一、ArrayList简介

实现：数组，非线程安全，可以使用Collections.synchronizedList(new ArrayList())包装为线程安全List。

使用场景：一次写、追加写、随机读。

时间复杂度：追加add，指定位置set，指定位置get为$O(1)$，retainAll、removeAll为$O(n^2)$，其余为$O(n)$。

二、成员变量、常量解析

成员变量/常量名称	初始值	解析
private static final long serialVersionUID	8683452581122892189L	JDK序列化标识
private static final int DEFAULT_CAPACITY	10	默认容量大小
private static final Object[] EMPTY_ELEMENTDATA	{}	共享空集合常量
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA	{}	这里主要用于区别默认初始化和指定容量为0的初始化方式。如果是默认初始化，首次扩容至少为DEFAULT_CAPACITY，否则根据常规1.5倍逻辑扩容。区别默认和指定两种语义。
transient Object[] elementData	NULL	元素存储的地方
private int size	0	元素数量
private static final int MAX_ARRAY_SIZE	Integer.MAX_VALUE - 8	最大的容量限制，一些VM有头部标记字段之类的，这里要预留8个元素的空间

三、公共方法

构造方法

// 带初始化容量的构造方法
public ArrayList(int initialCapacity) {
    if (initialCapacity > 0) {
        this.elementData = new Object[initialCapacity];
    } else if (initialCapacity == 0) {
        this.elementData = EMPTY_ELEMENTDATA; // 注意这里（指令容量为0）
    } else {
        throw new IllegalArgumentException("Illegal Capacity: "+
                                           initialCapacity);
    }
}

// 默认构造方法
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA; // 注意这里（默认初始化）
}

// 从一个collection拷贝构造，顺序与原collection一致
// 注意：容器是完全新建的，但存储的元素并未copy，是一个浅拷贝
public ArrayList(Collection<? extends E> c) {
    Object[] a = c.toArray();
    if ((size = a.length) != 0) {
        // 如果原来就是ArrayList就直接将c.toArray()返回结果赋给elementData
        if (c.getClass() == ArrayList.class) {
            elementData = a;
        } else {
            // 否则还要再去copy一次，防止其他集合实现的toArray返回的是原数组的引用，
            // 从而引发两个collection使用一个elementData的问题
            elementData = Arrays.copyOf(a, size, Object[].class);
        }
    } else {
        elementData = EMPTY_ELEMENTDATA; // 注意这里（相当于指定容量为0）
    }
}

常用方法

// 调整List容量与其包含的元素个数一致，以达到节省内存的目的
public void trimToSize() {
    modCount++; // 防止并发修改
    if (size < elementData.length) {
        elementData = (size == 0)
            ? EMPTY_ELEMENTDATA
            : Arrays.copyOf(elementData, size);
    }
}

// 扩容操作
// 注意：如果我们使用默认构造方法初始化List，那么随后在这里指定容量时，
// 只有期望容量大于默认容量才生效。
public void ensureCapacity(int minCapacity) {
    int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
        ? 0
        : DEFAULT_CAPACITY;

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

// 计算扩容大小，默认初始化的List，最小也要DEFAULT_CAPACITY，即10个
private static int calculateCapacity(Object[] elementData, int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        return Math.max(DEFAULT_CAPACITY, minCapacity);
    }
    return minCapacity;
}

// 扩容
private void ensureCapacityInternal(int minCapacity) {
    ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
}

// 实施扩容操作
private void ensureExplicitCapacity(int minCapacity) {
    modCount++; // 防止并发修改

    // 只能往大扩容
    if (minCapacity - elementData.length > 0)
        grow(minCapacity);
}

// 最大的容量限制，一些VM有头部标记字段之类的，这里要预留8个元素的空间
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

// 扩容实现
private void grow(int minCapacity) {
    int oldCapacity = elementData.length;
    // 默认新容量为原容量1.5倍，增加原来一半的容量
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    // 如果扩容后还是小于需求容量，就以需求容量为准
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    // 如果扩容后超出最大容量限制：
    // 1. 期望容量未超出MAX_ARRAY_SIZE，就扩到MAX_ARRAY_SIZE
    // 2. 期望容量超出MAX_ARRAY_SIZE但未溢出，扩容到Integer.MAX_VALUE（可能因为头部字段溢出）
    // 3. 期望容量溢出，报错OutOfMemoryError
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // 复制元素到新数组
    elementData = Arrays.copyOf(elementData, newCapacity);
}

// 计算边界容量
private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
    MAX_ARRAY_SIZE;
}

// 返回元素个数
public int size() {
    return size;
}

// 判断数组是否为空
public boolean isEmpty() {
    return size == 0;
}

// 判断是否包含元素o，通过equals判断，o可以为null
public boolean contains(Object o) {
    return indexOf(o) >= 0;
}

// 查找元素o的第一次出现的位置，查询不到返回-1
public int indexOf(Object o) {
    if (o == null) {
        for (int i = 0; i < size; i++)
            if (elementData[i]==null)
                return i;
    } else {
        for (int i = 0; i < size; i++)
            if (o.equals(elementData[i]))
                return i;
    }
    return -1;
}

// 查找元素o的最后一次出现的位置，查询不到返回-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;
}

// 浅拷贝，元素本身没有copy，elementData是新容器
public Object clone() {
    try {
        ArrayList<?> v = (ArrayList<?>) super.clone();
        v.elementData = Arrays.copyOf(elementData, size);
        v.modCount = 0;
        return v;
    } catch (CloneNotSupportedException e) {
        // 不会发生该异常
        throw new InternalError(e);
    }
}

// 返回elementData的一个新copy，但是内部元素没有copy，其引用还是指向同一个对象
public Object[] toArray() {
    return Arrays.copyOf(elementData, size);
}

// 返回指定类型的元素数组
// 参数a有两层含义：传递泛型、传递容器
// 当a的容量小于list中元素个数时，会忽略a，进而返回一个全新的数组
// 否则将元素从前到后以此填充到a中，a中剩余的元素用null填充
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
    if (a.length < size)
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

// 访问下标为index的元素
@SuppressWarnings("unchecked")
E elementData(int index) {
    return (E) elementData[index];
}

// 获取index位置的元素，index不合法会抛出IndexOutOfBoundsException
public E get(int index) {
    rangeCheck(index);
    return elementData(index);
}

// 覆盖index位置的元素，index不合法会抛出IndexOutOfBoundsException
// 返回覆盖之前的旧值
public E set(int index, E element) {
    rangeCheck(index);
    E oldValue = elementData(index);
    elementData[index] = element;
    return oldValue;
}

// 向list尾部添加一个元素e
public boolean add(E e) {
    // 容量检查&扩容，modCount++，防止并发修改
    ensureCapacityInternal(size + 1);
    elementData[size++] = e;
    return true;
}

// 在指定位置插入一个元素
public void add(int index, E element) {
    // 检查插入位置是否合法
    rangeCheckForAdd(index);
    // 容量检查&扩容，modCount++，防止并发修改
    ensureCapacityInternal(size + 1);
    //将index之后的元素顺序向后移动一位
    System.arraycopy(elementData, index, elementData, index + 1,
                     size - index);
    elementData[index] = element;
    size++;
}

// 移除指定位置的元素，返回移除的元素
public E remove(int index) {
    rangeCheck(index);

    modCount++;
    E oldValue = elementData(index);

    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    // 解除引用，方便GC
    elementData[--size] = null;

    return oldValue;
}

// 移除最先出现的元素o，如果o不存在，则什么也不做
public boolean remove(Object o) {
    if (o == null) {
        for (int index = 0; index < size; index++)
            if (elementData[index] == null) {
                fastRemove(index);
                return true;
            }
    } else {
        for (int index = 0; index < size; index++)
            if (o.equals(elementData[index])) {
                fastRemove(index);
                return true;
            }
    }
    return false;
}

// 快速移除操作，内部方法，省略边界检查
private void fastRemove(int index) {
    modCount++;
    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    // 解除引用，方便GC
    elementData[--size] = null;
}

// 清除所有元素，注意list容量不会变
public void clear() {
    modCount++;
    for (int i = 0; i < size; i++)
        elementData[i] = null;
    size = 0;
}

// 将另外一个collection元素追加到list尾部，如果本list发生了变化就返回true
// 未定义行为：当添加的集合是list本身
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;
}

// 从index开始依次插入c中的元素，原始index以后的元素顺移（如果有的话）
public boolean addAll(int index, Collection<? extends E> c) {
    rangeCheckForAdd(index);
    Object[] a = c.toArray();
    int numNew = a.length;
    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;
}

// 删除[fromIndex, toIndex)的元素
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;
}

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

// 检查index是否越界，用于添加元素
private void rangeCheckForAdd(int index) {
    if (index > size || index < 0)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

// 构造异常消息
private String outOfBoundsMsg(int index) {
    return "Index: "+index+", Size: "+size;
}

// 移除集合c中包含的所有元素（求差集 O(n^2)）
public boolean removeAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, false);
}

// 仅保留集合c中的元素（求交集 O(n^2)）
public boolean retainAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, true);
}

// 批量移除与c相关的元素
// complement: true 移除c中不存在的元素， false 移除c中存在的元素
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 {
        // 如果发生异常终端，就要将未处理完的元素复制到当前写的位置
        if (r != size) {
            System.arraycopy(elementData, r,
                             elementData, w,
                             size - r);
            w += size - r;
        }
        // 然后将多余的元素解除引用，改变size大小
        if (w != size) {
            for (int i = w; i < size; i++)
                elementData[i] = null;
            modCount += size - w;
            size = w;
            modified = true;
        }
    }
    return modified;
}

// foreach 迭代，同样会快速失败
@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;
    // 就是for循环遍历并作用action
    for (int i=0; modCount == expectedModCount && i < size; i++) {
        action.accept(elementData[i]);
    }
    if (modCount != expectedModCount) {
        throw new ConcurrentModificationException();
    }
}

// 删除满足条件的元素
@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;
    // 检查需要删除的元素
    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;
        // 依次将剩下的元素左移
        for (int i=0, j=0; (i < size) && (j < newSize); i++, j++) {
            i = removeSet.nextClearBit(i);
            elementData[j] = elementData[i];
        }
        // 空余的位置置为null
        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;
}

// 将每个元素用operator作用后的值替换
@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（O(nlogn)）
@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++;
}

迭代器

// 获得一个从指定位置开始的迭代器
public ListIterator<E> listIterator(int index) {
    if (index < 0 || index > size)
        throw new IndexOutOfBoundsException("Index: "+index);
    return new ListItr(index);
}

// 获取一个从0开始的迭代器
public ListIterator<E> listIterator() {
    return new ListItr(0);
}

// 获取一个通用迭代器
public Iterator<E> iterator() {
    return new Itr();
}

// 通用迭代器
private class Itr implements Iterator<E> {
    int cursor;       // index of next element to return
    int lastRet = -1; // index of last element returned; -1 if no such
    int expectedModCount = modCount;

    Itr() {}

    public boolean hasNext() {
        return cursor != size;
    }

    @SuppressWarnings("unchecked")
    public E next() {
        //检查并发修改，快速失败
        checkForComodification(); 
        int i = cursor;
        if (i >= size)
            throw new NoSuchElementException();
        Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length)
            throw new ConcurrentModificationException();
        cursor = i + 1;
        return (E) elementData[lastRet = i];
    }

    // 迭代器删除元素
    public void remove() {
        if (lastRet < 0)
            throw new IllegalStateException();
        checkForComodification();

        try {
            ArrayList.this.remove(lastRet);
            cursor = lastRet;
            lastRet = -1;
            expectedModCount = modCount;
        } catch (IndexOutOfBoundsException ex) {
            throw new ConcurrentModificationException();
        }
    }

    // 对剩下的元素依次作用consumer
    @Override
    @SuppressWarnings("unchecked")
    public void forEachRemaining(Consumer<? super E> consumer) {
        Objects.requireNonNull(consumer);
        final int size = ArrayList.this.size;
        int i = cursor;
        if (i >= size) {
            return;
        }
        final Object[] elementData = ArrayList.this.elementData;
        if (i >= elementData.length) {
            throw new ConcurrentModificationException();
        }
        while (i != size && modCount == expectedModCount) {
            consumer.accept((E) elementData[i++]);
        }
        // update once at end of iteration to reduce heap write traffic
        cursor = i;
        lastRet = i - 1;
        checkForComodification();
    }

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

// ListItr，可以指定起始迭代位置，可以往复迭代
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];
    }

    // 迭代过程中覆盖元素
    public void set(E e) {
        if (lastRet < 0)
            throw new IllegalStateException();
        checkForComodification();

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

    // 迭代过程中添加元素
    public void add(E e) {
        checkForComodification();

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

子列表视图(SubList)

// 返回一个子序列视图，对这个SubList的任何修改都将反应到原始list
public List<E> subList(int fromIndex, int toIndex) {
    subListRangeCheck(fromIndex, toIndex, size);
    return new SubList(this, 0, fromIndex, toIndex);
}

// 边界检查
static void subListRangeCheck(int fromIndex, int toIndex, int size) {
    if (fromIndex < 0)
        throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
    if (toIndex > size)
        throw new IndexOutOfBoundsException("toIndex = " + toIndex);
    if (fromIndex > toIndex)
        throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                           ") > toIndex(" + toIndex + ")");
}

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

    // 注意这里的offset，这是用于subList中再求subList使用的
    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);
    }
}

分割器

// Java8 分割器，主要用于并行安全的迭代List
// 构造一个新的分割器
@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; // 当前子序列的起始位置
    private int fence; // -1 until used; then one past last index
    private int expectedModCount; //检查并发修改

    // 用给定的区间创建一个分割器
    ArrayListSpliterator(ArrayList<E> list, int origin, int fence,
                         int expectedModCount) {
        this.list = list; // OK if null unless traversed
        this.index = origin;
        this.fence = fence;
        this.expectedModCount = expectedModCount;
    }

    // 获取区间结束位置，-1表示整个集合的结尾位置
    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) {
            if ((lst = list) == null)
                hi = fence = 0;
            else {
                expectedModCount = lst.modCount;
                hi = fence = lst.size;
            }
        }
        return hi;
    }

    // 一个分割器一分为二，返回一个新的分割器（二分法）
    public ArrayListSpliterator<E> trySplit() {
        int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
        return (lo >= mid) ? null : 
        // 注意这里的index=mid，返回的index较小的部分，留下的是index较大的部分
        new ArrayListSpliterator<E>(list, lo, index = mid,
                                    expectedModCount);
    }

    // 将子序列起始位置(index)的元素作用于action，然后子序列区间向末尾缩小一位
    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;
    }

    // 对子序列剩余元素依次作用于action
    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();
    }

    // 返回区间元素个数
    public long estimateSize() {
        return (long) (getFence() - index);
    }

    // 返回分割器特征
    public int characteristics() {
        return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
    }
}

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