spliterators
         * with Arrays.spliterator. Instead we detect as much
         * interference during traversal as practical without
         * sacrificing much performance. We rely primarily on
         * modCounts. These are not guaranteed to detect concurrency
         * violations, and are sometimes overly conservative about
         * within-thread interference, but detect enough problems to
         * be worthwhile in practice. To carry this out, we (1) lazily
         * initialize fence and expectedModCount until the latest
         * point that we need to commit to the state we are checking
         * against; thus improving precision.  (This doesn't apply to
         * SubLists, that create spliterators with current non-lazy
         * values).  (2) We perform only a single
         * ConcurrentModificationException check at the end of forEach
         * (the most performance-sensitive method). When using forEach
         * (as opposed to iterators), we can normally only detect
         * interference after actions, not before. Further
         * CME-triggering checks apply to all other possible
         * violations of assumptions for example null or too-small
         * elementData array given its size(), that could only have
         * occurred due to interference.  This allows the inner loop
         * of forEach to run without any further checks, and
         * simplifies lambda-resolution. While this does entail a
         * number of checks, note that in the common case of
         * list.stream().forEach(a), no checks or other computation
         * occur anywhere other than inside forEach itself.  The other
         * less-often-used methods cannot take advantage of most of
         * these streamlinings.
         */

        private final ArrayList<E> list;
        private int index; // current index, modified on advance/split
        private int fence; // -1 until used; then one past last index
        private int expectedModCount; // initialized when fence set

        /** Create new spliterator covering the given  range */
        ArrayListSpliterator(ArrayList<E> list, int origin, int fence,
                             int expectedModCount) {
            this.list = list; // OK if null unless traversed
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }

        private int getFence() { // initialize fence to size on first use
            int hi; // (a specialized variant appears in method forEach)
            ArrayList<E> lst;
            if ((hi = fence) < 0) {
                if ((lst = list) == null)
                    hi = fence = 0;
                else {
                    expectedModCount = lst.modCount;
                    hi = fence = lst.size;
                }
            }
            return hi;
        }

        public ArrayListSpliterator<E> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null : // divide range in half unless too small
                new ArrayListSpliterator<E>(list, lo, index = mid,
                                            expectedModCount);
        }

        public boolean tryAdvance(Consumer<? super E> action) {
            if (action == null)
                throw new NullPointerException();
            int hi = getFence(), i = index;
            if (i < hi) {
                index = i + 1;
                @SuppressWarnings("unchecked") E e = (E)list.elementData[i];
                action.accept(e);
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            int i, hi, mc; // hoist accesses and checks from loop
            ArrayList<E> lst; Object[] a;
            if (action == null)
                throw new NullPointerException();
            if ((lst = list) != null && (a = lst.elementData) != null) {
                if ((hi = fence) < 0) {
                    mc = lst.modCount;
                    hi = lst.size;
                }
                else
                    mc = expectedModCount;
                if ((i = index) >= 0 && (index = hi) <= a.length) {
                    for (; i < hi; ++i) {
                        @SuppressWarnings("unchecked") E e = (E) a[i];
                        action.accept(e);
                    }
                    if (lst.modCount == mc)
                        return;
                }
            }
            throw new ConcurrentModificationException();
        }

        public long estimateSize() {
            return (long) (getFence() - index);
        }

        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        // figure out which elements are to be removed
        // any exception thrown from the filter predicate at this stage
        // will leave the collection unmodified
        int removeCount = 0;
        final BitSet removeSet = new BitSet(size);
        final int expectedModCount = modCount;
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            @SuppressWarnings("unchecked")
            final E element = (E) elementData[i];
            if (filter.test(element)) {
                removeSet.set(i);
                removeCount++;
            }
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }

        // shift surviving elements left over the spaces left by removed elements
        final boolean anyToRemove = removeCount > 0;
        if (anyToRemove) {
            final int newSize = size - removeCount;
            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
                i = removeSet.nextClearBit(i);
                elementData[j] = elementData[i];
            }
            for (int k=newSize; k < size; k++) {
                elementData[k] = null;  // Let gc do its work
            }
            this.size = newSize;
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            modCount++;
        }

        return anyToRemove;
    }

    @Override
    @SuppressWarnings("unchecked")
    public void replaceAll(UnaryOperator<E> operator) {
        Objects.requireNonNull(operator);
        final int expectedModCount = modCount;
        final int size = this.size;
        for (int i=0; modCount == expectedModCount && i < size; i++) {
            elementData[i] = operator.apply((E) elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }

    @Override
    @SuppressWarnings("unchecked")
    public void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, size, c);
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
}

 贴上类的结构图：

内容太长，分2次截图。

直接看看里面有些什么吧！

（1）存放集合数据的数组！　　

private static final Object[] EMPTY_ELEMENTDATA = {};//用于空实例的共享空数组实例。
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};//扩展数组
transient Object[] elementData;//存放数据的原始数组

 (2）实例化集合对象

//【1】传入集合大小，初始化集合容量，不够时自动扩展
public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA;
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}
//【2】直接拿到集合实例，不固定集合大小
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}
//【3】用另外一个集合来实例化当前集合
public ArrayList(Collection<? extends E> c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

 分析说明一下：

　　（1）如果采用【1】中方式拿到集合对象，那么当超过设置的初始容量的时候是如何扩容的呢？

看下面这个例子：

public static void main(String[] args) throws Exception {
        List list=new ArrayList(2);
        System.out.println("扩展前集合容量1："+getCapacity(list));
        list.add(1);
        list.add(2);
        System.out.println("扩展前集合容量2："+getCapacity(list));
        list.add(3);
        list.add(4);
        System.out.println("扩展后集合容量3："+getCapacity(list));
        list.add(5);
        list.add(6);
        System.out.println("扩展后集合容量4："+getCapacity(list));
        list.add(7);
        list.add(8);
        System.out.println("扩展后集合容量5："+getCapacity(list));
        list.add(9);
        System.out.println("扩展后集合容量6："+getCapacity(list));
        list.add(10);
        System.out.println("扩展后集合容量7："+getCapacity(list));
        for(Object o:list){
            System.out.println(o);
        }
        System.out.println("扩展后集合容量8："+getCapacity(list));
    }
    //反射拿到当前集合保存数据的数组的容量
    static int getCapacity(List<?> l) throws Exception {  
        Field dataField = ArrayList.class.getDeclaredField("elementData");  
        dataField.setAccessible(true);  
        return ((Object[]) dataField.get(l)).length;  
    }

 输出结果：

扩展前集合容量1：2
扩展前集合容量2：2
扩展后集合容量3：4
扩展后集合容量4：6
扩展后集合容量5：9
扩展后集合容量6：9
扩展后集合容量7：13
1
2
3
4
5
6
7
8
9
10
扩展后集合容量8：13

　从结果上，可以知道，扩展容量并不是均匀扩展的（每次都递增相同的容量），而是发生着变化的，那么这种变化规则是怎么样的呢？

继续分析一下源码中是怎么定义的：

//将指定的元素追加到列表的末尾
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // 增量模式计数!!
    elementData[size++] = e;
    return true;
}
//确定内部容量的方法
private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    ensureExplicitCapacity(minCapacity);
}
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;
    //判断当前集合是否超多最大容量，即是否有容量溢出现象
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}
//集合内部维护的是数组扩容
private void grow(int minCapacity) {
    int oldCapacity = elementData.length;//拿到当前集合大小
    int newCapacity = oldCapacity + (oldCapacity >> 1);//计算集合新的容量大小
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    //复制旧集合数据到新的集合中（内部维护的数组扩容-产生新的数组）
    elementData = Arrays.copyOf(elementData, newCapacity);
}

 那么在详细看以下2行代码是如何计算增加的容量的吧：

 int oldCapacity = elementData.length;
 int newCapacity = oldCapacity + (oldCapacity >> 1);

简单说明一下：“>>”运算符

　　>>运算规则：按二进制形式把所有的数字向右移动对应巍峨位数，低位移出（舍弃），高位的空位补符号位，即正数补零，负数补1.
　　  语法格式：
　　  需要移位的数字 >> 移位的次数
　　  例如11 >> 2，则是将数字11右移2位
　　  计算过程：11的二进制形式为：0000 0000 0000 0000 0000 0000 0000 1011，然后把低位的最后两个数字移出，因为该数字是正数，所以在高位补零。则得到的最终结果是0000 0000 0000 0000 0000 0000 0000 0010.转换为十进制是3.数学意义：右移一位相当于除2，右移n位相当于除以2的n次方。

举一个简答的例子：

public static void main(String[] args) throws Exception {
        for(int i=0;i<=10;i++){
            System.out.println("******当前i="+i+"***********");
            System.out.println("递增数值："+((i>>1)));
            System.out.println("递增数后值："+(i+(i>>1)));
        }
}

 运行结果：

******当前i=0***********
递增数值：0
递增数后值：0
******当前i=1***********
递增数值：0
递增数后值：1
******当前i=2***********
递增数值：1
递增数后值：3
******当前i=3***********
递增数值：1
递增数后值：4
******当前i=4***********
递增数值：2
递增数后值：6
******当前i=5***********
递增数值：2
递增数后值：7
******当前i=6***********
递增数值：3
递增数后值：9
******当前i=7***********
递增数值：3
递增数后值：10
******当前i=8***********
递增数值：4
递增数后值：12
******当前i=9***********
递增数值：4
递增数后值：13
******当前i=10***********
递增数值：5
递增数后值：15

 分析可以知道：每次递增的数值为：源数值的一半，且向下取整得到的数值。

（3）如何添加数据待集合中

　　ArrayList实现类中提供了2种方法，供添加数据使用：

public boolean add(E e) {//【1】直接在集合末尾添加一个数据
    ensureCapacityInternal(size + 1);  // 判断数据是否会溢出
    elementData[size++] = e;
    return true;
}
public void add(int index, E element) {//【2】添加一个数据到集合的指定位置
    rangeCheckForAdd(index);//检查位置是否合法
    ensureCapacityInternal(size + 1);  // 判断数据是否会溢出
    System.arraycopy(elementData, index, elementData, index + 1,//System提供了一个静态方法arraycopy(),我们可以使用它来实现数组之间的复制
                     size - index);
    elementData[index] = element;
    size++;
}

 方法【1】直接在集合末尾添加一个数据，上面已经给出了分析，这里就不在赘述了，那么来看一下方法【2】是怎么处理的！

（1）在指定位置添加一个数据，这个指定的位置，就需要检查是否合法：

 rangeCheckForAdd(index);//检查位置是否合法

 private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

 （2）然后就是判断，如果新增了这个元素，会不会导致整个数组的数据溢出，如果会导致溢出现象，则要扩容数组。

具体的方法上面已经给出分析：这里再放一遍代码

private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

 （3）此时，位置索引值合法，数组也不会溢出了，就开始要实现如何插入。

System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);

 上面这句代码：

System提供了一个静态方法arraycopy(),我们可以使用它来实现数组之间的复制。其函数原型是：

public static void (Object src,
                             int srcPos,
                             Object dest,
                             int destPos,
                             int length)

 参数说明：

src:源数组； srcPos:源数组要复制的起始位置；
dest:目的数组； destPos:目的数组放置的起始位置； length:复制的长度。

注意：src and dest都必须是同类型或者可以进行转换类型的数组．
有趣的是这个函数可以实现自己到自己复制，比如：
int[] fun ={0,1,2,3,4,5,6};
System.arraycopy(fun,0,fun,3,3);
则结果为：{0,1,2,0,1,2,6};
实现过程是这样的，先生成一个长度为length的临时数组,将fun数组中srcPos
到srcPos+length-1之间的数据拷贝到临时数组中，再执行System.arraycopy(临时数组,0,fun,3,3).

（4）实现了，指定位置之后（包含指定位置）的数据，向后移动的功能后，开始在指定的位置插入数据了。并且数据长度+1

 elementData[index] = element;
        size++;

 (4)如何从集合中删除一个数据

　　ArrayList实现类中提供了2种方法，供删除数据使用：

public E remove(int index) {//【1】删除指定位置的数据
    rangeCheck(index);//检查位置是否合法
    modCount++;
    E oldValue = elementData(index);//拿到指定位置的数据
    int numMoved = size - index - 1;//判断当前删除的位置是否是最后一位，如果是，则不需要移动数组元素，否则就需要移动
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; //置空该位置的数据
    return oldValue;
}
 public boolean remove(Object o) {//【2】删除指定的数据
    if (o == null) {//当前要删除的数据为null
        for (int index = 0; index < size; index++)
            if (elementData[index] == null) {//遍历数组容量，拿到要删除数据的位置索引值
                fastRemove(index);//调用删除元素的方法
                return true;
            }
    } else {//当前要删除的数据不为null
        for (int index = 0; index < size; index++)
            if (o.equals(elementData[index])) {//遍历数组容量，拿到要删除数据的位置索引值
                fastRemove(index);//调用删除元素的方法
                return true;
            }
    }
    return false;
}
private void fastRemove(int index) {//快速删除指定位置的元素
    modCount++;
    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work
}
private void rangeCheck(int index) {//判断当前位置的索引值是否合法
    if (index >= size)//要删除的数据的位置不可超过数组容量，说明：如果过小的话，也会造成异常的发生
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

 总结一下;

　　(1)具体如何删除集合数据，上面代码+说明已经很明显了，这里也不在赘述了。

有意思的是：　　

private void rangeCheck(int index) {//判断当前位置的索引值是否合法
    if (index >= size)//要删除的数据的位置不可超过数组容量，说明：如果过小的话，也会造成异常的发生
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

 

 在删除指定位置的数据的时候，对这个位置索引值做了合法性判断。但是只做了上限判断，下限判断 没有做。

（1）当索引值超过数组上限容量的时候，会抛出：throw new IndexOutOfBoundsException(outOfBoundsMsg(index));

（2）当索引值低于数组下限容量值0的时候，如何处理呢？　　

public static void main(String[] args) throws Exception {
        List alist=Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        alist.remove(-1);        
    }

 运行结果：

Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractList.remove(AbstractList.java:161)
at com.xfwl.test.Test4.main(Test4.java:17)

分析说明：

　　根据运行结果，发现的确会抛出异常，但是并不是在ArrayList这个实现子类中抛出的，而是在ArrayList继承的AbstractList父类中抛出的，ArrayList重写了父类的方法，虽然其中并没有抛出： java.lang.UnsupportedOperationException

但是查看了源码，就可以知道：父类抽象类：AbstractList的remove(int index)方法中，抛出了异常。

public E remove(int index) {
        throw new UnsupportedOperationException();
    }

 还有一种现象：就是我们删除数据的位置超过数组上限值，也抛出：java.lang.UnsupportedOperationException

public static void main(String[] args) {
        List alist=Arrays.asList(1,2,3,4,5,6,7,8,9,10);
        alist.remove(100);
    }

 运行结果：

Exception in thread "main" java.lang.UnsupportedOperationException
at java.util.AbstractList.remove(AbstractList.java:161)
at com.xfwl.test.Test2.main(Test2.java:11)

分析说明：

　　可以看到，超过上限值，也会抛出父类方法中的异常。

 总结一下：

　　 首先ArrayList是子类，AbstractList是父类。

（1）当子类中因为超过上限，而抛出：throw new IndexOutOfBoundsException(outOfBoundsMsg(index));   但是合格异常并没有被处理，会继续往上抛，就会抛到父类中去。而父类AbstractList的实现接口List和继承AbstractCollection类中都没有：

 public E remove(int index)这个方法，换句话说就是， public E remove(int index)这个方法在AbstractList这里就终止了，也就抛出了AbstractList中的这个异常。

（2）当子类中因为地域下限时，由于没有异常捕获或者抛出处理，所以同样会到父类中捕获抛出异常。

（5）集合中是如何把数据传递给迭代器中的链表的呢？

分析JDK1.8的源码，可以知道：ArrayList中提供了，三个获取迭代器的方法，2个内置的具有继承关系的迭代器类。

三个获取迭代器的方法：

public ListIterator<E> listIterator(int index) {//【1】获取数组指定位置（不是索引值）之后的数组数据存放入迭代器链表中
    if (index < 0 || index > size)
        throw new IndexOutOfBoundsException("Index: "+index);
    return new ListItr(index);
}
public ListIterator<E> listIterator() {//【2】默认的方法。获取所有的数组数据放入迭代器链表中
    return new ListItr(0);
}

public Iterator<E> iterator() {//【3】默认方法。获取所有的数组数据放入迭代器链表中
    return new Itr();
}
//说明：【2】和【3】的区别是返回的类型不懂，Iterator是ListIterator的父类

 2个内置的迭代器类：

private class Itr implements Iterator<E> {}

Itr类内部结构图：

 private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor != size;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;
            if (i >= size)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

 private class ListItr extends Itr implements ListIterator<E> {}

ListItr类内部结构图：

 private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }

        public boolean hasPrevious() {
            return cursor != 0;
        }

        public int nextIndex() {
            return cursor;
        }

        public int previousIndex() {
            return cursor - 1;
        }

        @SuppressWarnings("unchecked")
        public E previous() {
            checkForComodification();
            int i = cursor - 1;
            if (i < 0)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i;
            return (E) elementData[lastRet = i];
        }

        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.set(lastRet, e);
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        public void add(E e) {
            checkForComodification();

            try {
                int i = cursor;
                ArrayList.this.add(i, e);
                cursor = i + 1;
                lastRet = -1;
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }
    }

 从代码中，我们可以看到，这两种迭代器，从根本上来说都是使用实现了 Iterator接口来存储数据，定位数据，移动数据。

关于链表的数据结构，以后会在算法章节做另做模拟、测试和分析。

好了，此篇到此结束了！