ArrayList简介

          ArrayList简介

ArrayList简介

    ArrayList是基于数组实现的，是一个动态数组，其容量能自动增长，类似于C语言中的动态申请内存，动态增长内存。

    ArrayList不是线程安全的，只能用在单线程环境下，多线程环境下可以考虑用Collections.synchronizedList(List l)函数返回一个线程安全的ArrayList类，也可以使用concurrent并发包下的CopyOnWriteArrayList类。

    ArrayList实现了Serializable接口，因此它支持序列化，能够通过序列化传输，实现了RandomAccess接口，支持快速随机访问，实际上就是通过下标序号进行快速访问，实现了Cloneable接口，能被克隆。

ArrayList源码剖析

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

   // 默认容量大小
    private static final int DEFAULT_CAPACITY = 10;
	//一个空的数组
    private static final Object[] EMPTY_ELEMENTDATA = {};

    //缺省的空数组
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

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

    // ArrayList中所包含元素的个数
    private int size;

     // 带初始容量参数的构造函数
    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);
        }
    }

     // 默认构造函数，其默认初始容量为10
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

   // 带Collection参数的构造函数
    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;
        }
    }
  // 将此 ArrayList 实例的容量调整为列表的当前大小(实际元素个数)
    public void trimToSize() {
        modCount++;
        if (size < elementData.length) {
            elementData = (size == 0)
              ? EMPTY_ELEMENTDATA
              : Arrays.copyOf(elementData, size);
        }
    }

  // 如有必要，增加此 ArrayList 实例的容量，以确保它至少能够容纳最小容量参数所
    // 指定的元素数
    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);
        }
    }

    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == DEFAULTCAPACITY_EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }

    private void ensureExplicitCapacity(int minCapacity) {
        modCount++;

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

    //数组的最大分配为 Integer.MAX_VALUE - 8;
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

   //数组扩容，利用右移变为原来1.5倍，如果扩容后大于最大分配限度，则扩容为minCapacity
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        if (newCapacity - MAX_ARRAY_SIZE > 0)
            newCapacity = hugeCapacity(minCapacity);
        // minCapacity is usually close to size, so this is a win:
        elementData = Arrays.copyOf(elementData, newCapacity);
    }
	//扩容为minCapacity
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }

   // 返回ArrayList中的元素个数
    public int size() {
        return size;
    }

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

 // 判断ArrayList是否包含Object(o)
    public boolean contains(Object o) {
        return indexOf(o) >= 0;
    }

     // 返回ArrayList中首次出现的指定元素的索引，或如果此列表不包含元素，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;
    }

  // 返回ArrayList中最后一次出现的指定元素的索引，或如果此列表不包含索引，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;
    }

    // 克隆函数， 返回此 ArrayList 实例的浅表副本
    public Object clone() {
        try {
			  // 将当前ArrayList的全部元素拷贝到v中
            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);
        }
    }

    // 按适当顺序（从第一个到最后一个元素）返回包含此列表中所有元素的数组
    public Object[] toArray() {
        return Arrays.copyOf(elementData, size);
    }

   // 返回ArrayList的模板数组。所谓模板数组，即可以将T设为任意的数据类型
    @SuppressWarnings("unchecked")
    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;
    }

   // 位置访问操作 

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

    
    // 返回ArrayList中指定位置上的元素
    public E get(int index) {
        rangeCheck(index);

        return elementData(index);
    }

    // 用指定的元素替代ArrayList中指定位置上的元素，并返回替代前的元素
    public E set(int index, E element) {
        rangeCheck(index);

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

    // 将指定的元素添加到ArrayList的尾部
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

  
	 // 将e添加到ArrayList的指定位置
    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ArrayList指定位置的元素 
    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);
        elementData[--size] = null; // clear to let GC do its work

        return oldValue;
    }

	 // 删除ArrayList的指定元素 
    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;
    }

    /*
     * Private remove method that skips bounds checking and does not
     * return the value removed.
     */
	 // 快速删除第index个元素 
    private void fastRemove(int index) {
        modCount++;
        int numMoved = size - index - 1;
        if (numMoved > 0)
			// 从"index+1"开始，用后面的元素替换前面的元素。
            System.arraycopy(elementData, index+1, elementData, index,
                             numMoved);
			// 将最后一个元素设为null 
        elementData[--size] = null; // clear to let GC do its work
    }

 
	 // 清空ArrayList，将全部的元素设为null
    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追加到ArrayList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;
    }

       // 从index位置开始，将集合c添加到ArrayList
    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fromIndex到toIndex之间的全部元素。
    protected void removeRange(int fromIndex, int toIndex) {
        modCount++;
        int numMoved = size - toIndex;
		//把toIndex到末尾的元素复制到fromIndex到最后
        System.arraycopy(elementData, toIndex, elementData, fromIndex,
                         numMoved);

        // 把newSize个元素为null
        int newSize = size - (toIndex-fromIndex);
        for (int i = newSize; i < size; i++) {
            elementData[i] = null;
        }
        size = newSize;
    }

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

    //检查下标是否越界
    private void rangeCheckForAdd(int index) {
        if (index > size || index < 0)
            throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
    }

   
    private String outOfBoundsMsg(int index) {
        return "Index: "+index+", Size: "+size;
    }

 // 移除ArrayList中Collection所包含的所有元素
    public boolean removeAll(Collection c) {
        Objects.requireNonNull(c);
        return batchRemove(c, false);
    }

   // 保留所有ArrayList和Collection共有的元素
    public boolean retainAll(Collection c) {
        Objects.requireNonNull(c);
        return batchRemove(c, true);
    }

    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;
    }

   // java.io.Serializable的写入函数    
    // 将ArrayList的“容量，所有的元素值”都写入到输出流中
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException{
        // Write out element count, and any hidden stuff
        int expectedModCount = modCount;
        s.defaultWriteObject();

        // 写入“数组的容量” 
        s.writeInt(size);

         // 写入“数组的每一个元素”  
        for (int i=0; i

参考资料：

http://blog.csdn.net/ns_code/article/details/35568011以及jdk 1.8