ArrayList的源码解析

private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
}

    private static final Object[] EMPTY_ELEMENTDATA = {};

    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);
        }
    }

transient Object[] elementData; // non-private to simplify nested class access

private static final Object[] EMPTY_ELEMENTDATA = {};

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;
        }
}

    /**  
     * ArrayList的容量就是该数组的长度.
     */
    transient Object[] elementData; // non-private to simplify nested class access

    /**
     *  一个空数组.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the list‘s
     * current size. An application can use this operation to minimize the
     * storage of an <tt>ArrayList</tt> instance.
     * 
     * 将数组缓冲区大小调整到实际ArrayList存储元素的大小，即 elementData = Arrays.copyOf(elementData,size);
     */
    public void trimToSize() {
        modCount++;
        // 当实际大小 < 数组缓冲区大小时
        // 如调用默认构造函数后，刚添加一个元素，此时elementData.length=10，而size=1
        // 通过这一步，可以使得空间得到有效利用，而不会出现资源浪费的情况。
        if (size < elementData.length) {
            elementData = (size == 0) ? EMPTY_ELEMENTDATA : Arrays.copyOf(elementData,size);
        }
    }