Java集合系列[2]-ArrayList分析与源码解析

概述

之前介绍了 Collection 的整体架构。本文将通过Collection的具体类来加深理解。首先我们来看List，而ArrayList是List中最常用的，所以我们从他下手。主要分为以下部分：

• 第一部分 简介
• 第二部分 数据结构分析
• 第三部分 源码分析
• 第四部分 遍历方式
• 第五部分 toArray()异常

ArrayList简介

ArrayList的定义如下：

public class ArrayList<E> extends AbstractList<E> 
implements List<E>, RandomAccess, Cloneable, java.io.Serializable

ArrayList是一个数组列表，相当于动态数组。与Java中的数组相比，它的容量能够动态增长。它继承于AbstractList，实现了List、RandomAccess、Cloneable、java.io.Serializable这几个接口。

ArrayList继承了AbstractList，实现了List接口。它是一个数组列表，提供了相关的添加、修改、删除、遍历等功能。

ArrayList实现了RandomAccess接口，即提供了随机访问的功能。RandomAccess是Java中用来被List实现，为List提供快速访问功能的。除了各种List外，Stack、Vector也实现了RandomAccess接口，这其实也说明了Stack和Vector也是List。在ArrayList中，我们可以通过索引来快速地访问元素，这就是快速随机访问，也可以通过迭代器来访问，稍后我们会比较通过”快速随机访问“和“迭代器访问”的效率。

ArrayList实现了Cloneable,这就表明ArrayList覆盖了clone()方法，可以被克隆。

ArrayList实现了java.io.Serializable，这就表明ArrayList支持被序列化，可以通过序列化被传输。

和Vector不同，ArrayList访问不是线程安全的！所以，建议在单线程中才使用ArrayList，而在多线程中使用Vector或者CopyOnWriteArrayList。

ArrayList的构造方法

public ArrayList(); //构造一个空数组列表，容量为10
public ArrayList(int initialCapacity);//构造一个空数组列表，容量为指定大小。DEFAULT_CAPACITY是ArrayList的默认容量，当由于增加数据而导致容量不足的时候，容量会增加为上次容量的1.5倍。
public ArrayList(Collection<? extends E> c); // 构建一个包含c集合的数组列表。

ArrayList的API

public int size(); // 获取当前数组列表元素的个数；
public boolean isEmpty(); // 判断当前数组列表是否为空；
public boolean contains(Object o); // 判断当前数组列表是否至少有一个o对象；
public Object[] toArray(); // 返回一个包含数组列表所有元素的数组，并保持了相应的顺序；
public <T> T[] toArray(T[] a); // 返回一个包含数组列表所有元素的数组，并保持了相应的顺序；此外返回的元素类型转化为运行时类型T，下面会详细说这个；
public boolean add(E e); // 在数组列表的尾部附加上指定元素；
public boolean remove(Object o); // 删除在数组列表最小处的该元素；
public void clear(); // 清空当前数组列表的所有元素（设置为null），执行后size=0；
public boolean addAll(Collection<? extends E> c); // 根据迭代器顺序，把指定集合添加到当前数组列表尾部；
public boolean removeAll(Collection<?> c); // 在当前数组列表中删除元素，这些元素都包含指定集合中；
public boolean retainAll(Collection<?> c); // 仅保留在指定集合中的元素，删除不在指定集合中的元素；
public Iterator<E> iterator(); // 返回迭代器，不支持向前迭代
public void forEach(Consumer<? super E> action)；
public Spliterator<E> spliterator()；
public boolean removeIf(Predicate<? super E> filter)；
public void replaceAll(UnaryOperator<E> operator)；


/* AbstractList新增的接口 */
public E get(int index); // 返回指定索引处的元素；
public E set(int index, E element); // 替换指定索引处的元素；
public void add(int index, E element); // 在指定索引处替换为指定元素，指定索引处的原来元素及后面元素依次往后推；
public E remove(int index); // 删除指定索引处的元素，后面的元素往前推；
public boolean addAll(int index, Collection<? extends E> c)；// 根据迭代器顺序，把指定集合添加到当前数组列表尾部，原索引处元素及后面元素依次往后推；
public ListIterator<E> listIterator(int index); // 在指定索引处返回列表迭代器，当该迭代器第一次执行next()的时候返回该索引处元素，执行previous()返回该索引处的前一个元素；
public ListIterator<E> listIterator(); // 返回列表迭代器；
public List<E> subList(int fromIndex, int toIndex); // 返回一个包含fromIndex，不包含toIndex的子列表；如果fromIndex=toIndex，返回空列表；子列表的修改也会影响原数组列表；
public void sort(Comparator<? super E> c)；


/* ArrayList新增的接口 */
public Object clone(); // 返回数组列表的一个浅克隆(a shallow copy)；
public void ensureCapacity(int minCapacity); // 增加ArrayList对象的容量，使其容量不低于minCapacity；
public void trimToSize(); //整理ArrayList对象的容量为它当前的size，去掉预留元素位置，内存紧张的时候会用到。

---

数据结构分析

ArrayList的继承关系

            java.lang.Object
                    |
                    v
                    java.util.AbstractCollection 
                        |
                        v
                        java.util.AbstractList
                            |
                            v
                            java.util.ArrayList
                            
public class ArrayList<E> extends AbstractList<E> 
implements List<E>, RandomAccess, Cloneable, java.io.Serializable

ArrayList与Collection的关系

ArrayList包含2个重要数据：elementData和size。

• elementData 是”Object[]类型的数组”，它是ArrayList实际存储元素的地方。实际上，elementData是一个动态数组，默认的容量是10，我们可以通过ArrayList(int initialCapacity)来设定它的容量(不小于10)。容量的大小会根据数组列表的长度增长而增长，具体的增长可以查看ensureCapacity()方法。
• size 是动态数组的实际大小。

---

ArrayList源码分析(基于JDK 1.8.0_40)

源码分析

package java.util;

import java.util.function.Consumer;
import java.util.function.Predicate;
import java.util.function.UnaryOperator;

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

    /**
     * 默认初始化容量大小.
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 空实例共享的空数组实例
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 空实例共享的空数组实例
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * 保存ArrayList数据的数组
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     * ArrayList实际数据的数量
     */
    private int size;

    /**
     * 指定ArrayList容量大小的构造器
     */
    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);
        }
    }

    /**
     * 构造一个空ArrayList，默认容量为其实是0，当第一次调用add()的时候，该数组大小会被扩展到10(DEFAULT_CAPACITY)
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 用指定集合来构造一个ArrayList，ArrayList的顺序是集合的迭代器的迭代顺序
     */
    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;
        }
    }

    /**
     * 整理ArrayList对象的容量设置为它当前的size，去掉预留元素位置，内存紧张的时候会用到。
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    /**
     * 增加ArrayList对象的容量，确保容量不低于minCapacity；
     * 如果ArrayList当前容量不足以容纳当前的全部元素，设置 新的容量=旧的容量*1.5
     */
    public void ensureCapacity(int minCapacity) {
        int minExpand = (elementData != DEFAULTCAPACITY_EMPTY_ELEMENTDATA) ? 0 : DEFAULT_CAPACITY;

        if (minCapacity > minExpand) {
            ensureExplicitCapacity(minCapacity);
        }
    }
    
    private void ensureCapacityInternal(int minCapacity) {
        // 如果当前列表的数组是空数组，那么最小容量就选minCapacity和默认容量的较大值
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    
    private void ensureExplicitCapacity(int minCapacity) {
        // 修改次数+1
        modCount++;
    
        // 如果最小容量大于数组的实际大小，则增长数组容量
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * 数组可以分配的最大的大小。有些虚拟机在一个数组中保留了头部信息，尝试去分配最大的数
     * 组大小可能会导致OutOfMemoryError错误：请求的数组大小超过了虚拟机的限制。
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 增加数组容量，确保ArrayList可以容纳至少minCapacity个元素
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        
        // 新容量 = 旧容量 + (旧容量 / 2)
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        
        // 如果新容量小于期望最小容量，那么新容量重新修改为期望最小容量
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        
        // 重新设置新容量后，把数组元素重新构造为含全部元素的新容量数组 
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

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

    /**
     * ArrayList可分配的最大的大小
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 返回ArrayList的实际大小
     */
    public int size() {
        return size;
    }

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

    /**
     * 判断当前ArrayList是否包含指定元素
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 正向查找，返回指定元素在ArrayList中最小索引，如果指定元素不在当前ArrayList中则返回-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;
    }

    /**
     * 反向查找，返回指定元素的ArrayList中的最大索引，如果指定元素不存在于当前ArrayList中则返回-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;
    }

    /**
     * 返回当前ArrayList的一个浅复制实例，浅复制ArrayList实例的元素和当前ArrayList实例的元素是对应的， 如果一个地方修改，另一个地方也会相应变化
     */
    public Object clone() {
        try {
            ArrayList<?> v = (ArrayList<?>) super.clone();
            v.elementData = Arrays.copyOf(elementData, size);
            v.modCount = 0;
            return v;
        } catch (CloneNotSupportedException e) {
            // this shouldn't happen, since we are Cloneable
            throw new InternalError(e);
        }
    }

    /**
     * 返回一个数组，包含了当前ArrayList的全部元素(依次从头部到尾部)
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * 返回一个包含数组列表所有元素的数组，并保持了相应的顺序；此外返回的元素类型转化为运行时类型T
     */
    @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;
    }

    // 位置访问操作

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

    /**
     * 返回当前ArrayList中指定索引处的元素
     */
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    /**
     * 替换当前ArrayList中指定索引处的元素
     */
    public E set(int index, E element) {
        rangeCheck(index);

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

    /**
     * 在ArrayList的尾部附加上指定的元素
     */
    public boolean add(E e) {
        // 确认容量是否足够
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        
        // 把当前ArrayList的末尾设置为当前值
        elementData[size++] = e;
        return true;
    }

    /**
     * 在指定索引处替换为指定元素，指定索引处的原来元素及后面元素依次往后推；
     */
    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++;
    }

    /**
     * 删除指定索引处的元素，后面的元素往前推；
     */
    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;
    }

    /**
     * 删除在数组列表最小处的该元素；
     */
    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;
    }

    /*
     * 私有的移除方法，跳过了边界检查，不会返回删除的值
     */
    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
    }

    /**
     * 清空当前数组列表的所有元素（设置为null），执行后size=0；
     */
    public void clear() {
        modCount++;

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

        size = 0;
    }

    /**
     * 根据迭代器顺序，把指定集合添加到当前数组列表尾部；
     */
    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;
    }

    /**
     * 根据迭代器顺序，把指定集合添加到当前数组列表尾部，原索引处元素及后面元素依次往后推；
     */
    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)中所有的元素，后面的元素往前推
     */
    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;
    }

    /**
     * 检查指定索引是否在当前ArrayList的范围中，如果不在则抛出一个运行时异常；该方法不会检查指定索引是否是负数
     */
    private void rangeCheck(int index) {
        if (index >= size)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 给add和addAll使用的另一个版本的rangeCheck
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 构造一个IndexOutOfBoundsException的异常消息
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
     * 在当前数组列表中删除元素，这些元素都包含指定集合中；
     */
    public boolean removeAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    /**
     * 仅保留在指定集合中的元素，删除不在指定集合中的元素；
     */
    public boolean retainAll(Collection<?> c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    /**
     * 批量删除
     */
    private boolean batchRemove(Collection<?> c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                if (c.contains(elementData[r]) == complement)
                    elementData[w++] = elementData[r];
        } finally {
            // 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;
    }

    /**
     * 保存当前ArrayList实例到一个流中
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        //先写入容量
        s.writeInt(size);

        // 再依次写入每个元素
        for (int i=0; i<size; i++) {
            s.writeObject(elementData[i]);
        }

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

    /**
     * 从一个流中重新设定当前的ArrayList实例
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        elementData = EMPTY_ELEMENTDATA;

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

        // 先读取容量
        s.readInt(); // ignored

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

            Object[] a = elementData;
            // 再一次读取每个元素
            for (int i=0; i<size; i++) {
                a[i] = s.readObject();
            }
        }
    }

    /**
     * 在指定索引处返回列表迭代器，当该迭代器第一次执行next()的时候返回该索引处元素，执行previous()返回该索引处的前一个元素；
     */
    public ListIterator<E> listIterator(int index) {
        if (index < 0 || index > size)
            throw new IndexOutOfBoundsException("Index: "+index);
        return new ListItr(index);
    }

    /**
     * 返回列表迭代器；
     */
    public ListIterator<E> listIterator() {
        return new ListItr(0);
    }

    /**
     * 返回迭代器，不支持向前迭代
     */
    public Iterator<E> iterator() {
        return new Itr();
    }

    /**
     * 一个AbstractList.Itr的优化版本
     */
    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;

        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();
        }
    }

    /**
     * 一个AbstractList.ListItr的优化版本
     */
    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();
            }
        }
    }

    /**
     * 返回一个包含fromIndex，不包含toIndex的子列表；如果fromIndex=toIndex，返回空列表；子列表的修改也会影响原数组列表；
     */
    public List<E> subList(int fromIndex, int toIndex) {
        subListRangeCheck(fromIndex, toIndex, size);
        return new SubList(this, 0, fromIndex, toIndex);
    }

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

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

        SubList(AbstractList<E> parent,
                int offset, int fromIndex, int toIndex) {
            this.parent = parent;
            this.parentOffset = fromIndex;
            this.offset = offset + fromIndex;
            this.size = toIndex - fromIndex;
            this.modCount = ArrayList.this.modCount;
        }

        public E set(int index, E e) {
            rangeCheck(index);
            checkForComodification();
            E oldValue = ArrayList.this.elementData(offset + index);
            ArrayList.this.elementData[offset + index] = e;
            return oldValue;
        }

        public E get(int index) {
            rangeCheck(index);
            checkForComodification();
            return ArrayList.this.elementData(offset + index);
        }

        public int size() {
            checkForComodification();
            return this.size;
        }

        public void add(int index, E e) {
            rangeCheckForAdd(index);
            checkForComodification();
            parent.add(parentOffset + index, e);
            this.modCount = parent.modCount;
            this.size++;
        }

        public E remove(int index) {
            rangeCheck(index);
            checkForComodification();
            E result = parent.remove(parentOffset + index);
            this.modCount = parent.modCount;
            this.size--;
            return result;
        }

        protected void removeRange(int fromIndex, int toIndex) {
            checkForComodification();
            parent.removeRange(parentOffset + fromIndex,
                               parentOffset + toIndex);
            this.modCount = parent.modCount;
            this.size -= toIndex - fromIndex;
        }

        public boolean addAll(Collection<? extends E> c) {
            return addAll(this.size, c);
        }

        public boolean addAll(int index, Collection<? extends E> c) {
            rangeCheckForAdd(index);
            int cSize = c.size();
            if (cSize==0)
                return false;

            checkForComodification();
            parent.addAll(parentOffset + index, c);
            this.modCount = parent.modCount;
            this.size += cSize;
            return true;
        }

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

        public ListIterator<E> listIterator(final int index) {
            checkForComodification();
            rangeCheckForAdd(index);
            final int offset = this.offset;

            return new ListIterator<E>() {
                int cursor = index;
                int lastRet = -1;
                int expectedModCount = ArrayList.this.modCount;

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

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

                public boolean hasPrevious() {
                    return cursor != 0;
                }

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

                @SuppressWarnings("unchecked")
                public void forEachRemaining(Consumer<? super E> consumer) {
                    Objects.requireNonNull(consumer);
                    final int size = SubList.this.size;
                    int i = cursor;
                    if (i >= size) {
                        return;
                    }
                    final Object[] elementData = ArrayList.this.elementData;
                    if (offset + i >= elementData.length) {
                        throw new ConcurrentModificationException();
                    }
                    while (i != size && modCount == expectedModCount) {
                        consumer.accept((E) elementData[offset + (i++)]);
                    }
                    // 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);
        }
    }

    @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();
        }
    }

    /**
     * 生成一个支持late-binding和fail-fast的分割迭代器
     */
    @Override
    public Spliterator<E> spliterator() {
        return new ArrayListSpliterator<>(this, 0, -1, 0);
    }

    static final class ArrayListSpliterator<E> implements Spliterator<E> {

        private final ArrayList<E> list;
        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

        /** Create new spliterator covering the given  range */
        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;
        }

        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;
        }

        public ArrayListSpliterator<E> trySplit() {
            int hi = getFence(), lo = index, mid = (lo + hi) >>> 1;
            return (lo >= mid) ? null : // divide range in half unless too small
                new ArrayListSpliterator<E>(list, lo, index = mid,
                                            expectedModCount);
        }

        public boolean tryAdvance(Consumer<? super E> action) {
            if (action == null)
                throw new NullPointerException();
            int hi = getFence(), i = index;
            if (i < hi) {
                index = i + 1;
                @SuppressWarnings("unchecked") E e = (E)list.elementData[i];
                action.accept(e);
                if (list.modCount != expectedModCount)
                    throw new ConcurrentModificationException();
                return true;
            }
            return false;
        }

        public void forEachRemaining(Consumer<? super E> action) {
            int i, hi, mc; // hoist accesses and checks from loop
            ArrayList<E> lst; Object[] a;
            if (action == null)
                throw new NullPointerException();
            if ((lst = list) != null && (a = lst.elementData) != null) {
                if ((hi = fence) < 0) {
                    mc = lst.modCount;
                    hi = lst.size;
                }
                else
                    mc = expectedModCount;
                if ((i = index) >= 0 && (index = hi) <= a.length) {
                    for (; i < hi; ++i) {
                        @SuppressWarnings("unchecked") E e = (E) a[i];
                        action.accept(e);
                    }
                    if (lst.modCount == mc)
                        return;
                }
            }
            throw new ConcurrentModificationException();
        }

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

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

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

    @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++;
    }

    @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++;
    }
}

总结

• ArrayList实际是通过一个数组去保存元素的；如果使用默认的构造器，则ArrayList的默认容量是10；
• 当ArrayList容量不足以容纳全部元素的时候，会重新扩容：新的容量 = 原始容量 + （原始容量 / 2）。（JDK1.6版本是：新的容量 = (原始容量 * 3) / 2 + 1）。
• ArrayList的克隆函数，即是将所有全部元素克隆到一个数组中。
• ArrayList实现java.io.Serializable的方式。当写入到输出流的时候，先写入容量，再一次写入每个元素；当读取输入流的时候，先读取容量，再依次读取每个元素。

---

ArrayList的遍历

第一种：通过迭代器遍历

Integer value = null;
Iterator iter = list.iterator();
while (iter.hasNext()) {
    value = iter.next();
}

第二种：随机访问，通过索引值遍历

Integer value = null;
int size = list.size();
for (int i = 0; i < size; i++) {
    value = list.get(i);        
}

第三种：通过forEach遍历

Integer value = null;
for (Integer integ:list) {
    value = integ;
}

下面通过实例来测试这3种遍历方式的耗时：

package com.archerda;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * ArrayList遍历方式和效率的测试程序。
 */
public class ArrayListTest {

    public static void main(String[] args) {
        Integer size = 100000;

        List<Integer> list = new ArrayList<>();
        for (int i = 0; i < size; i++) {
            list.add(i);
        }

        randomAccessToTraverse(list);
        iteratorToTraverse(list);
        foreachToTraverse(list);

    }

    public static void randomAccessToTraverse(List list) {
        long startTime;
        long endTime;

        startTime = System.currentTimeMillis();
        for (int i = 0; i < list.size(); i++) {
            list.get(i);
        }
        endTime = System.currentTimeMillis();

        long interval = endTime - startTime;
        System.out.println("randomAccessToTraverse：" + interval + " ms");
    }

    public static void iteratorToTraverse(List list) {
        long startTime;
        long endTime;

        startTime = System.currentTimeMillis();
        for (Iterator iter = list.iterator(); iter.hasNext(); ) {
            iter.next();
        }
        endTime = System.currentTimeMillis();

        long interval = endTime - startTime;
        System.out.println("iteratorToTraverse：" + interval + " ms");
    }


    public static void foreachToTraverse(List list) {
        long startTime;
        long endTime;

        startTime = System.currentTimeMillis();
        for (Object obj : list)
            ;
        endTime = System.currentTimeMillis();

        long interval = endTime - startTime;
        System.out.println("foreachToTraverse：" + interval + " ms");
    }
}

执行结果：

randomAccessToTraverse：5 ms
iteratorToTraverse：9 ms
foreachToTraverse：6 ms

结论：
遍历ArrayList的时候，效率的排序依次是：随机访问遍历 > forEach遍历 > 迭代器遍历

---

toArray()异常

Arraylist有2个toArray()方法：

public Object[] toArray();
public <T> T[] toArray(T[] a);

调用toArray()的时候，可能会抛出ClassCastException异常，例如下面这种情况：

Integer[] integers2 = (Integer[]) list.toArray();

这个时候就会抛出：

java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.Integer;

因为toArray()返回的是Object[]，而Java 不支持向下转型，把Object[]转换为Integer[]就会出现异常。但是Object[]里面每个单独的Object是可以强转成Integer的，因为Object指向的实际类型就是Integer，所以强转没有问题。而Object[]本身就只是Object类型的数组，强转成Integer[]就会出错。

可以用toArray(T[] a)改进，方式如下：

Integer[] integers = arraylist.toArray(new Integer[arraylist.size()]);

---

参考文档