java8 ArrayList源码阅读

/*
*  ArrayList 源码阅读
*  Created by wbin on 2017/4/15.
*/
package java.util;

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

//默认初始容量为10
private static final int DEFAULT_CAPACITY = 10;

//空数组实例
private static final Object[] EMPTY_ELEMENTDATA = {};

//存储数据的数组，由此可看出ArrayList内部是用数组实现的
transient Object[] elementData; // non-private to simplify nested class access

//容量
private int size;

//构造方法，创建指定容量大小的数组。如果容量大小为负则会抛出异常
public ArrayList(int initialCapacity) {
super();
if (initialCapacity < 0)
throw new IllegalArgumentException("Illegal Capacity: " +
initialCapacity);
this.elementData = new Object[initialCapacity];
}

//默认构造方法，使用空数组初始化，容量大小默认为0
public ArrayList() {
super();
this.elementData = EMPTY_ELEMENTDATA;
}

//构造一个包含指定集合的元素的列表，按照它们由集合的迭代器返回的顺序。
public ArrayList(Collection<? extends E> c) {
elementData = c.toArray();
size = elementData.length;
// c.toArray 可能不返回Object[] (see 6260652)
if (elementData.getClass() != Object[].class)
elementData = Arrays.copyOf(elementData, size, Object[].class);
}

//释放数组空余的空间，容量为数组实际元素数量（因为容量通常会大于实际元素数量，
//扩充方法为增大1.5倍，见下grow()方法），可以在内存紧张时调用。
public void trimToSize() {
modCount++;     //涉及数组的改变，modCount都会改变，留意该变量，后面会频繁出现
if (size < elementData.length) {    //如果数组实际元素数量比数组容量小，则重新赋值数组释放多余空间
elementData = Arrays.copyOf(elementData, size);
}
}

//使用指定参数设定最小容量
public void ensureCapacity(int minCapacity) {
//如果数组为空，容量预取0，否则去默认值10
int minExpand = (elementData != EMPTY_ELEMENTDATA)
// any size if real element table
? 0
// larger than default for empty table. It's already supposed to be
// at default size.
: DEFAULT_CAPACITY;

//如果参数大于预设的最小值，则进一步设置容量大小
if (minCapacity > minExpand) {
ensureExplicitCapacity(minCapacity);
}
}

//添加新元素时使用，使用指定参数设定最小容量
private void ensureCapacityInternal(int minCapacity) {
//如果数组为空，使用默认值和参数中较大者作为容量预设值
if (elementData == EMPTY_ELEMENTDATA) {
minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
}

ensureExplicitCapacity(minCapacity);
}

//根据指定参数增加数组容量
private void ensureExplicitCapacity(int minCapacity) {
modCount++; //上面提到，只要涉及数组结构的改变(这里是数组大小改变)，该变量改变

//如果参数大于数组容量，就增加数组容量
if (minCapacity - elementData.length > 0)
grow(minCapacity);
}

//数组的最大容量，可能会导致内存溢出(VM内存限制)
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

//增加容量，以确保它可以至少持有由参数指定的元素的数目
private void grow(int minCapacity) {
// overflow-conscious code
int oldCapacity = elementData.length;

//新容量为原来的1.5倍
int newCapacity = oldCapacity + (oldCapacity >> 1);

//如果仍比参数小，则新容量大小取参数的值
if (newCapacity - minCapacity < 0)
newCapacity = minCapacity;
//若新容量大于默认的最大值，则检查是否溢出
if (newCapacity - MAX_ARRAY_SIZE > 0)
newCapacity = hugeCapacity(minCapacity);

elementData = Arrays.copyOf(elementData, newCapacity);
}

//检查参数是否溢出
private static int hugeCapacity(int minCapacity) {
if (minCapacity < 0) // 溢出
throw new OutOfMemoryError();
//如果参数比默认的最大值大，则取最大整数值，否则取容量默认最大值
return (minCapacity > MAX_ARRAY_SIZE) ?
Integer.MAX_VALUE :
MAX_ARRAY_SIZE;
}

//返回数组大小(实际元素数量)
public int size() {
return size;
}

//判断是否为空
public boolean isEmpty() {
return size == 0;
}

//是否包含特定元素，利用indexOf方法
public boolean contains(Object o) {
return indexOf(o) >= 0;
}

//返回特定元素在数组首次出现的位置（遍历的方式），会根据是否为null使用不同方式判断。不存在就返回-1
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;  //不存在返回-1
}

//返回特定元素在数组最后一次出现的位置（遍历的方式），会根据是否为null使用不同方式判断。不存在就返回-1
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;
}

//返回副本，浅拷贝
//复制过程数组发生改变会抛出异常
public Object clone() {
try {
//java.lang.Object.clone()只是一种浅复制，所以，v的elementData引用的还是当前ArrayList的elementData的引用
java.util.ArrayList<?> v = (java.util.ArrayList<?>) super.clone();
//对原来ArrayList的elementData进行一个数组拷贝，然后赋值给v的elementData
//这样，v的elementData就是原先ArrayList的elementData的一个副本，不再是内存中同一块数组
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);
}
}

//返回一个包含此列表中所有元素的数组,使用Arrays.copyOf()方法(浅拷贝),与clone情况类似
public Object[] toArray() {
return Arrays.copyOf(elementData, size);
}

//返回一个数组，使用运行时确定类型，该数组包含在这个列表中的所有元素（从第一到最后一个元素）
public <T> T[] toArray(T[] a) {
//如果参数数组容量比原数组容量小，则返回原数组的拷贝
if (a.length < size)
// Make a new array of a's runtime type, but my contents:
return (T[]) Arrays.copyOf(elementData, size, a.getClass());
//否则复制参数数组的前n个元素为原数组元素
System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
}

//直接返回指定位置的值
@SuppressWarnings("unchecked")
E elementData(int index) {
return (E) elementData[index];
}

//检查位置是否合法后返回指定位置的值
public E get(int index) {
//检查位置是否合法
rangeCheck(index);

return elementData(index);
}

//设置指定位置为一个新值，并返回之前的值，会检查这个位置是否合法
public E set(int index, E element) {
//检查位置是否合法
rangeCheck(index);

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

//添加一个新元素
public boolean add(E e) {
//确保容量，需要改变modCount值
ensureCapacityInternal(size + 1);
elementData[size++] = e;
return true;
}

//在指定位置添加新元素
public void add(int index, E element) {
//检查位置合法
rangeCheckForAdd(index);

ensureCapacityInternal(size + 1);  // Increments modCount!!
//从添加位置整体右移一位，因此arrayList添加元素是需要代价的
System.arraycopy(elementData, index, elementData, index + 1,
size - index);
elementData[index] = element;
size++;
}

//删除指定位置的元素，返回删除的元素
public E remove(int index) {
//检查位置合法
rangeCheck(index);

//数组改变，modCount需要++
modCount++;
E oldValue = elementData(index);

int numMoved = size - index - 1;
//如果删除位置不是最后一位，则从删除位置整体左移一位
if (numMoved > 0)
System.arraycopy(elementData, index + 1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work

return oldValue;
}

//删除指定元素首次出现的位置，若不存在则不改变
public boolean remove(Object o) {
if (o == null) {
for (int index = 0; index < size; index++)
if (elementData[index] == null) {
//由于位置一定合法，因此可以不用检查位置合法性，调用fastRemove方法即可。
fastRemove(index);
return true;
}
} else {
for (int index = 0; index < size; index++)
if (o.equals(elementData[index])) {
fastRemove(index);
return true;
}
}
return false;
}

//私有的快速删除指定位置的值，因为没有检查位置是否合法，调用方会保证位置合法性
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
}

//清空数组，全部赋值为null，方便垃圾回收
public void clear() {
//数组改变，modCount++
modCount++;

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

size = 0;
}

//添加一个集合的元素到末端，若要添加的集合为空返回false
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;
}

//从指定位置添加一个集合的元素，若要添加的集合为空返回false
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);

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

//添加方式为：先将指定位置后的元素后移，再复制参数集合的元素到指定位置后
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;
}

//删除指定范围内元素
protected void removeRange(int fromIndex, int toIndex) {
//数组改变，modCount++
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;
}

//检查指定位置是否超出数组长度，用于添加元素时
private void rangeCheck(int index) {
if (index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

//检查指定位置是否合法（小于0或超出数组长度）
private void rangeCheckForAdd(int index) {
if (index > size || index < 0)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

//抛出的异常的详情
private String outOfBoundsMsg(int index) {
return "Index: " + index + ", Size: " + size;
}

//删除指定集合的元素
public boolean removeAll(Collection<?> c) {
Objects.requireNonNull(c);  //检查集合是否为空
return batchRemove(c, false);   //调用batchRemove，complement为false
}

//与removeAll相反，仅保留指定集合的元素
public boolean retainAll(Collection<?> c) {
Objects.requireNonNull(c);  //检查集合是否为空
return batchRemove(c, true);    //调用batchRemove，complement为true
}

//complement true时从数组保留指定集合中元素的值，为false时从数组删除指定集合中元素的值。
private boolean batchRemove(Collection<?> c, boolean complement) {
final Object[] elementData = this.elementData;
int r = 0, w = 0;   //w为数组更新后的大小
boolean modified = false;
try {
//遍历数组，并检查元素是否在指定集合中，根据complement的值保留特定值到数组
//若complement为true即保留，则将相同元素移动到数组前端
//若complement为false即删除，则将不同元素移动到数组前端
for (; r < size; r++)
if (c.contains(elementData[r]) == complement)
elementData[w++] = elementData[r];
} finally {
//如果r!=size则说明c.contains(elementData[r])抛出异常
if (r != size) {
//将数组未遍历的部分添加
System.arraycopy(elementData, r,
elementData, w,
size - r);
w += size - r;
}
//如果w!=size说明进行了删除操作，故需将删除的值赋为null
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;   //更新数组容量
modified = true;
}
}
return modified;
}

//保存数组实例的状态到一个流（即它序列化）。写入过程数组被更改（利用modCount的值）会抛出异常
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
// Write out element count, and any hidden stuff
int expectedModCount = modCount;    //写入前modCount的值，将依据此检查写入过程数组是否被更改
s.defaultWriteObject();

// 写入容量大小
s.writeInt(size);

// 按顺序写入数组元素
for (int i = 0; i < size; i++) {
s.writeObject(elementData[i]);
}

// 若写入前后modCount值不同，说明写入过程数组被更改，则抛出异常
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
}

//读操作，跟上述写操作类似
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();
}
}
}

//返回ListIterator，开始位置为指定参数
public ListIterator<E> listIterator(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException("Index: " + index);
return new java.util.ArrayList.ListItr(index);
}

//返回ListIterator，开始位置为0
public ListIterator<E> listIterator() {
return new java.util.ArrayList.ListItr(0);
}

//返回普通迭代器
public Iterator<E> iterator() {
return new java.util.ArrayList.Itr();
}

// 迭代器实现，AbstractList.Itr的优化版本
private class Itr implements Iterator<E> {
int cursor;       //游标，下一个元素的索引，默认初始化为0
int lastRet = -1; //上次访问的元素的位置，初始化为-1
int expectedModCount = modCount;    //迭代过程不允许修改数组

//是否还有下一个
public boolean hasNext() {
return cursor != size;
}

//下一个元素
@SuppressWarnings("unchecked")
public E next() {
//检查数组是否修改，根据expectedModCount和modCount判断
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = java.util.ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}

//删除元素
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
//检查数组是否修改，根据expectedModCount和modCount判断
checkForComodification();

try {
java.util.ArrayList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = modCount;    //由于数组修改，重新设置expectedModCount
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

//为每个剩余元素执行给定的操作,直到所有的元素都已经被处理或行动将抛出一个异常
@Override
@SuppressWarnings("unchecked")
public void forEachRemaining(Consumer<? super E> consumer) {
Objects.requireNonNull(consumer);   //consumer不为空
final int size = java.util.ArrayList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = java.util.ArrayList.this.elementData;
if (i >= elementData.length) {
throw new ConcurrentModificationException();
}
//为每个剩余元素执行给定的操作
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();
}

//检查数组是否修改，根据expectedModCount和modCount判断
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}

//ListIterator迭代器实现，可以双向移动，AbstractList.ListItr的优化版本
private class ListItr extends java.util.ArrayList.Itr implements ListIterator<E> {
ListItr(int index) {
super();
cursor = index;
}

//是否有前一个
public boolean hasPrevious() {
return cursor != 0;
}

//返回下一个索引
public int nextIndex() {
return cursor;
}

//返回上一个索引
public int previousIndex() {
return cursor - 1;
}

//返回上一个元素
@SuppressWarnings("unchecked")
public E previous() {
checkForComodification();   //检查数组是否修改
int i = cursor - 1;
if (i < 0)
throw new NoSuchElementException();
Object[] elementData = java.util.ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i;
return (E) elementData[lastRet = i];
}

//更改当前元素的值
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();   //检查数组是否修改

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

//在当前位置添加新元素
public void add(E e) {
checkForComodification();   //检查数组是否修改

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

//返回指定范围的子数组
public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);    //检查参数合法
return new java.util.ArrayList.SubList(this, 0, fromIndex, toIndex);
}

//检查指定范围的参数合法，是否越界或小于0等
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 + ")");
}

//子list的实现，方法于上述类似，类比即可
private class SubList extends AbstractList<E> implements RandomAccess {
private final AbstractList<E> parent;   //父数组
private final int parentOffset;     //父数组的偏移，即基于子数组的起点
private final int offset;
int size;   //子数组大小

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 = java.util.ArrayList.this.modCount;
}

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

public E get(int index) {
rangeCheck(index);
checkForComodification();
return java.util.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 = java.util.ArrayList.this.modCount;

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

@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= java.util.ArrayList.SubList.this.size)
throw new NoSuchElementException();
Object[] elementData = java.util.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 = java.util.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 = java.util.ArrayList.SubList.this.size;
int i = cursor;
if (i >= size) {
return;
}
final Object[] elementData = java.util.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 {
java.util.ArrayList.SubList.this.remove(lastRet);
cursor = lastRet;
lastRet = -1;
expectedModCount = java.util.ArrayList.this.modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}

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

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

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

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

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

public List<E> subList(int fromIndex, int toIndex) {
subListRangeCheck(fromIndex, toIndex, size);
return new java.util.ArrayList.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 (java.util.ArrayList.this.modCount != this.modCount)
throw new ConcurrentModificationException();
}

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

//遍历每个元素做指定操作，java8新增方法
@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();
}
}

//返回Spliterator,用于多线程，java8新添加的
@Override
public Spliterator<E> spliterator() {
return new java.util.ArrayList.ArrayListSpliterator<>(this, 0, -1, 0);
}

//Spliterator的实现，java8新增方法，该类还需学习了解。
static final class ArrayListSpliterator<E> implements Spliterator<E> {

private final java.util.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(java.util.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)
java.util.ArrayList<E> lst;
if ((hi = fence) < 0) {
if ((lst = list) == null)
hi = fence = 0;
else {
expectedModCount = lst.modCount;
hi = fence = lst.size;
}
}
return hi;
}

//采用二分法，将前一半数据返回
public java.util.ArrayList.ArrayListSpliterator<E> trySplit() {
int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
return (lo >= mid) ? null : // divide range in half unless too small
new java.util.ArrayList.ArrayListSpliterator<E>(list, lo, index = mid,
expectedModCount);
}

//对下一个为处理的操作执行action并返回true， 如果没有下一个元素，返回false
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
java.util.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;
}
}

//判断条件是否满足，满足则删除，java8新增方法
@Override
public boolean removeIf(Predicate<? super E> filter) {
Objects.requireNonNull(filter);     //检查filter是否为空
int removeCount = 0;    //记录要删除的数目
final BitSet removeSet = new BitSet(size);  //利用BitSet来记录删除元素的下标
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];
//如果当前元素符合条件，则BitSet标记当前下标
if (filter.test(element)) {
removeSet.set(i);
removeCount++;
}
}
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}

final boolean anyToRemove = removeCount > 0;    //根据removeCount变量判断是否有满足条件的元素
if (anyToRemove) {
final int newSize = size - removeCount; //删除后数组元素的个数
//遍历数组，将不满足的元素前移到数组前端
for (int i = 0, j = 0; (i < size) && (j < newSize); i++, j++) {
i = removeSet.nextClearBit(i);
elementData[j] = elementData[i];
}
//将更新后的数组没用的元素置null
for (int k = newSize; k < size; k++) {
elementData[k] = null;  // Let gc do its work
}
this.size = newSize;
//检查数组是否修改
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}

return anyToRemove;
}

//根据operator进行替换，java8新增方法
@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]);
}
//检查数组是否修改
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}

//根据Comparator排序，java8新增方法
@Override
@SuppressWarnings("unchecked")
public void sort(Comparator<? super E> c) {
final int expectedModCount = modCount;
Arrays.sort((E[]) elementData, 0, size, c);
//检查数组是否修改
if (modCount != expectedModCount) {
throw new ConcurrentModificationException();
}
modCount++;
}
}