java源码--ArrayList
1.1、ArrayList概述
1）ArrayList是可以动态增长和缩减的索引序列，它是基于数组实现的List类。
2）该类封装了一个动态再分配的Object[]数组，每一个类对象都有一个capacity属性，表示它们所封装的Object[]数组的长度，当向ArrayList中添加元素时，该属性值会自动增加。
如果想ArrayList中添加大量元素，可使用ensureCapacity方法一次性增加capacity，可以减少增加重分配的次数提高性能。
3）ArrayList的用法和Vector向类似，但是Vector是一个较老的集合，具有很多缺点，不建议使用。
另外，ArrayList和Vector的区别是：ArrayList是线程不安全的，当多条线程访问同一个ArrayList集合时，程序需要手动保证该集合的同步性，而Vector则是线程安全的。
1.2、ArrayList的数据结构
分析一个类的时候，数据结构往往是它的灵魂所在，理解底层的数据结构其实就理解了该类的实现思路，具体的实现细节再具体分析。
ArrayList的数据结构是：
说明：底层的数据结构就是数组，数组元素类型为Object类型，即可以存放所有类型数据。我们对ArrayList类的实例的所有的操作底层都是基于数组的。
源码
package java.util; import java.util.function.Consumer; import java.util.function.Predicate; import java.util.function.UnaryOperator; import sun.misc.SharedSecrets; public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { private static final long serialVersionUID = 8683452581122892189L; /** * 默认初始容量. */ private static final int DEFAULT_CAPACITY = 10; /** * 用于空实例的共享空数组实例 */ private static final Object[] EMPTY_ELEMENTDATA = {}; /** * 用于默认大小的空实例的共享空数组实例。我们将其与EMPTY_ELEMENTDATA区分开来，以了解添加第一个元素时的膨胀程度。 */ private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {}; /** * 存储ArrayList元素的数组缓冲区。ArrayList的容量是这个数组缓冲区的长度。当添加第一个元素时，任何带有elementData==default电容_empty_elementdata的空ArrayList将被扩展为DEFAULT_CAPACITY。 */ transient Object[] elementData; // 非私有以简化嵌套类访问 /** * ArrayList的大小 * @serial */ private int size; /** * 构造具有指定初始容量的空列表*/ 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); } } /** * 构造一个初始容量为10的空列表 */ public ArrayList() { this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; } /**
* 构造一个包含指定集合的元素的列表，按集合的迭代器返回元素的顺序排列 */ 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 { // 用空数组替换 this.elementData = EMPTY_ELEMENTDATA; } } /** * 将这个ArrayList实例的容量调整为列表的当前大小 */ public void trimToSize() { modCount++; if (size < elementData.length) { elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size); } } /** * 如果需要，增加这个ArrayList实例的容量，以确保它至少可以容纳由最小容量参数指定的元素数量。 * ensureCapacity单词意思为：设置缓冲区大小*/ public void ensureCapacity(int minCapacity) //看，判断初始化的elementData是不是空的数组，也就是没有长度
//因为如果是空的话，minCapacity=size+1；其实就是等于1，空的数组没有长度就存放不了，所以就将minCapacity变成10，也就是默认大小，但是带这里，还没有真正的初始化这个elementData的大小
int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY; //应该已经是默认大小了 if (minCapacity > minExpand) {
//确认实际的容量，上面只是将minCapacity=10，这个方法就是真正的判断elementData是否够用 ensureExplicitCapacity(minCapacity); } } //找出列表大小的最大值 private static int calculateCapacity(Object[] elementData, int minCapacity) { if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) { return Math.max(DEFAULT_CAPACITY, minCapacity); } return minCapacity; } // private void ensureCapacityInternal(int minCapacity) { ensureExplicitCapacity(calculateCapacity(elementData, minCapacity)); } private void ensureExplicitCapacity(int minCapacity) { modCount++;
/** minCapacity如果大于了实际elementData的长度，那么就说明elementData数组的长度不够用，不够用那么就要增加elementData的length。这里有的同学就会模糊minCapacity到底是什么呢，这里给你们分析一下
* 第一种情况：由于elementData初始化时是空的数组，那么第一次add的时候，minCapacity=size+1；也就minCapacity=1，在上一个方法(确定内部容量ensureCapacityInternal)就会判断出是空的数组，就会给将minCapacity=10，
* 到这一步为止，还没有改变elementData的大小。
*
* 第二种情况：elementData不是空的数组了，那么在add的时候，minCapacity=size+1；也就是minCapacity代表着elementData中增加之后的实际数据个数，拿着它判断elementData的length是否够用，如果length不够用，那么肯定要扩大容量，
* 不然增加的这个元素就会溢出
**/ // overflow-conscious code if (minCapacity - elementData.length > 0)
//arrayList能自动扩展大小的关键方法就在这里了 grow(minCapacity); } /** * 要分配的数组的最大大小。一些vm在数组中保留一些标题词。试图分配更大的数组可能会导致OutOfMemoryError:请求的数组大小超过VM限制 */ private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8; /** * 增加容量，以确保它至少可以容纳由最小容量参数指定的元素数量*/ private void grow(int minCapacity) { // overflow-conscious code int oldCapacity = elementData.length;//将扩充前的elementData大小给oldCapacity int newCapacity = oldCapacity + (oldCapacity >> 1); //newCapacity就是1.5倍的oldCapacity，右移一位相当于除以2 if (newCapacity - minCapacity < 0)//这句话就是适应于elementData就空数组的时候，length=0，那么oldCapacity=0，newCapacity=0，所以这个判断成立，在这里就是真正的初始化elementData的大小了，就是为10.前面的工作都是准备工作。 newCapacity = minCapacity; if (newCapacity - MAX_ARRAY_SIZE > 0)//如果newCapacity超过了最大的容量限制，就调用hugeCapacity，也就是将能给的最大值给newCapacity newCapacity = hugeCapacity(minCapacity); //新的容量大小已经确定好了，就copy数组，改变容量大小咯 elementData = Arrays.copyOf(elementData, newCapacity); }
//如果minCapacity都大于MAX_ARRAY_SIZE，那么就Integer.MAX_VALUE返回，反之将MAX_ARRAY_SIZE返回。因为maxCapacity是三倍的minCapacity，可能扩充的太大了，就用minCapacity来判断了。
//Integer.MAX_VALUE:2147483647 MAX_ARRAY_SIZE：2147483639 也就是说最大也就能给到第一个数值。还是超过了这个限制，就要溢出了。相当于arraylist给了两层防护 private static int hugeCapacity(int minCapacity) { if (minCapacity < 0) // overflow throw new OutOfMemoryError(); return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; } /** * 返回元素数目*/ public int size() { return size; } /** * 列表是否为空*/ public boolean isEmpty() { return size == 0; } //是否包含对应内容 public boolean contains(Object o) { return indexOf(o) >= 0; } //返回指定元素第一次出现的索引 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; } /** * 最后一次出现的索引 */ 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; } /** * 返回此ArrayList实例的浅拷贝。(元素本身不会被复制。)*/ 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); } } /** *返回一个包含列表中所有元素的数组*/ public Object[] toArray() { return Arrays.copyOf(elementData, size); } //返回一个数组，该数组包含列表中的所有元素 @SuppressWarnings("unchecked") public <T> T[] toArray(T[] a) { if (a.length < size) return (T[]) Arrays.copyOf(elementData, size, a.getClass()); System.arraycopy(elementData, 0, a, 0, size); if (a.length > size) a[size] = null; return a; } // 位置访问操作 @SuppressWarnings("unchecked") E elementData(int index) { return (E) elementData[index]; } public E get(int index) { rangeCheck(index); //检查索引越界 return elementData(index); } public E set(int index, E element) { rangeCheck(index); //检查索引越界 E oldValue = elementData(index); elementData[index] = element; return oldValue; } public boolean add(E e) { ensureCapacityInternal(size + 1); // 增量modCount!! elementData[size++] = e; return true; } public void add(int index, E element) { rangeCheckForAdd(index); //检查索引越界 ensureCapacityInternal(size + 1); // 增量 modCount!! System.arraycopy(elementData, index, elementData, index + 1, size - index); elementData[index] = element; size++; } public E remove(int index) { rangeCheck(index); //检查索引越界 modCount++; E oldValue = elementData(index); int numMoved = size - index - 1; if (numMoved > 0) System.arraycopy(elementData, index+1, elementData, index, numMoved); elementData[--size] = null; // 清除，让GC做它的工作 return oldValue; } //感觉这个不怎么要分析吧，都看得懂，就是通过元素来删除该元素，就依次遍历，如果有这个元素，就将该元素的索引传给fastRemobe(index)，使用这个方法来删除该元素，
//fastRemove(index)方法的内部跟remove(index)的实现几乎一样，这里最主要是知道arrayList可以存储null值 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; } /* * 私有移除方法，该方法跳过边界检查且不返回被移除的值。 */ 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; // 清除 让GC做他的工作 } public void clear() { modCount++; // clear to let GC do its work for (int i = 0; i < size; i++) elementData[i] = null; size = 0; } public boolean addAll(Collection<? extends E> c) { Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); //增量modCount System.arraycopy(a, 0, elementData, size, numNew); size += numNew; return numNew != 0; } public boolean addAll(int index, Collection<? extends E> c) { rangeCheckForAdd(index); //检查越界 Object[] a = c.toArray(); int numNew = a.length; ensureCapacityInternal(size + numNew); //增量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; } // 范围删除 protected void removeRange(int fromIndex, int toIndex) { modCount++; int numMoved = size - toIndex; System.arraycopy(elementData, toIndex, elementData, fromIndex, numMoved); // 清除 让GC做它的工作 int newSize = size - (toIndex-fromIndex); for (int i = newSize; i < size; i++) { elementData[i] = null; } size = newSize; } /** * 越界检查 */ private void rangeCheck(int index) { if (index >= size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } /** * 添加时越界检查，参数不可等于自身大小 */ private void rangeCheckForAdd(int index) { if (index > size || index < 0) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } // 越界说明 private String outOfBoundsMsg(int index) { return "Index: "+index+", Size: "+size; } //删除指定元素 public boolean removeAll(Collection<?> c) { Objects.requireNonNull(c); //检查是否为空 return batchRemove(c, false); //批量删除 } //保留指定元素，其余删除 public boolean retainAll(Collection<?> c) { Objects.requireNonNull(c); return batchRemove(c, true); } //这个方法，用于两处地方，如果complement为false 则用于removeAll，如果为true，则给retainAll()用，retainAll()是用来检测两个集合是否有交集的 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; } /** * 将ArrayList实例的状态保存到一个流中(即序列化它)。*/ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{ // 写出元素计数和任何隐藏的东西 int expectedModCount = modCount; s.defaultWriteObject(); //将大小作为与克隆的行为兼容性的容量() s.writeInt(size); // 按适当的顺序写出所有的元素。 for (int i=0; i<size; i++) { s.writeObject(elementData[i]); } if (modCount != expectedModCount) { throw new ConcurrentModificationException(); } } /** * 从流(反序列化流)中重新构造ArrayList实例。 */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { elementData = EMPTY_ELEMENTDATA; //阅读大小，和任何隐藏的东西 s.defaultReadObject(); // 阅读的能力 s.readInt(); // ignored if (size > 0) { // 就像clone()，根据大小而不是容量来分配数组 int capacity = calculateCapacity(elementData, size); SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity); ensureCapacityInternal(size); Object[] a = elementData; // 按适当的顺序阅读所有元素。 for (int i=0; i<size; i++) { a[i] = s.readObject(); } } } /** *返回此列表中元素的列表迭代器，从列表中的指定位置开始。*/ public ListIterator<E> listIterator(int index) { if (index < 0 || index > size) throw new IndexOutOfBoundsException("Index: "+index); return new ListItr(index); } public ListIterator<E> listIterator() { return new ListItr(0); } /** * 按适当的顺序对列表中的元素返回一个迭代器。*/ public Iterator<E> iterator() { return new Itr(); } /** * 一个优化版的AbstractList.Itr */ private class Itr implements Iterator<E> { int cursor; // 下一个要返回的元素的索引 int lastRet = -1; // 最后一个返回元素的索引 int expectedModCount = modCount; Itr() {} 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++]); } // 在迭代结束时更新一次，以减少堆写流量 cursor = i; lastRet = i - 1; checkForComodification(); } //并发异常检查 final void checkForComodification() { if (modCount != expectedModCount) throw new ConcurrentModificationException(); } } /** * 一个优化版的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();//并发检查 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();//并发检查 try { ArrayList.this.set(lastRet, e); } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } public void add(E e) { checkForComodification();//并发检查 try { int i = cursor; ArrayList.this.add(i, e); cursor = i + 1; lastRet = -1; expectedModCount = modCount; } catch (IndexOutOfBoundsException ex) { throw new ConcurrentModificationException(); } } } /** * */ public List<E> subList(int fromIndex, int toIndex) { subListRangeCheck(fromIndex, toIndex, size); //越界检查 return new SubList(this, 0, fromIndex, toIndex); } //越界检查方法 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 + ")"); } //将截取的内容封装成一个集合 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++)]); } // 在迭代结束时更新一次，以减少堆写流量 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); //检验越界 return new SubList(this, offset, fromIndex, toIndex); } //检验越界 private void rangeCheck(int index) { if (index < 0 || index >= this.size) throw new IndexOutOfBoundsException(outOfBoundsMsg(index)); } //检验越界 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; } //并发异常 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(); } } @Override public Spliterator<E> spliterator() { return new ArrayListSpliterator<>(this, 0, -1, 0); } /** 基于索引的、二分的、懒加载器 */ static final class ArrayListSpliterator<E> implements Spliterator<E> { private final ArrayList<E> list; private int index; // 当前索引，预先修改/分割 private int fence; // 1,直到使用;然后是最后一个索引 private int expectedModCount; // 设置栅栏时初始化 /** 创建覆盖给定范围的新spliterator */ ArrayListSpliterator(ArrayList<E> list, int origin, int fence, int expectedModCount) { this.list = list; // 如果为空，则为OK，除非遍历 this.index = origin; this.fence = fence; this.expectedModCount = expectedModCount; } private int getFence() { // 在第一次使用时将栅栏初始化为大小 int hi; // 在方法forEach中出现一个专门的变体 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 : // 把范围分成两半，除非太小 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; // 从循环中提升入口和检查 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(); } // 估算大小 public long estimateSize() { return (long) (getFence() - index); } // 返回特征值 public int characteristics() { return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED; } } @Override public boolean removeIf(Predicate<? super E> filter) { Objects.requireNonNull(filter);
//找出需要删除的元素 在此阶段从筛选器谓词抛出的任何异常 将不修改集合
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(); } // 将剩余的元素移动到被移除元素所留下的空间上 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++; } }
总结
1）arrayList可以存放null。
2）arrayList本质上就是一个elementData数组。
3）arrayList区别于数组的地方在于能够自动扩展大小，其中关键的方法就是gorw()方法。
4）arrayList中removeAll(collection c)和clear()的区别就是removeAll可以删除批量指定的元素，而clear是全是删除集合中的元素。
5）arrayList由于本质是数组，所以它在数据的查询方面会很快，而在插入删除这些方面，性能下降很多，有移动很多数据才能达到应有的效果
6）arrayList实现了RandomAccess，所以在遍历它的时候推荐使用for循环。
相关连接：https://www.cnblogs.com/zhangyinhua/p/7687377.html#_lab2_0_1
