一点一点看JDK源码（三）java.util.ArrayList 前偏

一点一点看JDK源码（三）java.util.ArrayList

liuyuhang原创，未经允许禁止转载

本文举例使用的是JDK8的API

目录：一点一点看JDK源码（〇）

1.综述

　　ArrayList是一个容量不固定的容器，为单列，有序集合，容量可扩容，扩容系数为1.5

　　有最大值，一般达不到。

　　ArrayList是线程不安全的，其扩容发生于集合修改的时候，如add，addAll等

　　ArrayList底层使用的是Object数组，初始化内容为10个容量的元素

　　使用ArrayList的时候，几种情况下实例化将更加提高效率

　　　　①仅仅使用基础增加功能作为容器临时使用

　　　　　　List list = new ArrayList（）

　　　　　　这样可以使用较少的实例化方法

　　　　②使用ArrayList中的所有实例化方法时使用

　　　　　　ArrayList list = new ArrayList（）

　　　　　　这样可以使用ArrayList所有的方法

　　　　③如果你能确定这个集合的容量，最好指定其容量，

　　　　　　扩容也是一种算法，而且可能扩容多次，效率不高，如：

　　　　　　ArrayList list = new ArrayList（22）

　　　　④如果该ArrayList的长度巨大，并且不确定，只用于容器临时使用

　　　　　　建议使用LinkedList进行接收参数，然后再转化为ArrayList，如：

　　　　　　LinkedList list = new LinkedList（）；

　　　　　　//some operations

　　　　　　ArrayList listArr = new ArrayList(list);

　　

2.关注点

• Collection
• List
• LinkedList
• Vector

　　关注点

　　　　Collection为父接口的父接口

　　　　List为父接口

　　　　LinkedList为List的链表实现，ArrayList为List的数组实现

　　　　Vector几乎不去用，他是List的另一种数组实现，但是线程安全

　　虽然类注释上的@see只有这些，但是个人认为，

　　应该重点关注一下AbstractList抽象类，RandomAccess，Cloneable这两个接口，序列化接口关注度真心不高

　　对于接口的实现关系，哪些是List通用的，哪些内容已经在AbstractList抽象类中已经定义了，应该过一下的

3.源码解析

　　放源码如下，注解有些删掉，对于类，内部类，方法，构造等进行了简要的说明注释

　　内容比较多，建议慢慢看完，如果jdk1.8新增的看不懂，可以暂时搁置，略过

　　我也不保证对于jdk1.8的部分注释是正确的。

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable{
    /**
     * 序列化版本号
     */
    private static final long serialVersionUID = 8683452581122892189L;
    /**
     * 默认容量。
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 以object数组作为类内共享实例容器。
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 以object数组作为类内共享实例容器，使用默认容量实例化时使用。
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 以object数组作为容器缓自排序冲数组，注意没有private关键字，用于排序，当地一个元素添加的时候将转为默认容量
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * 缓存的size大小，
     */
    private int size;

    /**
     * 带参构造，手动设置容器初始大小，若实例化时能确定大小，最好别使用默认扩展容量，将提高运行效率
     */
    public ArrayList(int initialCapacity) {
        if (initialCapacity > 0) {
            this.elementData = new Object[initialCapacity]; //使用自排序缓冲数组
        } else if (initialCapacity == 0) {
            this.elementData = EMPTY_ELEMENTDATA; //使用默认数组
        } else {
            throw new IllegalArgumentException("Illegal Capacity: "+ initialCapacity); //容量扩充参数异常
        }
    }

    /**
     * 无参构造，使用默认容器初始化大小
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 带参构造器,将参数集合用数组工具类转成数组,并存入缓冲数组,对缓冲数组进行校验.
* 若转换后的数组并非object数组（内嵌套无法直接引用情况下），则使用数组工具类将该数组使用深度拷贝
     */
    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;
        }
    }

    /**
     * 去除多余的数组申请空间，在内存紧张时会用到
     */
    public void trimToSize() {
        modCount++;//修改次数
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    /**
     * 对底层缓冲数组进行扩容的优化方法,如果已知该ArrayList容量,则可执行一次性扩容,
     * 否则将在数组add的过程中进行动态扩容,效率较低
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA)? 0: DEFAULT_CAPACITY;
        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }
    /**
     * 重新计算ArrayList的size,在使用构造和扩容时调用
     */
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }

    /**
     * 被add调用,内部调用calculatecapacity,用于扩容
     */
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

    /**
     * 被ensureCapacity调用,内部调用calculatecapacity,用于扩容或优化
     */
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;
        // overflow-conscious code
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * 目标分配数组的容器大小,最大值要小于Integer.MAX_VALUE-8
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 扩容
     */
    private void grow(int minCapacity) {
        int oldCapacity = elementData.length; //原容量
        int newCapacity = oldCapacity + (oldCapacity >> 1); //新容量,扩容1.5倍,自查>>位运算
        if (newCapacity - minCapacity < 0) //容量不足,则扩容
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0) //扩容超支,使用最大容
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
/**
     * 被grow方法调用,扩容为最大
     */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) //参数错误,抛出异常
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ? Integer.MAX_VALUE : MAX_ARRAY_SIZE; //返回最大容量
    }

    /**
     * 获取当前size
     */
    public int size() {
        return size;
    }

    /**
     * 判断当前是size是否为0,容器是否为空
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 判断是否包含元素o,返回该元素的index是否大于等于0的判断,是为包含,否为不包含
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 获取查询元素的index,若备查元素为null,则寻找空元素,返回index
     * 若备查元素不为null，则使用equal判断该元素是否存在，并返回index
     * 不存在则返回-1
     * 被contains调用
     */
    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;
    }

    /**
     * 倒叙查询第一个出现的被查元素
     */
    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;
    }

    /**
     * clone本实例数组，返回类型为Object，使用根类意为可转换为其他格式容器
     */
    public Object clone() {
        try {
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            throw new InternalError(e);
        }
    }

    /**
     * 将本实例转换为object数组，调用数组工具类拷贝
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * 将本实例转换为指定类型数组，调用数组工具类拷贝，并获得泛型的Class
     */
    @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)
            a[size] = null;
        return a;
    }

    /**
     * 获取指定index的元素，被get方法调用，get方法内进行check，util包内的类可调用
     */
    @SuppressWarnings("unchecked")
    E elementData(int index) {
        return (E) elementData[index];
    }

    /**
     * 获取指定元素的方法，使用rangeCheck进行index的check
     */
    public E get(int index) {
        rangeCheck(index);
        return elementData(index);
    }

    /**
     * 将指定index的值设置为指定值，使用rangeCheck进行index的check
     */
    public E set(int index, E element) {
        rangeCheck(index);
        E oldValue = elementData(index);
        elementData[index] = element;
        return oldValue;
    }

    /**
     * 容量+1的，在容器尾部添加元素
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e; // 这个在数组尾部追加元素的写法很好用,前提是扩容
        return true; // 有返回值的,应当接收
    }

    /**
     * 在指定index添加指定元素,首先使用rangeCheckForAdd进行check,为保证数组操作的正确性,使用arraycopy来移动数组元素
     */
    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++;
    }

    /**
     * remove指定index的元素,并重构本实例
     */
    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;
    }

    /**
     * remove指定元素,返回是否找到该元素并移除成功的boolean标记
     */
    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;
    }

    /*
     * 内部使用,快速移除指定index的元素
     */
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        elementData[--size] = null; // clear to let GC do its work
    }

    /**
     * 清除本实例内部的所有元素,加速gc回收
     */
    public void clear() {
        modCount++;
        // clear to let GC do its work
        for (int i = 0; i < size; i++)
            elementData[i] = null;
        size = 0;
    }

    /**
     * 添加指定集合到本实例中Object数组末尾
     */
    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;
    }

    /**
     * 从指定元素开始添加指定集合到本实例
     */
    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;
    }

    /**
     * 本类和子类可使用,移除指定范围的index的所有元素
     */
    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;
    }

    /**
     * 内部的index超限检测方法,当index超过size的时候,抛出index超限异常,在本类中被多次调用
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 同上,但是在add和addAll中专用
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * index超限抛异常时使用,用来提示size和index的值
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
     * 从本类实例中移除指定集合中所有元素的方法,返回值应接收
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    /**
     * 从本类实例中保留指定集合中所有元素的方法,与removeAll相反
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    /**
     * 内部调用,如果参数集合发生更改,则返回true
     */
    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData; //获取本实例的所有元素
        int r = 0, w = 0; //r用于遍历,w用于记录相同元素的数量,也用于记录容器的扩容标记
        boolean modified = false; //标志位
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement) //判断该元素是否被包含
                    elementData[w++] = elementData[r]; //包含则w++
        } finally {
            // Preserve behavioral compatibility with AbstractCollection,
            // even if c.contains() throws.
            if (r != size) {//集合有变化
                System.arraycopy(elementData, r, elementData, w, size - r);
                w += size - r;
            }
            if (w != size) {//容量不相等
                // clear to let GC do its work
                for (int i = w; i < size; i++)
                    elementData[i] = null;
                modCount += size - w;
                size = w;
                modified = true; //容量有变化,说明有交集,则返回true
            }
        }
        return modified;
    }

    /**
     * 将本实例对象写入流中,实际上只写入了modCount,size和element[i],其余的内容在实例化时候是不必要的,或者可恢复的
     */
    private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();
        // Write out size as capacity for behavioural compatibility with clone()
        s.writeInt(size);
        // Write out all elements in the proper order.
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }
        if (modCount != expectedModCount) {
            throw new ConcurrentModificationException();
        }
    }

    /**
     * 从流中读取本实例对象
     */
    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
            int capacity = calculateCapacity(elementData, size);
            SharedSecrets.getJavaOISAccess().checkArray(s, Object[].class, capacity);
            ensureCapacityInternal(size);
            Object[] a = elementData;
            // Read in all elements in the proper order.
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }

    /**
     * 获得指定长度额本实例的List迭代器
     */
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    /**
     * 获得本实例的默认List迭代器
     */
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    /**
     * 获得本实例的默认迭代器
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * 内部使用的迭代器,重写自Iterator接口,为内部类
     */
    private class Itr implements Iterator<E> {
        int cursor;       // index of next element to return    迭代器指针
        int lastRet = -1; // index of last element returned; -1 if no such    末尾标记
        int expectedModCount = modCount;
        Itr() {}
        public boolean hasNext() {
            return cursor != size;
        }
        @SuppressWarnings("unchecked")
      //重写迭代器提供next方法,可能unchecked报错
        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];
        }
      //重写迭代器提供remove方法
        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")//重写迭代器接口下的forEachRemaining方法,可能unchecked报错,可使用lambda迭代
        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();
        }
      //校验modCount修改次数的方法
        final void checkForComodification() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }

    /**
     * 另一个内部提供ListItr的迭代器,继承自上述内部类,实现ListIteraotr接口,本内部类将允许使用固定长度的迭代器
     */
    private class ListItr extends Itr implements ListIterator<E> {
        ListItr(int index) {
            super();
            cursor = index;
        }
      //是否有前一个元素
        public boolean hasPrevious() {
            return cursor != 0;
        }
      //获取下一个index
        public int nextIndex() {
            return cursor;
        }
      //获取前一个index
        public int previousIndex() {
            return cursor - 1;
        }
      //获取前一个元素,可能因为unchecked报错
        @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];
        }
      //对迭代器内当index的元素设置为指定值
        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();
            }
        }
    }

    /**
     * 从fromIndex到toIndex拆分数组，内部调用subListRangeCheck方法对本实例进行check
     */
    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex); //调用复写的内部类,十分巨大
    }
/**
     * 被内部调用，进行拆分时的参数check校验
     */
    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 + ")");
    }

/**
     * 重写继承的AbstractList中的SubList类,对于该类提供和list基本相同的功能进行重新封装
     */
    private class SubList extends AbstractList<E> implements RandomAccess {
        private final AbstractList<E> parent;
        private final int parentOffset;
        private final int offset;
        int size;
      //构造
        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;
        }
      //内部类的set方法
        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;
        }
      //内部类的get方法
        public E get(int index) {
            rangeCheck(index);
            checkForComodification();
            return ArrayList.this.elementData(offset + index);
        }
       //内部类的size方法
        public int size() {
            checkForComodification();//复用
            return this.size;
        }
      //内部类的add方法
        public void add(int index, E e) {
            rangeCheckForAdd(index); //复用
            checkForComodification();//复用
            parent.add(parentOffset + index, e);
            this.modCount = parent.modCount;
            this.size++;
        }
      //内部类的remove方法
        public E remove(int index) {
            rangeCheck(index); //复用
            checkForComodification();//复用
            E result = parent.remove(parentOffset + index);
            this.modCount = parent.modCount;
            this.size--;
            return result;
        }
      //从本内部类实例中移除指定范围的元素并重构本实例
        protected void removeRange(int fromIndex, int toIndex) {
            checkForComodification();//复用
            parent.removeRange(parentOffset + fromIndex,
                               parentOffset + toIndex); //使用父类
            this.modCount = parent.modCount;
            this.size -= toIndex - fromIndex;
        }
      //内部类的addAll
        public boolean addAll(Collection<? extends E> c) {
            return addAll(this.size, c);
        }
      //内部类的addAll,从指定index开始
        public boolean addAll(int index, Collection<? extends E> c) {
            rangeCheckForAdd(index); //复用
            int cSize = c.size();
            if (cSize==0)
                return false;
            checkForComodification();//复用
            parent.addAll(parentOffset + index, c);
            this.modCount = parent.modCount;
            this.size += cSize;
            return true;
        }
      //内部类的迭代器
        public Iterator<E> iterator() {
            return listIterator();
        }
      //内部类的迭代器,被内部调用
        public ListIterator<E> listIterator(final int index) {
            checkForComodification();//复用
            rangeCheckForAdd(index); //复用
            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;
                }
            //内部类的迭代器的next
                @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)];
                }
            //内部类的迭代器的判断是否有前一个元素
                public boolean hasPrevious() {
                    return cursor != 0;
                }
            //内部类的获取前一个元素
                @SuppressWarnings("unchecked")
                public E previous() {
                    checkForComodification();
                    int i = cursor - 1;
                    if (i < 0)
                        throw new NoSuchElementException();
                    Object[] elementData = ArrayList.this.elementData;
                    if (offset + i >= elementData.length)
                        throw new ConcurrentModificationException();
                    cursor = i;
                    return (E) elementData[offset + (lastRet = i)];
                }
            //剩余元素的迭代,可以使用lambda表达式进行遍历
                @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();
                }
            //内部类的迭代器中获取下一个元素的index
                public int nextIndex() {
                    return cursor;
                }
            //内部类的迭代器中获取上一个元素的index
                public int previousIndex() {
                    return cursor - 1;
                }
            //内部类的迭代器中移除当前元素
                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();
                    }
                }
            //内部类的迭代器对当前元素设置成指定值
                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();
                    }
                }
            //内部类的迭代器中add方法添加元素到迭代器末尾
                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();
                    }
                }
            //内部类的modCount校验
                final void checkForComodification() {
                    if (expectedModCount != ArrayList.this.modCount)
                        throw new ConcurrentModificationException();
                }
            };
        }
      
/**
         * 内部类的拆分list的方法
         */
        public List<E> subList(int fromIndex, int toIndex) {
            subListRangeCheck(fromIndex, toIndex, size);
            return new SubList(this, offset, fromIndex, toIndex);
        }
      /**
            * 内部类的index越界检查方法
         */
        private void rangeCheck(int index) {
            if (index < 0 || index >= this.size)
                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }
      /**
            * 同上,add和addAll专用
         */
        private void rangeCheckForAdd(int index) {
            if (index < 0 || index > this.size)
                throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
        }
      /**
            * 内部类rangeCheckForAdd中抛异常调用,写明index和size
         */
        private String outOfBoundsMsg(int index) {
            return "Index: "+index+", Size: "+this.size;
        }
      /**
            * 内部类的modCount校验
         */
        private void checkForComodification() {
            if (ArrayList.this.modCount != this.modCount)
                throw new ConcurrentModificationException();
        }
      /**
            * 内部类的分离迭代器函数,调用ArrayList的ArrayListSpliterator内部类的构造器
         */
        public Spliterator<E> spliterator() {
            checkForComodification();
            return new ArrayListSpliterator<E>(ArrayList.this, offset, offset + this.size, this.modCount);
        }
    }

/**
     * 重写继承的forEach方法,用于迭代
     */
    @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();
        }
    }

    /**
     * jd1.8更新
     * 重写自父类,分离数组,默认使用本类内容,从0-末尾,
     */
    @Override
    public Spliterator<E> spliterator() {
        return new ArrayListSpliterator<>(this, 0, -1, 0);
    }

    /**
     * jd1.8更新
     * 重写自父类,实现Spliterator接口
     * 本类用于分离ArrayList,前提是ArrayList暂定是不变的,否则将可能出现线程错误
     */
    static final class ArrayListSpliterator<E> implements Spliterator<E> {

        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

       /**
         * 内部类ArrayListSpliterator的构造器
         */
        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;
        }
       /**
         * 内部类ArrayListSpliterator中很多方法都使用的初始化方法
         */
        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;
        }
       /**
         * 内部类ArrayListSpliterator的拆分arraylist方法,不保证结果正确
         */
        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);
        }

       /**
         * 内部类ArrayListSpliterator的预拆分?尝试是否能够拆分
         */
        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;
        }

       /**
         * 内部类ArrayListSpliterator迭代
         */
        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();
        }
      //内部类ArrayListSpliterator的size预估
        public long estimateSize() {
            return (long) (getFence() - index);
        }

      //分离参数初始化
        public int characteristics() {
            return Spliterator.ORDERED | Spliterator.SIZED | Spliterator.SUBSIZED;
        }
    }

    /**
     * 重写自父类,使用重写的filter接口进行筛选,符合条件的则移除
     */
    @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;
    }

   /**
     * 重写自父类,使用重写的operator接口进行替代本实例中匹配的元素
     */
    @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++;
    }

   /**
     * 重写自父类,使用comparator接口对本实例进行排序
     */
    @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++;
    }
}

　　本篇注释前偏，只是粗略的了解，中篇 和 后篇 将对其中的使用进行总结和举例。

以上！