1. 默认容量为10，用于在add方法添加第一个元素的时候初始化数组大小
    2. private static final int DEFAULT_CAPACITY = 10;
    1. private static final Object[] EMPTY_ELEMENTDATA = {};
    2. private static final Object[] DEFAULTCAPACITY_EMPTY_ELEMENTDATA = {};
    1. transient Object[] elementData; // non-private to simplify nested class access //实际存放元素的数组
    1. private int size; 包含的元素个数