超详细JDK1.8 ArrayList集合默认长度及扩容分析

1、首先看ArrayList默认构造方法创建

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

从以上两段代码得知，List集合底层就是用数组（属性名：elementData）存储数据的，默认构造方法初始化数组为空，那么数组的长度即为0，所以通过默认构造方法创建集合，默认数组的长度为0

2、接着看集合add方法和addAll方法

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

/**
     * Appends the specified element to the end of this list.
     *
     * @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;
    }

/**
     * Appends all of the elements in the specified collection to the end of
     * this list, in the order that they are returned by the
     * specified