ArrayList源码阅读记录

ArrayList多是在开发中使用最多的集合类。ArrayList其实就是一个动态数组，它会根据元素的个数进行动态扩容。

下面是部分源码，本次源码基于jdk1.7：

public class ArrayList<E> extends AbstractList<E>
        implements List<E>, RandomAccess, Cloneable, java.io.Serializable
{
    private static final long serialVersionUID = 8683452581122892189L;

    /**
     * ArrayList默认初始容量
     */
    private static final int DEFAULT_CAPACITY = 10;

    /**
     * 使用无参构造方法时ArrayList使用的空数组
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * ArrayList维护用于存放数据的数组
     * 
     */
    private transient Object[] elementData;

    /**
     * 数组大小
     * The size of the ArrayList (the number of elements it contains).
     *
     * @serial
     */
    private int size;

    /**
     * 构造一个空数组，自定义指定初始容量
     *
     * @param  initialCapacity  自定义初始容量
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    public ArrayList(int initialCapacity) {
        super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
        this.elementData = new Object[initialCapacity];
    }

    /**
     * 构造一个空数组，初始容量是10
     */
    public ArrayList() {
        super();
        this.elementData = EMPTY_ELEMENTDATA;
    }

    /**
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.
     *
     * @param c the collection whose elements are to be placed into this list
     * @throws NullPointerException if the specified collection is null
     */
    public ArrayList(Collection<? extends E> c) {
        elementData = c.toArray();
        size = elementData.length;
        // c.toArray might (incorrectly) not return Object[] (see 6260652)
        if (elementData.getClass() != Object[].class)
            elementData = Arrays.copyOf(elementData, size, Object[].class);
    }

    /**
     * 私用方法用于保证数组容量，先判断数组是否为空，为空则容量最少为10
     */
    private void ensureCapacityInternal(int minCapacity) {
        if (elementData == EMPTY_ELEMENTDATA) {
            minCapacity = Math.max(DEFAULT_CAPACITY, minCapacity);
        }

        ensureExplicitCapacity(minCapacity);
    }
    /**
     * 该方法被调用一次modCount内部修改计数器会加1，modCount计数器被定义在父类AbstractList中
     * modCount被内部实现Iterator和ListIterator的类使用
     * 该方法考虑了溢出，当须要的容量超过了当前数组的容量开始调用grow方法进行扩容
     */
    private void ensureExplicitCapacity(int minCapacity) {
        
        modCount++;

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

    /**
     * 最大可分配容量
     */
    private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

    /**
     * 该方法用于增长数组的容量，确保可容纳指定的最小长度的元素个数
     * 按照数组元容量的1.5倍扩容，这个扩容量主要是综合考虑了扩容次数和内存使用方便考虑的，
     * 若是扩容倍数大了浪费内存，小了会增长扩容次数增长开销。
     * 该方法实际是计算好最终的扩容量而后交给Arrays.copyOf->System.arraycopy
     * Systeam的arraycopy是一个native的方法实现数组的拷贝生成新的数组
     * @param minCapacity the desired minimum capacity
     */
    private void grow(int minCapacity) {
        // overflow-conscious code
        int oldCapacity = elementData.length;
        //等价于int newCapacity = oldCapacity + oldCapacity/2,即按照当前容量的1.5倍扩容;
        int newCapacity = oldCapacity + (oldCapacity >> 1);
        //若是原数组1.5倍容量小于指定的最小容量，则使用指定的最小容量
        if (newCapacity - minCapacity < 0)
            newCapacity = minCapacity;
        //若是原数组1.5倍容量大于MAX_ARRAY_SIZE(ps:Integer.MAX_VALUE - 8),则使用MAX_ARRAY_SIZE作容量
        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);
    }
    /**
     * 确保最大容量不能超过MAX_ARRAY_SIZE，超过了直接强制使用MAX_ARRAY_SIZE做为容量
     */
    private static int hugeCapacity(int minCapacity) {
        if (minCapacity < 0) // overflow
            throw new OutOfMemoryError();
        return (minCapacity > MAX_ARRAY_SIZE) ?
            Integer.MAX_VALUE :
            MAX_ARRAY_SIZE;
    }
    /**
     * 
     * 添加指定的元素，添加元素前调用ensureCapacityInternal方法来保证容量正常
     * 固然每增长一个元素modCount会加1
     * @param e element to be appended to this list
     * @return <tt>true</tt> (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        elementData[size++] = e;
        return true;
    }

    /**
     * 它也增长了modCount，而后计算要移动的元素个数，从index日后的元素都往前移动一位，
     * 实际调用System.arraycopy方法移动元素。elementData[--size] = null;
     * 这行代码将size减一，同时将最后一个位置设为null方便被回收
     *
     * @param index the index of the element to be removed
     * @return the element that was removed from the list
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    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;
    }
}