JAVA JDK8 ArrayList阅读

192 阅读14分钟
写在前言
ArrayList的本质是由数组进行实现的,默认数组长度为10,也可手动指定数组长度或者去掉未使用的空间,除了提供对数组的增删改查、扩容、数组集合转换、批量增加、批量删除、范围删除、IO流序列化/反序列化、排序、按条件删除、循环按条件处理、按条件删除、替代等方法外,还包含了ArrayListSpliterator分割集合、listIterator迭代、SubList截取等类

清单列表

属性

DEFAULT_CAPACITY:默认数组长度、常量(10)。
EMPTY_ELEMENTDATA:空数组、常量。
DEFAULTCAPACITY_EMPTY_ELEMENTDATA:默认数组,在初始化扩容的时候数组长度不会低于默认数组长度、常量。
elementData:存储元素数组,transient修饰,不能被序列化,防止未使用空间写入。
size:数组实际长度,不包含未使用空间。
MAX_ARRAY_SIZE:要分配数组的最大大小,一些虚拟机在数组中保存一些头字,分配更大的数组会报OutOfMemoryError错误,内存溢出。

构造方法

ArrayList(int initialCapacity):initialCapacity > 0按参数长度初始化elementData,initialCapacity = 0空数组无长度初始化elementData。
ArrayList():将默认数组初始化elementData。
ArrayList(Collection<? extends E> c):先将集合转换成数组赋值给elementData,数组长度等于0赋值EMPTY_ELEMENTDATA,数组长度大于0,验证数组的类型,如果不是Object类型进行对元素转换。

公有方法

trimToSize():去掉未使用的数组空间。
ensureCapacity(int minCapacity):公有扩容方法,如果要增加大量元素,使用这个方法可以一次性增加,防止多次扩容,降低效率。
size():返回实际长度。
isEmpty():判断数组是否有元素。
contains(Object o):判断是否包含元素。
indexOf(Object o):返回元素第一个下标。
lastIndexOf(Object o):返回元素最后一个下标。
clone():数组拷贝方法,数组长度、数组、操作数置0.
toArray():集合转数组。
toArray(T[] a):集合按参数类型转数组。
get(int index):公有方法:获取下标的元素。
set(int index, E element):给下标的元素重新赋值。
add(E e):添加原素。
add(int index, E element):在下标处添加元素。
remove(int index):删除元素。
remove(Object o):删除第一个等于参数的元素。
clear():清空数组。
addAll(Collection c):将集合转换成数组进行扩容添加。
addAll(int index, Collection c):在下标处进行添加集合转换后的数组。
removeAll(Collection c):删除包含集合的元素。
retainAll(Collection<?> c):删除不包含集合的元素。
forEach(Consumer<? super E> action):循环执行accept处理。
removeIf(Predicate<? super E> filter):删除符合filter的元素。
replaceAll(UnaryOperator operator):循环将元素按operator进行替换。
sort(Comparator<? super E> c):对集合进行排序。

私有方法

ensureCapacityInternal(int minCapacity):私有扩容方法。
ensureExplicitCapacity(int minCapacity):扩容前置,进行操作计数和长度判断。
grow(int minCapacity):扩容方法。
hugeCapacity(int minCapacity):长度大于最大大小扩容处理长度方法。
fastRemove(int index):删除下标元素的方法。
rangeCheck(int index):验证数组下标越界。
rangeCheckForAdd(int index):验证数组下标越界。
outOfBoundsMsg(int index):输出数组的信息。
batchRemove(Collection<?> c, boolean complement):removeAll,retainAll调用的删除方法,区别是complement。
writeObject(java.io.ObjectOutputStream s):数组序列化。(ArrayList list = new ArrayList(); ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(file)); oos.writeObject(list); )
readObject(java.io.ObjectInputStream s):数组反序列化。

default方法

elementData(int index):默认方法,取下标的元素。

protected方法

removeRange(int fromIndex, int toIndex):删除范围内的元素,包含开始元素,不包含结束元素。

内部类

Itr类:迭代器。
ListItr类:继承Itr类,并对其进行了升级。
SubList类:可以截取源集合的视图,但不能操作源集合,不然会报异常。
ArrayListSpliterator类:基于索引二分法拆分数组。

源码
/**
 * 默认数组长度,常量
 */
private static final int DEFAULT_CAPACITY = 10;

/**
 * 空数组,常量
 */
private static final Object[] EMPTY_ELEMENTDATA = {};

/**
 * 赋值默认长度的数组,常量
 */
private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
 * 实际数组,变量
 */
transient Object[] elementData; // non-private to simplify nested class access

/**
 * 实际长度,变量
 */
private int size;

/**
 * 带参长度的构造函数,大于0将new一个该长度的数组赋值实际数组,等于0就把常量空数组赋值实际数组,小于0抛出数字不合法异常
 */
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);
    }
}

/**
 * 未传参数,将默认长度的数组常量赋值实际数组
 */
public ArrayList() {
    this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

/**
 * 传参集合,先将集合转换成数组,赋值给实际数组,判断数组长度是不是等于0,等于0将常量空数组赋值实际数组,否则,判断数组的类型是不是等于Object的类型,不是,进行一一转换
 */
public ArrayList(Collection<? extends E> c) {{}
    elementData = c.toArray();
    if ((size = elementData.length) != 0) {
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    } else {
        // replace with empty array.
        this.elementData = EMPTY_ELEMENTDATA;
    }
}

/**
 * 主要是用在集合数量不可控的情况下,去掉扩容后未使用的空间,减少占用内存
 */
public void trimToSize() {
    modCount++;
    if (size < elementData.length) {
        elementData = (size == 0)
          ? EMPTY_ELEMENTDATA
          : Arrays.copyOf(elementData, size);
    }
}

/**
 * 用于外部调用,如果要增加大量元素,使用这个方法可以一次性增加,防止多次扩容,降低效率
 */
public void ensureCapacity(int minCapacity) {
    int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)
        // any size if not default element table
        ? 0
        // larger than default for default empty table. It's already
        // supposed to be at default size.
        : DEFAULT_CAPACITY;

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

/**
 * 内部方法,判断如果是默认数组,给默认长度10,并和传参长度取最大值进入下个方法扩容
 */
private void ensureCapacityInternal(int minCapacity) {
    if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
        minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
    }

    ensureExplicitCapacity(minCapacity);
}

/**
 * 扩容方法前置,操作计数,如果要扩容的长度大于数组长度就执行扩容方法
 */
private void ensureExplicitCapacity(int minCapacity) {
    modCount++;

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

/**
 * 要分配数组的最大大小,一些虚拟机在数组中保存一些头字,分配更大的数组会报OutOfMemoryError错误,内存溢出。
 */
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

/**
 * 首先扩容1.5倍,如果扩完之后小于传参长度,就直接赋予传参长度,如果该长度大于最大数组长度,就要进行处理是赋予定义大小还是数组最大大小,增加操作数组的长度
 */
private void grow(int minCapacity) {
    // overflow-conscious code
    int oldCapacity = elementData.length;
    int newCapacity = oldCapacity + (oldCapacity >> 1);
    if (newCapacity - minCapacity < 0)
        newCapacity = minCapacity;
    if (newCapacity - MAX_ARRAY_SIZE > 0)
        newCapacity = hugeCapacity(minCapacity);
    // minCapacity is usually close to size, so this is a win:
    elementData = Arrays.copyOf(elementData, newCapacity);
}

/**
 * 如果传参长度小于0,报内存溢出错误,否则,如果传参长度大于最大长度,给最大长度,否则给定义去除头字的长度
 */
private static int hugeCapacity(int minCapacity) {
    if (minCapacity < 0) // overflow
        throw new OutOfMemoryError();
    return (minCapacity > MAX_ARRAY_SIZE) ?
        Integer.MAX_VALUE :
        MAX_ARRAY_SIZE;
}

/**
 * 返回数组的实际长度
 */
public int size() {
    return size;
}

/**
 * 判断实际长度从而判断是否为空
 */
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;
}

/**
 * 进行空和有值判断,拿取第一个下标返回,没有返回-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;
}

/**
 * 进行空和有值判断,拿取最后一个下标返回,没有返回-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;
}

/**
 * 拷贝当前数组的基础属性操作数、实际长度等,深度拷贝实际数组,将操作数置为0
 */
public Object clone() {
    try {
		// 已经拷贝了数组、长度、操作数
        ArrayList<?> v = (ArrayList<?>) super.clone();
		// 重置数组
        v.elementData = Arrays.copyOf(elementData, size);
		// 重置操作数
        v.modCount = 0;
        return v;
    } catch (CloneNotSupportedException e) {
        // this shouldn't happen, since we are Cloneable
        throw new InternalError(e);
    }
}

/**
 * 将集合转为数组返回,这种是OBJECT的,如果转换类型要一个个转换
 */
public Object[] toArray() {
    return Arrays.copyOf(elementData, size);
}

/**
 * 将集合转换成数组返回,可以返回固定数组类型的
 */
@SuppressWarnings("unchecked")
public <T> T[] toArray(T[] a) {
    if (a.length < size)
        // Make a new array of a's runtime type, but my contents:
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        // 将最后一个置空,因为最大下标=长度-1
        a[size] = null;
    return a;
}

// Positional Access Operations

/**
 * 内部方法,用来获取目的下标的值
 */
@SuppressWarnings("unchecked")
E elementData(int index) {
    return (E) elementData[index];
}

/**
 * 获取固定下标的值
 */
public E get(int index) {
    rangeCheck(index);

    return elementData(index);
}

/**
 * 更新固定下标的值为参数值,并返回旧值
 */
public E set(int index, E element) {
    rangeCheck(index);

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

/**
 * 增加一个元素为参数值,先扩容,再赋值
 */
public boolean add(E e) {
    ensureCapacityInternal(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

/**
 * 在目的下标处增加一个元素为参数值
 */
public void add(int index, E element) {
    rangeCheckForAdd(index);

    ensureCapacityInternal(size + 1);  // Increments modCount!!
    System.arraycopy(elementData, index, elementData, index + 1,
                     size - index);
    elementData[index] = element;
    size++;
}

/**
 * 删除目的下标处的值
 */
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);
    elementData[--size] = null; // clear to let GC do its work

    return oldValue;
}

/**
 * 删除数组中等于参数的元素,只删除第一个
 */
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;
}

/*
 * 内部删除方法,把数据往前移一位,最后一位设成空
 */
private void fastRemove(int index) {
    modCount++;
    int numMoved = size - index - 1;
    if (numMoved > 0)
        System.arraycopy(elementData, index+1, elementData, index,
                         numMoved);
    elementData[--size] = null; // clear to let GC do its work
}

/**
 * 清空集合,操作计数,实际数组每个设置成null,实际长度设置成0
 */
public void clear() {
    modCount++;

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

    size = 0;
}

/**
 * 将集合添加进当前集合中,从最后一个元素开始添加
 */
public boolean addAll(Collection<? extends E> c) {
    Object[] a = c.toArray();
    int numNew = a.length;
    ensureCapacityInternal(size + numNew);  // Increments modCount
    System.arraycopy(a, 0, elementData, size, numNew);
    size += numNew;
    return numNew != 0;
}

/**
 * 将集合增加目的下标后,先扩容,迁移要使用空间的数据置后,然后将集合添加进来
 */
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;
}

/**
 * 范围删除,删除从开始下标到结束下标的值
 */
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;
}

/**
 *判断传入参数index是不是大于实际长度,如果大于等于,抛出数组下标越界异常。
 */
private void rangeCheck(int index) {
    if (index >= size)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
 * 断下标如果大于实际长度或者下标小于0,抛出数组下标越界异常,并将size+1。
 */
private void rangeCheckForAdd(int index) {
    if (index > size || index < 0)
        throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}

/**
 * 返回return "Index: "+index+", Size: "+size;下标和实际长度。
 */
private String outOfBoundsMsg(int index) {
    return "Index: "+index+", Size: "+size;
}

/**
 * 方法判断集合是否为空,返回 return batchRemove(c, false);方法
 *
 * @param c collection containing elements to be removed from this list
 * @return {@code true} if this list changed as a result of the call
 * @throws ClassCastException if the class of an element of this list
 *         is incompatible with the specified collection
 * (<a href="Collection.html#optional-restrictions">optional</a>)
 * @throws NullPointerException if this list contains a null element and the
 *         specified collection does not permit null elements
 * (<a href="Collection.html#optional-restrictions">optional</a>),
 *         or if the specified collection is null
 * @see Collection#contains(Object)
 */
public boolean removeAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, false);
}

/**
 *方法判断集合是否为空,返回 return batchRemove(c, true);方法。
 *
 * @param c collection containing elements to be retained in this list
 * @return {@code true} if this list changed as a result of the call
 * @throws ClassCastException if the class of an element of this list
 *         is incompatible with the specified collection
 * (<a href="Collection.html#optional-restrictions">optional</a>)
 * @throws NullPointerException if this list contains a null element and the
 *         specified collection does not permit null elements
 * (<a href="Collection.html#optional-restrictions">optional</a>),
 *         or if the specified collection is null
 * @see Collection#contains(Object)
 */
public boolean retainAll(Collection<?> c) {
    Objects.requireNonNull(c);
    return batchRemove(c, true);
}

/**
 * 使用final定义一个新数组,将缓冲区数组赋值给新数组,定义两个变量r、w等于0,
 * 定义修改状态为false,try遍历实际长度,使用变量r作为下标,如果集合c包含定义常量的某一个值,并等于参数布尔状态,将定义常量的下标w++赋值缓冲区数组下标r,
 * (当布尔状态为false的时候,将所有参数集合和新数组不交集的值放至最前面,当布尔状态为false的时候,将所有参数集合和新数组交集的值放至最前面),finally最终处理
 * 防止contains异常,如果r!=size,深拷贝将w后的数据拷贝到新数组,将w+=size - r;如果w != size,遍历实际长度,将大于w的下标的值都设成null,并将操作次数+size-w次,
 * 将w赋值给最新长度,并标记修改状态为true,返回修改状态。
 * @param c
 * @param complement
 * @return
 */
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 {
        // Preserve behavioral compatibility with AbstractCollection,
        // even if c.contains() throws.
        if (r != size) {
            System.arraycopy(elementData, r,
                             elementData, w,
                             size - r);
            w += size - r;
        }
        if (w != size) {
            // clear to let GC do its work
            for (int i = w; i < size; i++)
                elementData[i] = null;
            modCount += size - w;
            size = w;
            modified = true;
        }
    }
    return modified;
}

/**
 * 方法定义的抛出IO输入输出异常,首先定义常数操作次数,
 * 然后调用IO的defaultWriteObject方法将除了transient的其它数据序列化,再将数组大小序列化,
 * 然后再把元素一个个的序列化,如果操作次数不等于常数,抛出在排序期间有进行修改数组的异常。
 *
 * @serialData The length of the array backing the <tt>ArrayList</tt>
 *             instance is emitted (int), followed by all of its elements
 *             (each an <tt>Object</tt>) in the proper order.
 */
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();
    }
}

/**
 * 参数传入IO流的ObjectInputStream,方法定义的抛出IO异常、类找不到异常,首先定义缓冲区数组为空数组(1),
 * 将除了transient的其它数据反序列化,再将数组长度反序列化,如果实际长度大于0,调用ensureCapacityInternal(size);对数组进行扩容,
 * 定义一个常数数组,将缓冲区数组赋值给常数数组,循环实际长度,给每个下标赋值a[i] = s.readObject();。
 */
private void readObject(java.io.ObjectInputStream s)
    throws java.io.IOException, ClassNotFoundException {
    elementData = EMPTY_ELEMENTDATA;

    // Read in size, and any hidden stuff
    s.defaultReadObject();

    // Read in capacity
    s.readInt(); // ignored

    if (size > 0) {
        // be like clone(), allocate array based upon size not capacity
        ensureCapacityInternal(size);

        Object[] a = elementData;
        // Read in all elements in the proper order.
        for (int i=0; i<size; i++) {
            a[i] = s.readObject();
        }
    }
}

/**
 * 如果下标小于0或者下标大于实际长度的时候,抛出数组下标越界异常,返回对象return new ListItr(index);。
 *
 * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 */
public ListIterator<E> listIterator(int index) {
    if (index < 0 || index > size)
        throw new IndexOutOfBoundsException("Index: "+index);
    return new ListItr(index);
}

/**
 * 返回对象return new ListItr(0);。
 *
 * <p>The returned list iterator is <a href="#fail-fast"><i>fail-fast</i></a>.
 *
 * @see #listIterator(int)
 */
public ListIterator<E> listIterator() {
    return new ListItr(0);
}

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

/**
 * An optimized version of AbstractList.Itr
 */
private class Itr implements Iterator<E> {
    int cursor;       // 定义下一个要访问的元素下标
    int lastRet = -1; // 定义上一个要访问的元素下标
    int expectedModCount = modCount; // 定义常数接收操作次数

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

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

    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++]);
        }
        // update once at end of iteration to reduce heap write traffic
        cursor = i;
        lastRet = i - 1;
        checkForComodification();
    }

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

/**
 * An optimized version of AbstractList.ListItr
 */
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();
        }
    }
}

/**
 * 调用subListRangeCheck(fromIndex, toIndex, size);方法检查,返回对象new SubList(this, 0, fromIndex, toIndex);。
 *
 * @throws IndexOutOfBoundsException {@inheritDoc}
 * @throws IllegalArgumentException {@inheritDoc}
 */
public List<E> subList(int fromIndex, int toIndex) {
    subListRangeCheck(fromIndex, toIndex, size);
    return new SubList(this, 0, fromIndex, toIndex);
}

/**
 * 如果开始下标小于0,抛出数组下标越界异常,如果结束下标大于实际长度,抛出数组下标越界异常,如果开始下标大于结束下标,抛出数据不合法异常。
 * @param fromIndex
 * @param toIndex
 * @param size
 */
static void subListRangeCheck(int fromIndex, int toIndex, int size) {
    if (fromIndex < 0)
        throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
    if (toIndex > size)
        throw new IndexOutOfBoundsException("toIndex = " + toIndex);
    if (fromIndex > toIndex)
        throw new IllegalArgumentException("fromIndex(" + fromIndex +
                                           ") > toIndex(" + toIndex + ")");
}

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++)]);
                }
                // 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);
    }
}

/**
 * 首先检测集合是否存在,定义常数操作数等于操作数,定义一个数组等于缓冲区数组,
 * 定义一个变量等于实际长度,循环实际长度,并且操作数正确,将常数数组接收,如果操作数不相等,抛出数组修改操作数异常。
 * @param 参数为consumer类型,传参进去无返回值按照穿进去的方法进行处理
 */
@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();
    }
}

/**
 * 返回对象return new ArrayListSpliterator<>(this, 0, -1, 0);。
 *
 * @return a {@code Spliterator} over the elements in this list
 * @since 1.8
 */
@Override
public Spliterator<E> spliterator() {
    return new ArrayListSpliterator<>(this, 0, -1, 0);
}

/** Index-based split-by-two, lazily initialized Spliterator */
static final class ArrayListSpliterator<E> implements Spliterator<E> {
	/**
	属性:分割集合、当前下标、结束下标、操作数
	*/
    private final ArrayList<E> list;
    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

    // 构造函数
    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;
    }
	
	// 第一次使用实例化结束位置
    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;
    }
	
	// 分割list,返回一个返回一个新分割出的spilterator实例,二分法
    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);
    }
	
	// 返回true时,表示可能还有元素未处理/返回false时,没有剩余元素处理了
    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;
    }
	
	// 顺序遍历处理所有剩下的元素
    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();
    }
	
	// 估算大小
    public long estimateSize() {
        return (long) (getFence() - index);
    }
	
	// 返回特征值
    public int characteristics() {
        return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
    }
}

/**
 * 检测集合是否存在,定义删除总数量变量等于0,定义删除数据BitSet集合。定义常数操作数等于操作数,定义长度等于实际长度,
 * 遍历循环实际长度,如果操作数正确,定义一个元素接收缓冲区数组下标的值,如果检验参数有这个元素,将这个元素的下标放入删除集合,删除个数+1,如果操作次数不相符,
 * 抛出操作次数异常,定义布尔常数等于删除个数大于0,如果这个布尔参数为真,定义新的长度等于实际长度减去要删除的个数,循环新长度并且实际长度,
 * 将缓冲区数组的值进行修改,将多余的遍历为null,将新长度赋值给实际长度,如果操作次数有异常抛出异常,无就将操作次数加1,并且返回布尔状态。
 * @param 传参Predicate(处理方法,lamda表达式),传参返回布尔值
 * @return
 */
@Override
public boolean removeIf(Predicate<? super E> filter) {
    Objects.requireNonNull(filter);
    // figure out which elements are to be removed
    // any exception thrown from the filter predicate at this stage
    // will leave the collection unmodified
    int removeCount = 0;
    final BitSet removeSet = new BitSet(size);
    final int expectedModCount = modCount;
    final int size = this.size;
    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;
}

/**
 * 首先使用Objects.requireNonNull(operator);判断集合是否为空,定义一个常数等于操作次数,
 * 定义一个常数等于实际长度,遍历循环常数实际长度,判断条件常数操作次数与操作次数相等,以防替换数据的时候有进行修改,
 * 将数据进行替换elementData[i] = operator.apply((E) elementData[i]);,如果常数操作次数不等于操作次数,抛出数组有修改异常,无异常将操作次数进行加1。
 * @param 传什么类型的参数返回什么值,参数里面是执行方法(例:list.replaceAll(a->a.equals("zhangsan")?"张三":a);)
 */
@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++;
}

/**
 * 定义一个常数等于操作次数,调用Arrays.sort((E[]) elementData, 0, size, c);方法进行排序,如果操作次数不等于常数,抛出在排序期间有进行修改数组的异常,无异常抛出就将操作次数。
 * @param c
 */
@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++;
}