ArrayList源码及解析

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;

    //默认容量
    private static final int DEFAULT_CAPACITY = 10;

    //静态的一个属性，所有实例共享属性，当初始化容量为0的时候，就使用这个属性作为实例底层数组
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /*根据注释，这个大概意思就是构造一个空的对象数组，用来与EMPTY_ELEMENTDATA 这个数组进行对比
    来确定当第一次向ArrayList中添加数据时，应该如果进行扩容，就是增加多大的容量。*/
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    //实际上真正保存数据的数组，从此出可以看出ArrayList使用Object数组来保存数据
    transient Object[] elementData; // non-private to simplify nested class access

    //实际包含元素的个数
    private int size;

    /*
    传递一个初始化容量的构造函数，会判断传递的参数与0的关系
    如果大于0，会在ArrayList内部构建一个长度为initalCapacity的数组
    如果等于0，会将上述的静态EMPTY_ELEMENTDATA属性赋值给elementData，也不会产生新的数组。如果小于0，则抛出异常
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity];//注意此处并没有将initialCapacity赋值给size
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        }
    }

    /*
    无参的构造函数，在该构造函数中，会将上述的静态的DEFAULTCAPACITY_EMPTY_ELEMENTDATA属性，赋值给elementData属性
    也即我们用这种方法构造ArraList的时候，并不会真正产生实例化的数组，而是引用一个静态的空数组
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /*
    传递一个集合给ArrayList，它首先会将集合转换成数组赋值给elementData
    之后判断数组长度，如果等于0，则将elementData赋值为EMPTY_ELEMENTDATA
    如果不等于0，还需要判断接受过来的数组(现在是elementData)是否是Object[]类型的
    如果不是的化，将它转换成Object[]类型(根据注释，toArray方法有可能得到的不是Object[]类型)
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            if (elementData.getClass() != Object[].class)
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            // replace with empty array.
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    /*
    本质上是将数组的尾部删除掉形成新数组
    新数组的length与size一致，节约空间
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    /*
    增加这个ArrayList实例的能力，如果有必要，以确保它至少能容纳的最小容量参数指定元素个数。
    提供给外界的方法，是的使用者可以通过这个方法自己去扩容
     */
    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;//elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA意味着elementData可能不是一个length为0的数组

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    /*
    一个私有方法，确保minCapacity在容量范围内
    如果elementData等于DEFAULTCAPACITY_EMPTY_ELEMENTDATA，则minCapacity会取DEFAULT_CAPACITY, minCapacity中比较大的那个
    也即如果minCapacity小于10，则取10，如果大于10，则去minCapacity
    随后要执行ensureExplicitCapacity方法
     */
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    /*
    ensureExplicitCapacity要接受一个int类型的参数，意味着最少需要容量为minCapacity
    首先会对modCount+1,modCount是AbstractList类中的一个成员变量,该值表示对List的修改次数，主要是为了服务快速失败功能的
    随后如果minCapacity要大于现有数组elementData的长度的化，那么就执行grow方法，grow是扩容的方法
     */
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }


    /*
    数组所能开辟的最大长度
    因为有些虚拟机保留了一些header words在数组中
    尝试要开辟更大的长度的数组，可能会出现OOM异常(在一些虚拟机实现中)
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /*
    ArrayList的扩容，接收一个int类型参数，表示至少需要多少容量
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;//得到目前的容量
        //oldCapacity>>1表示除2取整数，该式子最终表示意思为newCapacity大于为oldCapacity的1.5倍数
        int newCapacity = oldCapacity + (oldCapacity >> 1);
           //判断newCapacity是否溢出
        if (newCapacity - minCapacity < 0)
            //溢出：newCapacity等于minCapacity
            newCapacity = minCapacity;
        //判断newCapacity是否超过了MAX_ARRAY_SIZE，超过了，则计算最大容量；具体原因是因为不同虚拟机的实现不同
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //执行Arrays.copyOf方法，传递原数组与新数组长度，由Arrays内部创建数组返回并接受给elementData
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    /*
    求出最大的容量值，首先判断minCapacity是否已经溢出了，溢出了就直接抛出OOM
    否则就去判断minCapacity 是否大于 MAX_ARRAY_SIZE 
       大于返回 Integer.MAX_VALUE ，不大于 返回MAX_ARRAY_SIZE
     */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

    //得到size，size是真正的保存的元素的数量
    public int size() {
        return size;
    }

    //判断容器是否为空(指是不包含元素)
    public boolean isEmpty() {
        return size == 0;
    }

    //判断容器是否包含某个元素
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    //indexOf是来获得o元素(包括null)在容器中的位置的,位置从0开始到size-1结束,如果返回-1表示不包含
    //对于重复的元素，只获取第一个所在的位置
    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;
    }

    //与indexOf功能一样，但是确实获得重复元素的最后一个位置
    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;
    }

    //重写了Object中的clone方法，用于赋值容器，浅复制
    public Object clone() {
        try {
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {//看来大神也感觉异常不好处理了...不可能发生异常的地方，却还是要处理...
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    //得到数组的副本
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /*
    给定一个指定数组，返回指定数组大小，类型的副本
     */
    @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);//此处是size == a.length
        if (a.length > size)
            a[size] = null;//如果a.length>size，则截取size的长度，但是如果a本身就是有数据的，可能会出现a[size+?]有数据，而a[size]为null
        return a;
    }

    // Positional Access Operations
    //不需要检查index的快速访问元素，但是是包权限，只允许内部使用
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /*
    判断一下是否index是否越界
    然后通过快速访问来返回元素
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /*
    判断一下是否越界
    然后得到处于index位置的原元素，随后将index位置置入新元素
    返回原来的元素
    要求 index<size
     */
    public E set(int index, E element) {
        rangeCheck(index);

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

    /*
    集合中新增一个元素，首先要确保在承受能力范围内
    之后将新加入进来的元素赋值到数组的第size的位置上
    随后size+1
    新增的元素，插入到数组的末尾
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /*
    插入一个元素element到指定index位置，原位置的元素依次向后移动一位
    改方法效率要低一些，如果并不是特定必须要塞入哪个位置的话，最好不要用
     */
    public void add(int index, E element) {
        //首先会去检查一下index是否可以使用
        rangeCheckForAdd(index);
        //确保数组可容纳
        ensureCapacityInternal(size + 1);  // Increments modCount!! 会修改modCount的值，modCount+1
        //随后调用System.arraycopy方法，将elementData的index位置元素依次向后移动，为接下来的插入预留空间
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;//真正的插入操作
        size++;//size+1
    }

    /*
    删除指定位置的元素，如果index>size的话，会出现数组越界
     */
    public E remove(int index) {
        rangeCheck(index);//index>size throw IndexOutOfBoundsException

        modCount++;
        E oldValue = elementData(index);//得到原来elementData中的元素

        int numMoved = size - index - 1;//计算删除之后需要移动元素的数量
        if (numMoved > 0)//移动元素
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);//移动的时候，就会覆盖原来的元素
        //清除最后一个元素的引用，因为原来的元素以及被删除了
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;//返回被删除的元素
    }

    /*
    删除某一个元素，传入要被删除的元素
     */
    public boolean remove(Object o) {
        if (o == null) {//删除null元素
            for (int index = 0; index < size; index++)//迭代ArrayList
                if (elementData[index] == null) {//如果在size之前的位置有存在空元素
                    fastRemove(index);//则快速删除(所谓快速删除，就是不去做越界检查以及不返回结果，完全给本类自己使用的private方法)
                    return true;
                }
        } else {//删除非空元素，与删除null元素逻辑相同
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {//此处使用equals方法来进行比较，所以在使用remove(Object o)的时候，要考虑是否重写了equals方法
                    fastRemove(index);//fastRemove也是会移动数组的，如果有删除重复元素的时候，效率很低
                    return true;
                }
        }
        return false;
    }

    /*
    快速删除
    不做index检查，只允许内部使用
     */
    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
    }

    /*
    清除数组，所有元素置为null
     */
    public void clear() {
        modCount++;

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

        size = 0;
    }

    /*
    添加一次性add多个元素，接受参数为集合类型
     */
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;//可能会产生空指针错误
        ensureCapacityInternal(size + numNew);  // Increments modCount
        //将a数组插入到elementData的size位置
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    /*
    指定index位置插入多个元素，原来位置的元素依次向后移动
    index不能大于size,如果大于size会产生数组越界
     */
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);

        Object[] a = c.toArray();
        int numNew = a.length;//可能会产生空指针错误
        ensureCapacityInternal(size + numNew);  // Increments modCount

        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew,
                             numMoved);

        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

    /*
    范围删除，删除从fromIndex~toIndex,包含fromIndex，不包含toIndex
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    //index检查判断，专门封装起来是因为很多地方使用
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //专门为add方法封装的rangeCheck方法
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //为IndexOutOfBoundsException提供信息的方法，告诉哪个位置出现了数组越界
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    //一次性删除多个元素
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);//判断c是否为空，为空抛出异常
        return batchRemove(c, false);//批量删除
    }

    //保留当前容器与c的并集，并返回
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    //批量删除方法，complement为true表示求交集，如果为false表示在elementData中保留原有的非c的集合
    //也即true: a属于elementData同时a属于c; false: a属于elementData同时a不属于c
    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;//一个读的index,一个是写的index
        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) {//只移动一次数组，比单独remove效果要好
                System.arraycopy(elementData, r,
                                 elementData, w,
                                 size - r);
                w += size - r;
            }
            if (w != size) {//清理数组中不需要的引用
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;//记录修改次数
                size = w;//重新定义size
                modified = true;
            }
        }
        return modified;
    }

    //保存数组实例的状态到一个流（即它序列化）
    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();
        }
    }

    //从一个流中读出数组实例的状态
    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();
            }
        }
    }

    //返回一个list迭代器，链表迭代器，可以双向迭代，并且还具有add方法，但是只有在list类型中才可以使用，别的集合类没有
    //接受一个Index，确定迭代器初始的位置
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)//先判断index是否合法
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    /**
     * Returns a list iterator over the elements in this list (in proper
     * sequence).
     *
     * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @see #listIterator(int)
     */
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    /**
     * Returns an iterator over the elements in this list in proper sequence.
     *
     * <p>The returned iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
     *
     * @return an iterator over the elements in this list in proper sequence
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * An optimized version of AbstractList.Itr
     * AbstractList.Itr的优化版本迭代器
     */
    private class Itr implements Iterator<E> {
        int cursor;       // 下一个要被返回元素的下标
        int lastRet = -1; // 上一个被返回的元素的下标，如果没有的话默认为-1
        int expectedModCount = modCount;

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

        //返回下一个元素，默认一开始的next是第一个元素
        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();//快速失败
            int i = cursor;
            if (i >= size)//会判断一次位置是否合法，因为cursor只是盲目的+1
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;//cursor设置为下一个要被返回的元素下标
            return (E) elementData[lastRet = i];//将lastRet设置为被返回的元素下标
        }

        //删除上一个元素，也即最近被next()出来的元素
        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

            try {
                ArrayList.this.remove(lastRet);//删除的是下标为lastRet元素
                cursor = lastRet;//回退
                lastRet = -1;//设置成为-1，也即不能连续的删除，该类不能够往回走，只能继续前进，因为继续删除，会抛出IllegalStateException异常
                expectedModCount = modCount;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        /*
        遍历余下的元素
         */
        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);//判断consumer不能为null
            final int size = ArrayList.this.size;
            int i = cursor;//余下的体现在这..
            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++]);//此处接受elementData元素，执行consumer中的方法，可能会去改变elementData元素
            }
            // 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();
        }
    }

    /**
     * An optimized version of AbstractList.ListItr
     * 一个对AbstractList.ListItr的优化版本链表迭代器
     */
    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;
        }

        //返回上一个元素
        @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];
        }

        //更新上一个位置的元素，将其置换成e
        public void set(E e) {
            if (lastRet < 0)
                throw new IllegalStateException();
            checkForComodification();

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

        //新增一个元素，处在上一个元素之后，下一个元素之前，会移动数组
        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();
            }
        }
    }

    //得到子列表 从fromIndex~toIndex位置
    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }

    //判断Index是否合法
    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 + ")");
    }

    //继承与AbstractList的SubList类，其实这个类，只是去封装了几个属性，实际上用的还是原来ArrayList类的数组，外观模式
    private class SubList extends AbstractList<E> implements RandomAccess {
        private final AbstractList<E> parent;
        private final int parentOffset;
        private final int offset;
        int size; 

        //参数：
        //parent 父类型
        //offset 父类型的偏移量
        //fromIndex 子列表的开始元素，位于父列表的位置
        //toIndex 子列表的结束元素，位于父列表的位置
        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++)]);
                    }
                    // 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<E> 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<E> spliterator() {
            checkForComodification();
            return new ArrayListSpliterator<E>(ArrayList.this, offset,
                                               offset + this.size, this.modCount);
        }
    }

    //与forEachRemaining很像，一个是迭代所有，一个是迭代剩余，都会去执行Consumer中定义的方法，可能会改变元素的值
    @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,用于并行计算中，splitable iterator可分割迭代器
    @Override
    public Spliterator<E> spliterator() {
        return new ArrayListSpliterator<>(this, 0, -1, 0);
    }

    static final class ArrayListSpliterator<E> implements Spliterator<E> {

        private final ArrayList<E> list;//原数组
        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(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() { // 第一次使用时，初始化fence大小
            int hi; // (a specialized variant appears in method forEach)
            ArrayList<E> lst;
            if ((hi = fence) < 0) { //-1表示初始化的值
                if ((lst = list) == null)
                    hi = fence = 0;
                else {
                    expectedModCount = lst.modCount;
                    hi = fence = lst.size;
                }
            }
            return hi;
        }

        //这就是为Spliterator专门设计的方法，区分与普通的Iterator，该方法会把当前元素划分一部分出去创建一个新的Spliterator作为返回，
        //两个Spliterator变会并行执行，如果元素个数小到无法划分则返回null
        public ArrayListSpliterator<E> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;//由于lo + hi都是整数，>>>相当于除2
            return (lo >= mid) ? null : // divide range in half unless too small
                new ArrayListSpliterator<E>(list, lo, index = mid,//注意index=min
                                            expectedModCount);
        }

        //tryAdvance就是顺序处理每个元素，类似Iterator，如果还有元素要处理，则返回true，否则返回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
            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();
        }

        //该方法用于估算还剩下多少个元素需要遍历
        public long estimateSize() {
            return (long) (getFence() - index);
        }

        //其实就是表示该Spliterator有哪些特性，用于可以更好控制和优化Spliterator的使用
        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

    //删除，增加过滤功能
    @Override
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);//判断过滤器是否为空
        // 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;//要删除元素的个数
        final BitSet removeSet = new BitSet(size);//使用BitSet类来保存要被删除的Set，BitSet是使用位图来保存数据，节省很大内存
        final int expectedModCount = modCount;//预期的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)) {//如果element匹配filter中的过滤条件的话，则会返回true
                removeSet.set(i);//使用位图来保存要被删除的index
                removeCount++;
            }
        }
        if (modCount != expectedModCount) {//快速失败机制，在多线程情况下，去报错，引起程序员注意
            throw new ConcurrentModificationException();
        }

        // shift surviving elements left over the spaces left by removed elements
        final boolean anyToRemove = removeCount > 0;//用于记录是否需要删除
        if (anyToRemove) {
            //需要删除...
            final int newSize = size - removeCount;//计算剩余的长度，也即新数组的长度
            for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {//(i < size) && (j < newSize)会节约一些效率
                i = removeSet.nextClearBit(i);//得到没有被拦截的index
                elementData[j] = elementData[i];
            }
            for (int k=newSize; k < size; k++) {//清除数组后面的多余引用，GC
                elementData[k] = null;  // Let gc do its work
            }
            this.size = newSize;
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            modCount++;//用于记录本条数据也改变了数组结构，从这个地方可以看出来，快速失败机制并不能完全确保一定会提醒到程序员，只是有可能
        }

        return anyToRemove;
    }

    //替换所有
    @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++;
    }
}