ArrayList源码分析

1. 下面是ArrayList的继承体系结构

 

　　从上图可以看出ArrayList的继承体系：

       1. 继承AbstractList,实现List,它是一个数组队列，提供了相关的添加、删除、修改、遍历等功能

       2. 实现RandomAccess接口,实现快速随机访问：通过元素的序号快速获取元素对象

       3. 实现Cloneable接口, 重写了clone(), 可以进行克隆(浅拷贝)

       4. 实现Serialiazable接口, 可以实现序列化

2. ArrayList的数据结构

    2.1  重要的全局变量

    /**
     * Default initial capacity.
     * 在初次进行元素添加的时候初始化的数组长度
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     * 默认的一个空数组
     */
    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.
     * 使用默认构造器创建ArrayList的对象时默认使用的一个空数组
     */
    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.
     * ArrayList的底层数组
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * The size of the ArrayList (the number of elements it contains).
     * ArrayList中已存在的元素个数
     * @serial
     */
    private int size;

　　
   /**
    * 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
    * ArrayList的允许的最大元素个数
    */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

2.2 构造器

    /**
     * Constructs an empty list with the specified initial capacity.
     * 创建一个指定容量的空列表，如果传入的参数为零，这使用一个默认已经创建好的空列表
     * 如果参数小于0，这会抛出异常
     * @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.
     * 默认的构造器，初始化容量为0，在第一个元素新增时
     * 使用DEFAULT_CAPACITY（10）完成有容量的初始化
     */
    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.
     * 接受一个Collection对象直接转换为ArrayList
     * @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) {
        
        //底层的动态数组
        elementData = c.toArray();
        
        //如果存在元素，且数组中的元素不是Object类型的，则进行转换并赋给ArrayList的底层数组
        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 {  //创建一个空容量的数组
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

 3. 常用方法的分析

/*
 * 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;

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

    /**
     * Default initial capacity.
     * 在初次进行元素添加的时候初始化的底层数组长度
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * Shared empty array instance used for empty instances.
     * 默认创建的一个没有容量的空数组
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 默认创建的一个没有容量的空数组
     * 使用默认构造器创建ArrayList的对象时
     * 底层数组是指向该对象
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * ArrayList的底层数组
     * 使用default修饰符，可以让本包的类访问
     */
    transient Object[] elementData;

    /**
     * The size of the ArrayList (the number of elements it contains).
     * 记录ArrayList中元素的个数，区别于底层数组的容量
     */
    private int size;

    /**
     * Constructs an empty list with the specified initial capacity.
     * 创建一个指定容量的空列表
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) { //等于0，则指向默认创建的没有容量的空数组对象
            this.elementData = EMPTY_ELEMENTDATA;
        } else {  //小于0则抛出异常
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     * 底层数组默认指向一个没有容量的空数组对象
     * 在第一个元素新增时
     * 使用DEFAULT_CAPACITY（10）完成有容量的初始化
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 接受一个Collection对象，并将其转换为ArrayList中的元素
     */
    public ArrayList(Collection<? extends E> c) {
        
        //底层的动态数组
        elementData = c.toArray();
        
        //如果存在元素，且数组中的元素不是Object类型的，则进行转换并赋给ArrayList的底层数组
        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 {  
            //创建一个空容量的数组
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    /**
     * 在底层数组的长度大于size的大小的时候，
     * 进行底层数组的长度的矫正
     */
    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.
     * 没有使用的方法
     * @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);
        }
    }
    
    /**
     * 在添加元素钱进行调用
     * 检查是否需要进行扩容
     * @param minCapacity
     */
    private void ensureCapacityInternal(int minCapacity) {
        //如果是第一次添加，则初始化底层数组的长度为10
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        
        //检查是否需要扩容
        ensureExplicitCapacity(minCapacity);
    }
    
    /**
     * 检查是否需要扩容
     * @param minCapacity
     */
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity); //执行扩容的方法
    }

    /**
     * ArrayList的允许的最大元素个数
     * 一些jvm的实现的java对象header会占据8位，所以这会进行-8.
     * 但其实是可以允许达到Integer的最大值的
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 进行底层数组的扩容
     * 将容量扩大到原数组长度的1.5倍
     *
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length; //底层数组现在的长度
        int newCapacity = oldCapacity + (oldCapacity >> 1); //扩容后的长度，将原数组的长度扩大到原来的1.5倍
        //对扩容后的底层数组长度进行校正
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity); //大于允许的最大长度进行校正
        // minCapacity is usually close to size, so this is a win:
        //复制元素到扩容后的数组里面
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    /**
     * 大于允许的元素的最大数量时进行校正
     * @param minCapacity
     * @return
     */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    /**
     * 获取ArrayList中元素的个数
     */
    public int size() {
        return size;
    }

    /**
     * 判断ArrayList是否为空，即是否包含元素
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 判断集合中是否包含某个元素
     * 包含返回true，不包含返回false
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 返回元素所在集合的位置
     * 不存在返回-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;
    }

    /**
     * ArrayList中的元素是允许重复的
     * 获取某个元素最后所在集合的位置
     * 不存在则返回-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;
    }

    /**
     * 克隆一个集合对象
     */
    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);
        }
    }

    /**
     * 将集合对象转换为一个Object类型的数组
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * 返回一个给定类型的数组
     */
    @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;
    }

    
    /**
     * 返回指定位置的底层数组的元素
     */
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /**
     * 获取指定位置的集合的对象
     */
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }

    /**
     *替换掉指定集合位置的元素
     *并返回旧的元素
     */
    public E set(int index, E element) {
        rangeCheck(index);

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

    /**
     * 往集合中添加元素
     * 默认是尾部添加法
     */
    public boolean add(E e) {
        //检查是否需要扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * 在集合的指定位置天添加元素
     */
    public void add(int index, E element) {
        //检查索引位置的合理性
        rangeCheckForAdd(index);
        //检查是否需要扩容
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //将数组中指定索引和其之后的元素向后移动
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        //将元素存放在指定的位置
        elementData[index] = element;
        size++;
    }

    /**
     * 移除指定索引位置的元素
     * 并返回移除的元素
     */
    public E remove(int index) {
        //检查索引的合理性
        rangeCheck(index);

        modCount++;
        //获取到要移除的元素
        E oldValue = elementData(index);
        //需要移动的元素的个数
        int numMoved = size - index - 1;
        //将该索引之后的元素往前移动
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

    /**
     * 移除指定的元素
     * 成功返回true
     * 失败则返回false
     */
    public boolean remove(Object o) {
        //元素为null的话，则移除第一个查找到的null
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    //真正执行移除的方法
                    fastRemove(index);
                    return true;
                }
        } else { //遍历查找要移除的元素
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

    /*
     * 移除指定索引的元素
     * 并移动索引之后的元素
     */
    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
    }

    /**
     * 清空集合中的元素
     */
    public void clear() {
        modCount++;

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

        size = 0;
    }

    /**
     * 添加一个Collection对象到集合中
     */
    public boolean addAll(Collection<? extends E> c) {
        //将collection对象转换为数组
        Object[] a = c.toArray();
        int numNew = a.length;
        //检查是否需要扩容
        ensureCapacityInternal(size + numNew);  // Increments modCount
        //添加元素
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * 在指定的位置添加一个collection对象
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        //检查索引的合理性
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;
        //检查是否需要扩容
        ensureCapacityInternal(size + numNew);  // Increments modCount

        int numMoved = size - index;
        //先把指定位置及其之后的元素往后移动
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);
        //在将collection对象的元素添加到集合中
        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

    /**
     * 移除指定范围的元素
     * 只能在子类中使用
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        //将需要移除的最后一个元素后面的所有元素往前移动
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);
        
        //将移动后后面的那些指定为null，加快gc的回收
        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    /**
     * 检查给定索引的合理性
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 检查给定索引的合理性.
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 下标越界时打印的信息
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
     * 如何集合中包含collection对象中的元素
     * 则进行移除，如果有重复的元素，则全部移除
     */
    public boolean removeAll(Collection<?> c) {
        //判断collection对象是否为null
        Objects.requireNonNull(c);
        //进行元素的移除
        return batchRemove(c, false);
    }

    /**
     * 只保留同时存在于集合和collection对象中的元素
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }
    
    /**
     * 移除集合中包含的collection对象中的元素
     * @param c
     * @param complement
     * @return
     */
    private boolean batchRemove(Collection<?> c, boolean complement) {
        //获取集合的底层数组
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            //循环判断collection中的元素是否存在于集合中
            //并根据complement的值来决定是移除存在 元素还是只保留存在的元素
            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.
            //将r之后的元素往前移动
            if (r != size) {
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            //将w之后的元素指定为null，并重新调整集合的size大小
            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;
    }

    /**
     * 序列化集合中的元素
     */
    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();
        }
    }

    /**
     * 反序列化集合中的元素
     */
    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();
            }
        }
    }

    /**
     * 从指定位置开始返回一个listIterator
     */
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    /**
     * 返回一个listIterator
     */
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    /**
     * 返回迭代器对象
     */
    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;
        
        //判断迭代器中是否还有元素
        public boolean hasNext() {
            return cursor != size;
        }
        
        //返回存在的元素
        @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; //修改cursor的值
            return (E) elementData[lastRet = i];
        }
        
        //移除元素
        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();
            }
        }
        
        /**
         * java8新增的方法
         * 对每个元素执行给定的方法，依次处理，直到所有元素已被处理或被异常终止
         * 默认方法调用tryAdvance方法
         */
        @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();
        }
    }

    /**
     * ArrayList独有的迭代器
     */
    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }
        
        //是否存在前一个元素
        public boolean hasPrevious() {
            return cursor != 0;
        }
        
        //当前元素的索引
        public int nextIndex() {
            return cursor;
        }

        //前一个元素的索引
        public int previousIndex() {
            return cursor - 1;
        }
        
        //获取前一个元素
        @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];
        }
        
        //替换元素
        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

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

    /**
     * 从指定位置开始截断集合，并返回截断后的集合
     */
    public List<E> subList(int fromIndex, int toIndex) {
        //检查截断的起始索引是否合理
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }
    
    /**
     * 检查起始索引的合理性
     * @param fromIndex
     * @param toIndex
     * @param size
     */
    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 + ")");
    }
    
    /**
     * 一个内部类，将集合截断后返回的对象
     * 跟ArrayList的结构一样
　　　* 省略掉内容
     * @author Administrator
     *
     */
    private class SubList extends AbstractList<E> implements RandomAccess {
       
    }
    
    /**
     * java8新增加的方法
     * 根据传入的lambda表达式
     * 遍历集合中的元素
     * 不是线程安全的
     */
    @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();
        }
    }

    /**
     * java8新增的方法
     * 返回一个并行的分片迭代器
     */
    @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> {

        private final ArrayList<E> list;
        //起始位置(包含)advance/split操作时会修改
        private int index; // current index, modified on advance/split
        //结束位置(不包含) -1 表示到最后一个元素
        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;
        }
        
        /**
         * 获取结束位置，主要用于第一次使用时对fence的初始化赋值
         */
        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) {
                //当list为空，fence=0
                if ((lst = list) == null)
                    hi = fence = 0;
                else { //否则fence等于list的长度
                    expectedModCount = lst.modCount;
                    hi = fence = lst.size;
                }
            }
            return hi;
        }
        
        /**
         * 对任务(list)分割，返回一个新的Spliterator迭代器
         */
        public ArrayListSpliterator<E> trySplit() {
            //二分法
            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);
        }
        
        /**
         * 对单个元素执行给定的执行方法
         * 若没有元素需要执行，返回false；若可能还有元素尚未执行，返回true
         */
        public boolean tryAdvance(Consumer<? super E> action) {
            if (action == null)
                throw new NullPointerException();
            int hi = getFence(), i = index;
            if (i < hi) { //起始位置 < 终止位置 -> 说明还有元素尚未执行
                index = i + 1; //起始位置往后移动
                @SuppressWarnings("unchecked") E e = (E)list.elementData[i];
                action.accept(e); //执行传入的lambda表达式
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }
        
        /**
         * 对每个元素执行给定的方法，依次处理，直到所有元素已被处理或被异常终止
         * 默认方法调用tryAdvance方法
         */
        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); //每个元素都执行传入的指定方法
                    }
                    if (lst.modCount == mc)
                        return;
                }
            }
            throw new ConcurrentModificationException();
        }
        
        /**
         * 计算尚未执行的任务个数
         */
        public long estimateSize() {
            return (long) (getFence() - index);
        }
        
        /**
         * 返回当前对象的特征量
         */
        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }
    
    /**
     * java8新增的方法
     * 传入一个lambda表达式
     * 移除满足规则的元素
     * 不是线程安全的
     */
    @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;
        //遍历查找满足规则的元素，并将元素的位置放入bitSet中
        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++;
            }
        }
        //在移除的过程中如果发生过其他操作，则抛出异常
        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;
            //遍历移除满足规则的元素
            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
                i = removeSet.nextClearBit(i);
                elementData[j] = elementData[i];
            }
            
            //将newSize后面的元素置为null，让gc进行垃圾回收
            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;
    }
    
    /**
     * java8新增的方法
     * 按照指定的规替换集合中的元素
     */
    @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++;
    }
    
    /**
     * java8新增的一个方法
     * 传入一个lambda表达式
     * 对集合中的元素进行排序
     */
    @Override
    @SuppressWarnings("unchecked")
    public void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, size, c);
        //排序时集合中的元素是否进行过其他操作，有则抛出异常
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
}