ArrayList的源码解析

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源码解析

• 注释在代码中

import 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 {
    //序列号
    private static final long serialVersionUID = 8683452581122892189L;
    //默认容量-10
    private static final int DEFAULT_CAPACITY = 10;
    //空数组，当用户指定arrayList容量为0时--该空数组
    private static final Object[] EMPTY_ELEMENTDATA = {};
    //空数组，当调用无参数构造方法时，返回该数组，new 一个arrayList 其容量是0
    //当第一次添加元素时，该数组会进行扩容 。编程默认容量为10的一个数组 ensureCapacityInternal()方法实现
    //与 EMPTY_ELEMENTDATA 的区别就是：DEFAULTCAPACITY_EMPTY_ELEMENTDATA 数组是默认返回的，而EMPTY_ELEMENTDATA是在用户指定容量为 0 时返回
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    //arrayList基于数组实现，用elementData保存数据，容量就是elementData数组的长度
    transient Object[] elementData;
    //arrayList实际存储的数据数量
    private int size;

    /**
     * 创建一个初试容量的、空的ArrayList
     *
     * @return
     * @throws 初始容量值非法(小于0)时抛出
     * @Param [initialCapacity] 初始容量
     **/
    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);
        }
    }

    /**
     * 无参构造 创建一个空的ArrayList
     *
     * @return
     * @Param []
     **/
    public ArrayList() {
        //数组大小为0,第一次添加元素时，会被扩容至10
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 创建一个包含Collection的ArrayList
     *
     * @return
     * @Param [c] 集合，集合内的元素全部添加进新建的ArrayList中
     **/
    public ArrayList(Collection<? extends E> c) {
        //集合转成数组
        elementData = c.toArray();
        //把转换后的数组长度赋值给ArrayList的size，并判断是否为0
        if ((size = elementData.length) != 0) {
            // c.toArray might (incorrectly) not return Object[] (see 6260652)
            //c.toArray可能不会反悔object[]，可以查看java 6260652的bug
            if (elementData.getClass() != Object[].class)
                //如果返回的不是Object[]类型，则利用 Arrays.copyOf重新构建一个size的数组
                elementData = Arrays.copyOf(elementData, size, Object[].class);
        } else {
            //替换空数组
            this.elementData = EMPTY_ELEMENTDATA;
        }
    }

    //将数组缓冲区调整到实际arrayList存储元素的大小 即elementData = Arrays.copyOf(elementData, size);
    //该方法用户调用，避免空间资源浪费
    public void trimToSize() {
        //modCount 是 AbstractList 的属性值：protected transient int modCount = 0;
        //？作用？？
        modCount++;
        //当元素大小，小于数据的长度
        if (size < elementData.length) {
            //判断size是否为0，为0，elementData实际存储EMPTY_ELEMENTDATA，如果有数据则调整实际arraylist的大小，就是Arrays.copyOf(elementData, size)
            elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData, size);
        }
    }


    //指定ArrayList 最小容量
    public void ensureCapacity(int minCapacity) {
        //判断是否为空，空为，最小扩容大小为0;不为空，最小扩容大小大小10
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY;
        //如果用户指定的容量大于最小扩充容量，则以用户指定为准，不然还是最10
        if (minCapacity > minExpand) {
            //进行扩充
            ensureExplicitCapacity(minCapacity);
        }
    }

    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        // 若 elementData == {}，则取 minCapacity 为 默认容量和参数 minCapacity 之间的最大值
        // 注：ensureCapacity() 是提供给用户使用的方法，在 ArrayList 的实现中并没有使用
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    //私有方法，明确arrayList的容量
    //用于内部优化
    //minCapacity 指定的最小容量
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    //私有方法，明确arrayList的容量
    //用于内部优化
    //minCapacity 指定的最小容量
    private void ensureExplicitCapacity(int minCapacity) {
        //将“修改统计数”+1,该变量是实现fail-fast机制的
        modCount++;
        //防止溢出，确保指定的最小容量大于数组的长度
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * 数组缓冲区最大存储容量
     * //- 一些 VM 会在一个数组中存储某些数据（元数据对象）--->为什么要减去 8 的原因?????
     * - 尝试分配这个最大存储容量，可能会导致 OutOfMemoryError(当该值 > VM 的限制时)
     * https://stackoverflow.com/questions/35756277/why-the-maximum-array-size-of-arraylist-is-integer-max-value-8
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    //扩容，以确保arrayList至少能存储minCapacity个元素
    //扩容计算-- oldCapacity + (oldCapacity >> 1)，扩容当前1.5倍
    private void grow(int minCapacity) {
        //防止溢出
        int oldCapacity = elementData.length;
        //运算符 >> 是带符号右移. 如 oldCapacity = 10,则 newCapacity = 10 + (10 >> 1) = 10 + 5 = 15
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)//如果newCapacity依旧小于minCapacity
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)//若大于最大存储容量，则进行大容量分配
            newCapacity = hugeCapacity(minCapacity);//
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

    //大容量分配 最大分配容量MAX_ARRAY_SIZE
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) throw new OutOfMemoryError();
        //比MAX_ARRAY_SIZE大 就分配Integer.MAX_VALUE
        return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE;
    }

    //返回arraylist的实际存储的元素数量
    public int size() {
        return size;
    }

    //判断arraylist是否有元素
    public boolean isEmpty() {
        return size == 0;
    }

    //arraylist是否包含对象
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    //顺序查找--返回索引值 存在=>0 不存在=-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;
    }

    //逆序查找 返回索引值 不存在返回-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;
    }

    //实现Cloneable接口 深度复制
    //返回一个深复制后的对象
    public Object clone() {
        try {
            //object的clone()方法，会复制本对象及所有基本类型成员和String类型成员，不复制对象的引用，对象的成员变量
            ArrayList<?> v = (ArrayList<?>) super.clone();
            //将需要复制的引用变量，进行独立copy到arrayList中
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    //把arrayList拷贝进一个数组中
    //包含员arrayList中的所有元素
    //对返回数组进行操作 不影响arrayList
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * @return T[]
     * @Param [a] 需要存储arrayList中所有元素的数组
     **/
    @SuppressWarnings("unchecked")
    public <T> T[] toArray(T[] a) {
        //数组长度小于arrayList的大小，重新拷贝一个新的数组
        if (a.length < size)
            return (T[]) Arrays.copyOf(elementData, size, a.getClass());
        //数组长度==arrayList的大小，则元素全部拷贝到数组中
        System.arraycopy(elementData, 0, a, 0, size);
        //大小arrayList大小，下标size至null
        if (a.length > size)
            a[size] = null;
        return a;
    }

    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    //获取指定下标上的元素
    public E get(int index) {
        rangeCheck(index);//检查是否越界
        return elementData(index);
    }

    //设置index下标元素的值
    public E set(int index, E element) {
        rangeCheck(index);//检查是否越界
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;//返回旧的数值
    }

    //添加元素，到arrayList的最后位置
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);
        elementData[size++] = e;
        return true;
    }

    //在指定位置，插入元素
    public void add(int index, E element) {
        rangeCheckForAdd(index);//判断下标是否越界
        //+1表示新增一个元素
        ensureCapacityInternal(size + 1);
        //要复制的数组，复制的数组从第几个开始，复制到哪？，复制过后下标从几开始，复制的长度
        System.arraycopy(elementData, index, elementData, index + 1,
                size - index);
        elementData[index] = element;
        size++;
    }

    //移除指定位置的元素
    //之后的元素一次左移一位
    public E remove(int index) {
        rangeCheck(index);//检查下标
        modCount++;
        E oldValue = elementData(index);
        int numMoved = size - index - 1;//要移动的长度
        if (numMoved > 0)
            System.arraycopy(elementData, index + 1, elementData, index,
                    numMoved);
        //将最后一个元素至null
        elementData[--size] = null;
        return oldValue;
    }

    //移除指定的第一个元素{符合条件索引最低的}
    //如果包含这个元素，index之后所有元素依次左移依次
    public boolean remove(Object o) {
        if (o == null) {
            for (int index = 0; index < size; index++)
                if (elementData[index] == null) {
                    fastRemove(index);
                    return true;
                }
        } else {
            for (int index = 0; index < size; index++)
                if (o.equals(elementData[index])) {
                    fastRemove(index);
                    return true;
                }
        }
        return false;
    }

    //根据index下标快速删除元素 无检查下标--所以快速
    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;
    }

    //移除arrayList中所有元素，
    //清空后，我们直接打印 list，却只会看见一个 [], 而不是 [null, null, ….] ==> toString() 和 迭代器进行了处理
    public void clear() {
        modCount++;
        //clear to let GC do its work
        //GC完成回收工作
        for (int i = 0; i < size; i++)
            //将数组至空
            elementData[i] = null;
        size = 0;
    }

    //将一个集合的中所有元素，追加到arrayList的末尾
    //ArrayList 是线程不安全的。
    //没有加锁，当一个线程对arrayList进行操作，另一个进程再改C的元素 会有问题
    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;
    }

    //list中原来位置的元素向后移
    // 从 List 中指定位置开始插入指定集合的所有元素,
    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;
    }

    //移除list中 [fromIndex,toIndex) 的元素
    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;
    }

    //检查下标是否越界
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    //构建异常的消息
    private String outOfBoundsMsg(int index) {
        return "Index: " + index + ", Size: " + size;
    }

    //移除list中指定集合包含的所有元素
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);//判断集合是否为空，如果空报NullPointerException
        return batchRemove(c, false);
    }

    //移除arrayList与集合并集的元素
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    //批处理移除
    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            if (r != size) {
                System.arraycopy(elementData, r,
                        elementData, w,
                        size - r);
                w += size - r;
            }
            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
    private void writeObject(java.io.ObjectOutputStream s)
            throws java.io.IOException {
        int expectedModCount = modCount;
        s.defaultWriteObject();
        s.writeInt(size);
        for (int i = 0; i < size; i++) {
            s.writeObject(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    //反序列化重构arrayList
    private void readObject(java.io.ObjectInputStream s)
            throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;
        s.defaultReadObject();
        s.readInt();
        if (size > 0) {
            int capacity = calculateCapacity(elementData, size);
            SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
            ensureCapacityInternal(size);
            Object[] a = elementData;
            for (int i = 0; i < size; i++) {
                a[i] = s.readObject();
            }
        }
    }

    //返回从指定索引开始到结束的带有元素的list迭代器
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: " + index);
        return new ListItr(index);
    }

    /**
     * 返回从0索引开始到结束的带有元素的list迭代器
     */
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    /**
     * 以一种合适的排序返回一个iterator到元素的结尾
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * Itr是AbstractList.Itr的优化版本
     * 为什么会报ConcurrentModificationException异常?
     * 1. Iterator 是工作在一个独立的线程中，并且拥有一个 mutex 锁。
     * 2. Iterator 被创建之后会建立一个指向原来对象的单链索引表，当原来的对象数量发生变化时，
     * 这个索引表的内容不会同步改变，所以当索引指针往后移动的时候就找不到要迭代的对象，
     * 3. 所以按照 fail-fast 原则 Iterator 会马上抛出 java.util.ConcurrentModificationException 异常。
     * 4. 所以 Iterator 在工作的时候是不允许被迭代的对象被改变的。
     * 但你可以使用 Iterator 本身的方法 remove() 来删除对象，
     * 5. Iterator.remove() 方法会在删除当前迭代对象的同时维护索引的一致性。
     */
    private class Itr implements Iterator<E> {
        int cursor; //下一个元素的索引
        int lastRet = -1; //最后一个元素返回的索引
        int expectedModCount = modCount;

        Itr() {
        }

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

        /**
         * 返回list中的值
         */
        @SuppressWarnings("unchecked")
        public E next() {
            checkForComodification();
            int i = cursor;//i当前元素的索引
            if (i >= size)////第一次检查：角标是否越界越界
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)////第二次检查,list集合中数量是否发生变化
                throw new ConcurrentModificationException();
            cursor = i + 1; //cursor 下一个元素的索引
            return (E) elementData[lastRet = i];//最后一个元素返回的索引
        }

        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();
            }
        }

        @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++]);
            }
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    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;
        }

        @SuppressWarnings("unchecked")
        public E previous() {
            checkForComodification();
            int i = cursor - 1;
            if (i < 0)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i;
            return (E) elementData[lastRet = i];
        }

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

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

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

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

    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }

    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 + ")");
    }

    private class SubList extends AbstractList<E> implements RandomAccess {
        private final AbstractList<E> parent;
        private final int parentOffset;
        private final int offset;
        int size;

        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++)]);
                    }
                    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);
        }
    }

    //---1.8
    @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();
        }
    }

    //分割器
    @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;
        private int index;
        private int fence;
        private int expectedModCount;

        ArrayListSpliterator(ArrayList<E> list, int origin, int fence,
                             int expectedModCount) {
            this.list = list;
            this.index = origin;
            this.fence = fence;
            this.expectedModCount = expectedModCount;
        }

        private int getFence() {
            int hi;
            ArrayList<E> lst;
            if ((hi = fence) < 0) {
                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 : 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;
            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;
        }
    }

    //根据Predicate条件来移除元素
    @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();
        }

        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;
            }
            this.size = newSize;
            if (modCount != expectedModCount) {
                throw new ConcurrentModificationException();
            }
            modCount++;
        }

        return anyToRemove;
    }

    //对数组中的每一个元素进行一系列的操作，返回同样的元素，
    @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++;
    }

    // 根据 Comparator条件进行排序
    @Override
    @SuppressWarnings("unchecked")
    public void sort(Comparator<? super E> c) {
        final int expectedModCount = modCount;
        Arrays.sort((E[]) elementData, 0, size, c);
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
        modCount++;
    }
}