ArrayList构造函数源码

 /**
     * Shared empty array instance used for empty instances.
     */
    private static final Object[] EMPTY_ELEMENTDATA = {};

    /**
     * Shared empty array instance used for default sized empty instances. We
     * distinguish this from EMPTY_ELEMENTDATA to know how much to inflate when
     * first element is added.
     */
    private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};

/**
     * Constructs an empty list with the specified initial capacity.
     *
     * @param  initialCapacity  the initial capacity of the list
     * @throws IllegalArgumentException if the specified initial capacity
     *         is negative
     */
    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);
        }
    }

    /**
     * Constructs an empty list with an initial capacity of ten.
     */
    public ArrayList() {
        this.elementData = DEFAULTCAPACITY_EMPTY_ELEMENTDATA;
    }

由jdk源码可知：

1，当不指定initialCapacity时创建空arraylist 此时内部空数组为DEFAULTCAPACITY_EMPTY_ELEMENTDATA，当添加第一个元素时 长度自动扩充至DEFAULT_CAPACITY也就是10

2，当指定initialCapacity=0时创建空arraylist 此时内部空数组为EMPTY_ELEMENTDATA，每添加一次元素容量扩充1

测试代码：

ArrayList<String> list1  = new ArrayList<String>();
		ArrayList<String> list2  = new ArrayList<String>(0);
		ArrayList<String> list3  = new ArrayList<String>(20);
		list1.add("test");
		list2.add("test");
		list3.add("test");