JDK源码学习笔记——ArrayList/Vector

一、类定义

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable

二、属性

　　// 序列化id
    private static final long serialVersionUID = 8683452581122892189L;
    // 默认初始的容量
    private static final int DEFAULT_CAPACITY = 10;
    // 一个空对象
    private static final Object[] EMPTY_ELEMENTDATA = new Object[0];
    // 一个空对象，如果使用默认构造函数创建，则默认对象内容默认是该值
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = new Object[0];
    // 当前数据对象存放地方，当前对象不参与序列化
    transient Object[] elementData;
    // 当前数组长度
    private int size;
    // 数组最大长度
    private static final int MAX_ARRAY_SIZE = 2147483639;

三、构造方法

    /**
     * 注意：此时我们创建的ArrayList对象中的elementData中的长度是1，size是0,当进行第一次add的时候，elementData将会变成默认的长度：10.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }
    
    /**
     * 如果传入参数，则代表指定ArrayList的初始数组长度，传入参数如果是大于等于0，则使用用户的参数初始化，如果用户传入的参数小于0，则抛出异常，构造方法如下：
     */
    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);
        }
    }

    /**
     * 1）将collection对象转换成数组，然后将数组的地址的赋给elementData。
     * 2）更新size的值，同时判断size的大小，如果是size等于0，直接将空对象EMPTY_ELEMENTDATA的地址赋给elementData
     * 3）如果size的值大于0，则执行Arrays.copy方法，把collection对象的内容（可以理解为深拷贝）copy到elementData中。
     * 注意：this.elementData = arg0.toArray(); 这里执行的简单赋值时浅拷贝，所以要执行Arrays,copy 做深拷贝
     */
    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;
        }
    }

四、主要方法

    // 扩容：
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }
         
    private void grow(int minCapacity) {
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);// 扩1.5倍
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;// 或者要求的容量
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    // get
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }
    E elementData(int index) {
        return (E) elementData[index];
    }
    
    // set
    public E set(int index, E element) {
        rangeCheck(index);

        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }
    
    // add
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }
    
    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++;
    }
    
    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;
    }
    
    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;
    }
    
    // remove
    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;
    }
    
    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;
    }
    
    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;
    }
    
    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;
    }

五、内部类

    private class Itr implements Iterator<E> // 迭代器 只能从头到尾 只有remove
    private class ListItr extends Itr implements ListIterator // 加强迭代器 可以指定开始位置、倒序、set、add
    static final class ArrayListSpliterator<E> implements Spliterator<E> // 并行迭代器
    
    // 截取部分 还是原数组的引用，只是记录了开始结束下标
    private class SubList extends AbstractList<E> implements RandomAcces { 
        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;
        }
    }

关于并行迭代器Spliterator：jdk8中Spliterator的作用

六、其他

1.Fail-Fast机制：java中的fail-fast(快速失败)机制

 2.ArrayList与Vector区别：实现原理一样主要区别

　　①线程安全：Vector增删改查的方法都有synchronized，线程安全，但性能较ArrayList差

　　②扩容：ArrayList每次扩1.5倍(int newCapacity = oldCapacity + (oldCapacity >> 1))

　　　　　　Vector默认扩成原来的2倍(int newCapacity = oldCapacity + ((capacityIncrement > 0) ? capacityIncrement : oldCapacity))

参考资料：

1.Java集合---ArrayList的实现原理

2.ArrayList源码分析（基于JDK8）

3.Java集合之ArrayList源码分析