Java编程中ArrayList源码分析

之前看过一句话，说的特别好。有人问阅读源码有什么用？学习别人实现某个功能的设计思路，提高自己的编程水平。

是的，大家都实现一个功能，不同的人有不同的设计思路，有的人用一万行代码，有的人用五千行。有的人代码运行需要的几十秒，有的人只需要的几秒。。下面进入正题了。

本文的主要内容：

· 详细注释了ArrayList的实现，基于JDK 1.8 。
·迭代器SubList部分未详细解释，会放到其他源码解读里面。此处重点关注ArrayList本身实现。
·没有采用标准的注释，并适当调整了代码的缩进以方便介绍

import java.util.AbstractList;
import java.util.Arrays;
import java.util.BitSet;
import java.util.Collection;
import java.util.Comparator;
import java.util.ConcurrentModificationException;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.Objects;
import java.util.RandomAccess;
import java.util.Spliterator;
import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

public class ArrayList extends AbstractList implements List, RandomAccess, Cloneable, java.io.Serializable
{
  private static final long serialVersionUID = 8683452581122892189L;
  //默认初始容量
  private static final int DEFAULT_CAPACITY = 10;
  //用于空实例共享空数组实例。
  private static final Object[] EMPTY_ELEMENTDATA = {};
  //默认的空数组
  private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
  //对的，存放元素的数组，包访问权限
  transient Object[] elementData;
  //大小，创建对象时Java会将int初始化为0
  private int size;
  //用指定的数设置初始化容量的构造函数，负数会抛出异常
  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);
    }
  }
  //默认构造函数，使用控数组初始化
  public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
  }
  //以集合的迭代器返回顺序，构造一个含有集合中元素的列表
  public ArrayList(Collection c) {
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
      // c.toarray可能（错误地）不返回对象[]（见JAVA BUG编号6260652）
      if (elementData.getClass() != Object[].class)
 elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
      // 使用空数组
      this.elementData = EMPTY_ELEMENTDATA;
    }
  }
  //因为容量常常会大于实际元素的数量。内存紧张时，可以调用该方法删除预留的位置，调整容量为元素实际数量。
  //如果确定不会再有元素添加进来时也可以调用该方法来节约空间
  public void trimToSize() {
    modCount++;
    if (size < elementData.length) {
      elementData = (size == 0) ? EMPTY_ELEMENTdata: Arrays.copyOf(elementData, size);
    }
  }
  //使用指定参数设置数组容量
  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 != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)? 0: DEFAULT_CAPACITY;
    //若参数大于预设的容量，在使用该参数进一步设置数组容量
    if (minCapacity > minExpand) {
      ensureExplicitCapacity(minCapacity);
    }
  }
  //用于添加元素时，确保数组容量
  private void ensureCapacityInternal(int minCapacity) {
    //使用默认值和参数中较大者作为容量预设值
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
      minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    ensureExplicitCapacity(minCapacity);
  }
  //如果参数大于数组容量，就增加数组容量
  private void ensureExplicitCapacity(int minCapacity) {
    modCount++;
    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) {
    int oldCapacity = elementData.length;
    //预设容量增加一半
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    //取与参数中的较大值
    if (newCapacity - minCapacity < 0)//即newCapacity 0)
      newCapacity = hugeCapacity(minCapacity);
    elementData = Arrays.copyOf(elementData, newCapacity);
  }
  //检查是否溢出，若没有溢出，返回最大整数值(java中的int为4字节，所以最大为0x7fffffff)或默认最大值
  private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) //溢出
      throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
  }
  //返回数组大小
  public int size() {
    return size;
  }
  //是否为空
  public boolean isEmpty() {
    return size == 0;
  }
  //是否包含一个数 返回bool
  public boolean contains(Object o) {
    return indexOf(o) >= 0;
  }
  //返回一个值在数组首次出现的位置，会根据是否为null使用不同方式判断。不存在就返回-1。时间复杂度为O(N)
  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;
  }
  //返回一个值在数组最后一次出现的位置，不存在就返回-1。时间复杂度为O(N)
  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，元素本身没有被复制，复制过程数组发生改变会抛出异常
  public Object clone() {
    try {
      ArrayList v = (ArrayList) super.clone();
      v.elementData = Arrays.copyOf(elementData, size);
      v.modCount = 0;
      return v;
    } catch (CloneNotSupportedException e) {
      throw new InternalError(e);
    }
  }
  //转换为Object数组，使用Arrays.copyOf()方法
  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）
  //返回的数组容量由参数和本数组中较大值确定
  @SuppressWarnings("unchecked")
  public  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;
  }
  //返回指定位置的值，因为是数组，所以速度特别快
  @SuppressWarnings("unchecked")
  E elementData(int index) {
    return (E) elementData[index];
  }
  //返回指定位置的值，但是会检查这个位置数否超出数组长度
  public E get(int index) {
    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超出数组长度
  public E set(int index, E element) {
    rangeCheck(index);
    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
  }
  //添加一个值，首先会确保容量
  public boolean add(E e) {
    ensureCapacityInternal(size + 1);
    elementData[size++] = e;
    return true;
  }
  //指定位置添加一个值，会检查添加的位置和容量
  public void add(int index, E element) {
    rangeCheckForAdd(index);
    ensureCapacityInternal(size + 1);
    //public static void arraycopy(Object src, int srcPos, Object dest, int destPos, int length) 
    //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
    System.arraycopy(elementData, index, elementData, index + 1,size - index);
    elementData[index] = element;
    size++;
  }
  //删除指定位置的值，会检查添加的位置，返回之前的值
  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; //便于垃圾回收期回收
    return oldValue;
  }
  //删除指定元素首次出现的位置
  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;
  }
  //快速删除指定位置的值，之所以叫快速，应该是不需要检查和返回值，因为只内部使用
  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，把每一个值设为null,方便垃圾回收(不同于reset，数组默认大小有改变的话不会重置)
  public void clear() {
    modCount++;
    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 c) {
    Object[] a = c.toArray();
    int numNew = a.length;
    ensureCapacityInternal(size + numNew); 
    System.arraycopy(a, 0, elementData, size, numNew);
    size += numNew;
    return numNew != 0;
  }
  //功能同上，从指定位置开始添加
  public boolean addAll(int index, Collection c) {
    rangeCheckForAdd(index);
    Object[] a = c.toArray();  //要添加的数组
    int numNew = a.length;   //要添加的数组长度
    ensureCapacityInternal(size + numNew); //确保容量
    int numMoved = size - index;//不会移动的长度(前段部分)
    if (numMoved > 0)      //有不需要移动的，就通过自身复制，把数组后部分需要移动的移动到正确位置
      System.arraycopy(elementData, index, elementData, index + numNew,numMoved);
    System.arraycopy(a, 0, elementData, index, numNew); //新的数组添加到改变后的原数组中间
    size += numNew;
    return numNew != 0;
  }
  //删除指定范围元素。参数为开始删的位置和结束位置
  protected void removeRange(int fromIndex, int toIndex) {
    modCount++;
    int numMoved = size - toIndex; //后段保留的长度
    System.arraycopy(elementData, toIndex, elementData, fromIndex,numMoved);
    int newSize = size - (toIndex-fromIndex);
    for (int i = newSize; i < size; i++) {
      elementData[i] = null;
    }
    size = newSize;
  }
  //检查数否超出数组长度 用于添加元素时
  private void rangeCheck(int index) {
    if (index >= size)
      throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
  }
  //检查是否溢出
  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);//检查参数是否为null
    return batchRemove(c, false);
  }
  //仅保留指定集合的元素
  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 {
      //遍历数组，并检查这个集合是否包含对应的值，移动要保留的值到数组前面，w最后值为要保留的元素的数量
      //简单点：若保留，就将相同元素移动到前段；若删除，就将不同元素移动到前段
      for (; r < size; r++)
 if (c.contains(elementData[r]) == complement)
   elementData[w++] = elementData[r];
    }finally {//确保异常抛出前的部分可以完成期望的操作，而未被遍历的部分会被接到后面
      //r!=size表示可能出错了：c.contains(elementData[r])抛出异常
      if (r != size) {
 System.arraycopy(elementData, r,elementData, w,size - r);
 w += size - r;
      }
      //如果w==size：表示全部元素都保留了，所以也就没有删除操作发生，所以会返回false；反之，返回true，并更改数组
      //而w!=size的时候，即使try块抛出异常，也能正确处理异常抛出前的操作，因为w始终为要保留的前段部分的长度，数组也不会因此乱序
      if (w != size) {
 for (int i = w; i < size; i++)
   elementData[i] = null;
 modCount += size - w;//改变的次数
 size = w;  //新的大小为保留的元素的个数
 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会抛出异常
  private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{
    int expectedModCount = modCount;
    s.defaultWriteObject(); //执行默认的反序列化/序列化过程。将当前类的非静态和非瞬态字段写入此流
    // 写入大小
    s.writeInt(size);
    // 按顺序写入所有元素
    for (int i=0; i 0) {
      ensureCapacityInternal(size);
      Object[] a = elementData;
      //读入所有元素
      for (int i=0; i listIterator(int index) {
    if (index < 0 || index > size)
      throw new IndexOutOfBoundsException("Index: "+index);
    return new ListItr(index);
  }
  //返回ListIterator，开始位置为0
  public ListIterator listIterator() {
    return new ListItr(0);
  }
  //返回普通迭代器
  public Iterator iterator() {
    return new Itr();
  }
  //通用的迭代器实现
  private class Itr implements Iterator {
    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
    int expectedModCount = modCount;//迭代过程不运行修改数组，否则就抛出异常
    //是否还有下一个
    public boolean hasNext() {
      return cursor != size;
    }
    //下一个元素
    @SuppressWarnings("unchecked")
    public E next() {
      checkForComodification();//检查数组是否被修改
      int i = cursor;
      if (i >= size)
 throw new NoSuchElementException();
      Object[] elementData = ArrayList.this.elementData;
      if (i >= elementData.length)
 throw new ConcurrentModificationException();
      cursor = i + 1; //向后移动游标
      return (E) elementData[lastRet = i];  //设置访问的位置并返回这个值
    }
    //删除元素
    public void remove() {
      if (lastRet < 0)
 throw new IllegalStateException();
      checkForComodification();//检查数组是否被修改
      try {
 ArrayList.this.remove(lastRet);
 cursor = lastRet;
 lastRet = -1;
 expectedModCount = modCount;
      } catch (IndexOutOfBoundsException ex) {
 throw new ConcurrentModificationException();
      }
    }
    @Override
    @SuppressWarnings("unchecked")
    public void forEachRemaining(Consumer 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++]);
      }
      cursor = i;
      lastRet = i - 1;
      checkForComodification();
    }
    //检查数组是否被修改
    final void checkForComodification() {
      if (modCount != expectedModCount)
 throw new ConcurrentModificationException();
    }
  }
  //ListIterator迭代器实现
  private class ListItr extends Itr implements ListIterator {
    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();
      }
    }
  }
  //返回指定范围的子数组
  public List subList(int fromIndex, int toIndex) {
    subListRangeCheck(fromIndex, toIndex, size);
    return new SubList(this, 0, fromIndex, toIndex);
  }
  //安全检查
  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 + ")");
  }
  //子数组
  private class SubList extends AbstractList implements RandomAccess {
    private final AbstractList parent;
    private final int parentOffset;
    private final int offset;
    int size;
    SubList(AbstractList 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 c) {
      return addAll(this.size, c);
    }
    public boolean addAll(int index, Collection 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 iterator() {
      return listIterator();
    }
    public ListIterator listIterator(final int index) {
      checkForComodification();
      rangeCheckForAdd(index);
      final int offset = this.offset;
      return new ListIterator() {
 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 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 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 spliterator() {
      checkForComodification();
      return new ArrayListSpliterator(ArrayList.this, offset,offset + this.size, this.modCount);
    }
  }
  @Override
  public void forEach(Consumer 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 spliterator() {
    return new ArrayListSpliterator<>(this, 0, -1, 0);
  }
  
  static final class ArrayListSpliterator implements Spliterator {
    
    private final ArrayList 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
    
    ArrayListSpliterator(ArrayList 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 lst;
      if ((hi = fence) < 0) {
 if ((lst = list) == null)
   hi = fence = 0;
 else {
   expectedModCount = lst.modCount;
   hi = fence = lst.size;
 }
      }
      return hi;
    }
    public ArrayListSpliterator trySplit() {
      int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
      return (lo >= mid) ? null : // divide range in half unless too small
 new ArrayListSpliterator(list, lo, index = mid,
 expectedModCount);
    }
    public boolean tryAdvance(Consumer 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 action) {
      int i, hi, mc; // hoist accesses and checks from loop
      ArrayList 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 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 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 c) {
    final int expectedModCount = modCount;
    Arrays.sort((E[]) elementData, 0, size, c);
    if (modCount != expectedModCount) {
      throw new ConcurrentModificationException();
    }
    modCount++;
  }
}

总结

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