集合框架知识系列03 ArrayList的源码分析和使用示例

摘要：每次迭代器进结构性修改的时候都将和进行对比如果两种相等则说明没有其他迭代器修改了对象，可以进行。

ArrayList的内部实现了动态数组，提供了动态的增加和减少元素，继承AbstractList类，并且实现了List、RandomAccess、Cloneable和java.io.Serializable接口。ArrayList是一个数组队列，提供添加、删除、修改和遍历元素的功能。因为实现RandomAccess接口，提供了随机访问的功能。现了Cloneable接口，即覆盖了函数clone()，能被克隆。现java.io.Serializable接口，这意味着ArrayList支持序列化。ArrayList不是线程安全的，建议在单线程中访问。
ArrayList有三个构造方法，定义如下：

//创建给定初始化大小的ArrayList
public ArrayList(int initialCapacity) {}
//默认无参构造方法创建的ArrayList
public ArrayList() {}
//创建给定初始化集合c的ArrayList
public ArrayList(Collection c) {}

ArrayList是通过动态数组实现的，下面通过源码分析ArrayList的实现：

public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable
{
   
    /**
     * 默认初始化大小
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 空数组实例
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * 判断是否为第一次添加元素
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

    /**
     * ArrayList保存元素数据，通过elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA 来判断是否是第一次添加元素
     */
    transient Object[] elementData;

    /**
     * ArrayList的实际大小
     */
    private int size;

    /**
     * 创建大小为initialCapacity的空ArrayList
     */
    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);
        }
    }

    /**
     * 创建初始化容量为10的list
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

    /**
     * 通过集合c创建list
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException 如果c为nulll,有空指针异常
     */
    public ArrayList(Collection 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;
        }
    }

    /**
     * 缩小list容量为当前真实大小
     */
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

    //外部调用方法，调整容量，确保list不会越界
    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);
        }
    }

         //计算容量
    private static int calculateCapacity(Object[] elementData, int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            return Math.max(DEFAULT_CAPACITY, minCapacity);
        }
        return minCapacity;
    }
     
         //内部调用方法，调整容量，确保list不会越界
    private void ensureCapacityInternal(int minCapacity) {
        ensureExplicitCapacity(calculateCapacity(elementData, minCapacity));
    }

         //扩展容量
    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

        // 如果最小容量大于数组大小，进行数组扩展
        if (minCapacity - elementData.length > 0)
            grow(minCapacity);
    }

    /**
     * 数组容量的最大值。部分虚拟机限制，大于MAX_ARRAY_SIZE，会导致OutOfMemoryError
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 数组按照1.5倍增加，如果增加后的值小于minCapacity，按照minCapacity增加
     * 
     */
    private void grow(int minCapacity) {
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;

           // 是否大于最大值
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        elementData = Arrays.copyOf(elementData, newCapacity);
    }

         //如果扩展容量大于最大值，按照最大值扩展
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0)
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

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

    /**
     * 如果实际大小为0，返回true.
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /**
     * 返回元素是否存在，indexOf(o)方法返回-1表示不存在.
     */
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

    /**
     * 返回元素的下标，-1表示元素不存在
     */
    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;
    }

    /**
     * 返回最后一个元素o的下标
     */
    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;
    }

    /**
     * copy一个list对象
     */
    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);
        }
    }

    /**
     * 将list转换为对象
     */
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

    /**
     * 将list转换为对应类型的数组，如果数组大小小于size,通过Arrays.copyOf转换，如果大于System.arraycopy转换
     */
    public  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;
    }

    //通过制定下标返回一个元素
    E elementData(int index) {
        return (E) elementData[index];
    }

    /**
     * 根据下标获取元素
     *
     */
    public E get(int index) {
     //检查是否越界
        rangeCheck(index);
        return elementData(index);
    }

    /**
     * 将指定位置的元素替换，返回老的元素
     */
    public E set(int index, E element) {
        rangeCheck(index);

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

    /**
     *  在list中添加一个元素
     */
    public boolean add(E e) {
        //调整大小
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * 在指定位置添加一个元素
     */
    public void add(int index, E element) {
        rangeCheckForAdd(index);

        ensureCapacityInternal(size + 1);  // Increments modCount!!
         //index之后的元素后移
        System.arraycopy(elementData, index, elementData, index + 1,
                         size - index);
        elementData[index] = element;
        size++;
    }

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

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

        int numMoved = size - index - 1;
        if (numMoved > 0)
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
        //将最后一个对象置空，便于GC
        elementData[--size] = null; 

        return oldValue;
    }

    /**
     * 根据指定的元素移除，调用fastRemove(index)方法
     */
    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;
    }

    /*
     * 不检查边界的快速移除元素
     */
    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
    }

    /**
     * 清除所有元素，size赋值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c中的元素添加到list中
     */
    public boolean addAll(Collection 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c中的元素添加到index开始的位置，原index之后的元素后移
     */
    public boolean addAll(int index, Collection 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;
    }

    /**
     * 删除指定区间的元素
     */
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // clear to let GC do its work
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

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

    /**
     * 添加时检查是否越界
     */
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

    /**
     * 越界后返回的异常详细信息
     */
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

    /**
     * 删除集合c中所有元素，首选检查c是否为空，调用batchRemove(c, false)方法
     */
    public boolean removeAll(Collection c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

    /**
     * 保留给定集合的元素，删除其他的
     *
     */
    public boolean retainAll(Collection c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

         /**
     * 根据complement判断是删除还是保留给定的集合元素
     *
     */
    private boolean batchRemove(Collection c, boolean complement) {
        final Object[] elementData = this.elementData;
        int r = 0, w = 0;
        boolean modified = false;
        try {
            for (; r < size; r++)
                 //将删除或者保留的元素移动到数据前面
                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;
            }
             //把w下标后的数据删除
            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;
    }

    /**
     * 将ArrayList保存到流中
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // 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

上面分析了ArrayList源码，其中Itr和ListItr这两个内部类没有详细介绍。Itr和ListItr在AbstractList中有实现，在ArrayList对其进行了优化。下面进行详细介绍：

Itr

Itr实现了Iterator接口，源码如下：

private class Itr implements Iterator {
         //下一个元素的下标          
        int cursor;       // index of next element to return
         //最后返回元素的下标，如果不存在，返回-1
        int lastRet = -1; 

     /**
      * 每个迭代器保存一个expectedModCount ，来记录这个迭代器对对象进行结构性修改的次数。
      * 每次迭代器进结构性修改的时候都将expectedModCount 和modCount进行对比
      * 如果两种相等则说明没有其他迭代器修改了对象，可以进行。如果不相等则说明有迭代进行了修改，立刻抛出异常
      */
        int expectedModCount = modCount;

        Itr() {}
        //下一个元素下标不等于size,表示还有下一个元素
        public boolean hasNext() {
            return cursor != size;
        }

        //获取到下一个元素
        public E next() {
            //检查其他迭代器对list是否有修改
            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 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();
            }
            //移动元素
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;
            checkForComodification();
        }

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

ListItr

ListItr继承自Itr,并且实现了ListIterator接口，源码如下：

private class ListItr extends Itr implements ListIterator {
        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();
            }
        }
    }

jdk 1.8以前的集合list遍历支持三种方式，在1.8中增加了java 8 forEach方法，下面分别分析这四种遍历方式以及效率：

public class ArrayListIteratorTest {
    
        public static void main(String[] args) {
    
            List list = new ArrayList();
            for (int i = 0; i < 1000000 ; i++) {
                list.add(i);
            }
    
            iteratorTest(list);
    
            foreashITest(list);
    
            foreashTest(list);
    
            java8ForeashTest(list);
    
    
        }
    
        /**
         * 通过迭代器遍历
         * @param list
         */
        static void iteratorTest(List list){
            long startTime;
            long endTime;
            startTime = System.currentTimeMillis();
            Iterator iterator = list.iterator();
            while (iterator.hasNext()){
                iterator.next();
            }
            endTime = System.currentTimeMillis();
            System.out.println("Iterator time :" + (endTime - startTime));
        }
    
        /**
         * 通过索引遍历
         * @param list
         */
        static void foreashITest(List list){
            long startTime;
            long endTime;
            startTime = System.currentTimeMillis();
            for (int i = 0, length = list.size(); i < length; i++) {
                list.get(i);
            }
            endTime = System.currentTimeMillis();
            System.out.println("fori time :" + (endTime - startTime));
        }
    
        /**
         * 通过foreash遍历
         * @param list
         */
        static void foreashTest(List list){
            long startTime;
            long endTime;
            startTime = System.currentTimeMillis();
            for (Object l: list) {
    
            }
            endTime = System.currentTimeMillis();
            System.out.println("foreash time :" + (endTime - startTime));
        }
    
        /**
         * 通过java 8 中提供的foreash遍历
         * @param list
         */
        static void java8ForeashTest(List list){
            long startTime;
            long endTime;
            startTime = System.currentTimeMillis();
            list.forEach(l->{});
            endTime = System.currentTimeMillis();
            System.out.println("java 8 foreash time :" + (endTime - startTime));
        }
    }

以上代码运行后的结果如下：

从运行结果看，foreash运行效率最高，java 8 中的foreash运行效率最差。

ArrayList中提供了连个方法将list转换为数组，分别是Object[] toArray()和 T[] toArray(T[] a)。调用第一个方法会有抛出“java.lang.ClassCastException”异常的情况，下面通过具体示例演示：

public class ArrayListToArraysTest {

    public static void main(String[] args) {
        List list = new ArrayList<>();
        Dog dog1 = new Dog();
        Dog dog2 = new Dog();

        list.add(dog1);
        list.add(dog2);
    //此处会抛出异常
        Dog[] dogs1 = (Dog[]) list.toArray();
        System.out.println(Arrays.toString(dogs1));

        Dog[] dogs2 = new Dog[list.size()];
        dogs2 = list.toArray(dogs2);

        System.out.println(Arrays.toString(dogs2));
    }

    private static class Dog{
        private String name;

        private String color;


        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getColor() {
            return color;
        }

        public void setColor(String color) {
            this.color = color;
        }
    }
}

当某一个线程A通过iterator去遍历某集合的过程中，若该集合的内容被其他线程所改变了；那么线程A访问集合时，就会抛出ConcurrentModificationException异常，产生fail-fast事件。
Fail-fast示例如下：

public class ArrayListFailFastTest {
        private static List list = new ArrayList();
        public static void main(String[] args) {
    
            Thread t1 = new Thread(new ThreadTest(),"t1");
            Thread t2 = new Thread(new ThreadTest(), "t2");
            t1.start();
            t2.start();
        }
    
        private static class ThreadTest implements Runnable{
            @Override
            public void run() {
                for (int i = 0; i < 20; i++) {
                    list.add(i);
                }
                Iterator iterator = list.iterator();
                while (iterator.hasNext()){
                    System.out.print(iterator.next() + " ");
                }
            }
        }
    
    }

可以看出，在多线程下，通过iterator去遍历某集合，会抛ConcurrentModificationException异常。

在本章中，分析了ArrayList集合。ArrayList的内部是通过动态数组存储数据的，默认初始大小是10，在jdk1.8中，默认构造方法创建对象，默认的数组为空，当第一次添加元素时，设置数组大小为10。在调整数组大小时，默认是增加原数组的1.5倍，如果传入的最小扩展数大于增加1.5倍后的大小，则按照此最小扩展数扩展，否则按照默认扩展。