3.Java集合框架剖析 之 ArrayList源码剖析

package java.util;

import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;
import sun.misc.SharedSecrets;

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

    //默认初始化大小
    private static final int DEFAULT_CAPACITY = 10;

    //空数组，当调用无参构造方法的时候，默认给一个空数组，所有实例共享的一个静态属性，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 = {};

    //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 = {};

    //真正保存数据的数组，所以说ArrayList是用数组来实现的
    transient Object[] elementData;

    //实际元素的个数
    private int size;

    //设置数组大小的构造方法
    //当大于0的时候，就实例化一个initialCapacity的数组
    //当等于0的时候，就将静态空数组EMPTY_ELEMENTDATA赋值给elementData，注意：EMPTY_ELEMENTDATA的length为0，不是为空！
    //当小于0的时候，抛出异常
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
        //此处注意：initialCapacity没有赋值给size。size是统计元素的个数，而initialCapacity是实例化数组的大小
            this.elementData = new Object[initialCapacity];
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA;
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity);
        }
    }

    //无参构造方法，将静态的空数组DEFAULTCAPACITY_EMPTY_ELEMENTDATA赋值给elementData
    //用此无参构造方法构造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，
        先将集合转换成数组赋值给elementData之后，再判断数组长度，
            若等于0，则将EMPTY_ELEMENTDATA赋值给elementData
            如果不等于0，还需要判断接受过来的数组(elementData)是否为Object[]类型。
            如果不是，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 {
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    //将数组尾部多余的删掉，形成新数组，而新数组的length和size相当，节约空间。中间产生的空间开销，JVM会自动回收
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size);
        }
    }

    // 作用：判断当前是否需要进行扩容。提供给外界的方法，使用者可以通过这个方法自己去扩容
    // 首先判断elementData是不是空数组，如果是，minExpand=10，如果不是，minExpand=0
    // 再判断minExpand跟指定的最小容量minCapacity比较大小，看是否需要进行扩容
    public void ensureCapacity(int minCapacity) {
        //minExpand：最小扩容数量
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY;
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }

    // 私有方法，保证minCapacity在容量大小范围内
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    // 私有方法，保证minCapacity在容量大小范围内，看是否有必要去扩容
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    //私有方法，当最小容量大于数组的长度时，就进行扩容
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }



    //扩容
    private void grow(int minCapacity) {
        //获取当前数组的容量
        int oldCapacity = elementData.length;
        //在当前数组容量的大小情况下，扩容50%
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //当扩容后的容量大小 还是小于最小容量，新的数组容量就采用最小容量的长度
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //处理新数组容量大于MAX_ARRAY_SIZE
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        //调用系统的copyOf方法，先开辟一个newCapacity的数组，然后把之前的旧数组的数据复制到新数组
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
    
    //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OutOfMemoryError异常
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
    
    //求出最大的容量值，先判断minCapacity是否小于0，溢出就抛出OutOfMemoryError异常
    //否则就去判断minCapacity 是否大于 MAX_ARRAY_SIZE，是，返回 Integer.MAX_VALUE ，否， 返回MAX_ARRAY_SIZE
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0)
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
    }
    

    
    //真正保存的数据个数
    public int size() {
        return size;
    }

    //判断数据个数是否为空
    public boolean isEmpty() {
        return size == 0;
    }

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

    // indexOf是来获得指定元素(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，但是获得指定元素的最后出现的位置
    //对于重复的元素，只获取最后一个所在的位置
    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) {
            // 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);
        //如果a.length>size，则截取size的长度，有可能a本身就是有数据的，可能会出现a[size+?]有数据，所以a[size]设置为null来区别
        if (a.length > size)
            a[size] = null;
        return a;
    }

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

    //获取指定下标的元素
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }

    //用element新值去覆盖index位置的旧值
    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) {
        //首先检查index是否可以使用
        rangeCheckForAdd(index);
        //确保数组可容纳
        ensureCapacityInternal(size + 1);  // Increments modCount!!
           //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);
        //插入数据
        elementData[index] = element;
        size++;
    }

    //删除指定位置的元素
    public E remove(int index) {
        //下标检查
        rangeCheck(index);
        modCount++;
        //暂时保留旧值
        E oldValue = elementData(index);
        //从index+1位置开始的numMoved个元素，全部向前移动一位
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index, numMoved);
        //最后一位设置为null                
        elementData[--size] = null;
        //返回被删除的值
        return oldValue;
    }

    //删除某个元素，对于重复的元素，只删除第一次出现的元素
    public boolean remove(Object o) {
        //删除null
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    快速删除(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)
                    fastRemove(index);
                    return true;
                }
        } else {//删除非null的元素
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    快速删除(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)
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

    //快速删除：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
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            //从index+1位置开始的numMoved个元素，全部向前移动一位
            System.arraycopy(elementData, index+1, elementData, index, numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

    //清空数组
    public void clear() {
        modCount++;
        for (int i = 0; i < size; i++)
            elementData[i] = null;
        size = 0;
    }

    //添加一个集合C的全部元素，就是把集合中的数据全部复制过来
    public boolean addAll(Collection<? extends E> c) {
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew); 
        System.arraycopy(a, 0, elementData, size, numNew);
        size += numNew;
        return numNew != 0;
    }

    //指定index位置插入集合C中的全部元素，原来位置的元素依次向后移动
    public boolean addAll(int index, Collection<? extends E> c) {
        rangeCheckForAdd(index);
        Object[] a = c.toArray();
        int numNew = a.length;
        ensureCapacityInternal(size + numNew);  
        int numMoved = size - index;
        if (numMoved > 0)
            System.arraycopy(elementData, index, elementData, index + numNew, numMoved);
        System.arraycopy(a, 0, elementData, index, numNew);
        size += numNew;
        return numNew != 0;
    }

    //  删除指定fromIndex~toIndex范围的元素，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);
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

    //index下标检查判断
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //专门为add方法的index检查判断
    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);
        return batchRemove(c, false);
    }

    //保留当前容器与c的交集
    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;
        boolean modified = false;
        try {
            //一个读的index,一个是写的index,
            //因为每次都是把elementData数组拿来遍历，
            //注意complement是true还是false，
            //比方：complement为true: a属于elementData同时a属于c; false: a属于elementData同时a不属于c
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // 当c.contains()方法抛出异常的时候，就中断遍历，然后把从下标r开始的元素到结尾，全部移动到新数组的末尾接上。
            if (r != size) {
                System.arraycopy(elementData, r,elementData, w, size - r);
                w += size - r;
            }
            //清理数组中不需要的引用
            if (w != size) {
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true;
            }
        }
        return modified;
    }

    //序列化：保存数组实例的状态到一个流
    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：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;
        s.defaultReadObject();
        s.readInt();
        if (size > 0) {
            // be like clone(), allocate array based upon size not capacity
            int capacity = calculateCapacity(elementData, size);
            SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, 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，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)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

 
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    public Iterator<E> iterator() {
        return new Itr();
    }

     //AbstractList.Itr的优化版本迭代器
    private class Itr implements Iterator<E> {
        int cursor;       // 下一个要被返回的元素下标
        int lastRet = -1; // 上一个要被返回的元素下标，如果没有，就默认为-1
        int expectedModCount = modCount;

        Itr() {}
        
        //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() {
            //快速失败检测
            checkForComodification();
            int i = cursor;
            //会判断一次位置是否合法，因为cursor只是盲目的+1
            if (i >= size)
                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
        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
                //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
                lastRet = -1;
                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);
            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();
            }
            ////此处接受elementData元素，执行consumer中的方法，可能会去改变elementData元素
            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();
        }
    }

       //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();
            }
        }
    }

    //得到从fromIndex~toIndex位置的子列表
    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

        
        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;
        }

        // 第一次使用时，初始化fence大小
        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) {//-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;
        }

        public ArrayListSpliterator<E> trySplit() {
            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);
        }

        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();
        }

        public long estimateSize() {
            return (long) (getFence() - index);
        }

        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

    @Override
    public boolean removeIf(Predicate<? super E> filter) {
        Objects.requireNonNull(filter);
        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;
    }

    //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++;
    }

    //排序
    @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++;
    }
}