/**

      * The array buffer into which the elements of the ArrayList are stored.

      * The capacity of the ArrayList is the length of this array buffer.

      */

     private transient Object[] elementData;  

     /**

      * The size of the ArrayList (the number of elements it contains).

      *

      * @serial

      */

     private int size;

// 将指定的元素添加到此列表的尾部。

    public boolean add(E e) {

        ensureCapacity(size + 1); //判断容量是否足够，不够就扩容

        elementData[size++] = e;

        return true;

    }

public void ensureCapacity(int minCapacity) {

    modCount++;

    int oldCapacity = elementData.length;

    if (minCapacity > oldCapacity) {  //size+1即添加后的长度，判断如果添加，会不会过长

        Object oldData[] = elementData;

        int newCapacity = (oldCapacity * 3)/2 + 1;  //1.5倍扩容

            if (newCapacity < minCapacity)

                newCapacity = minCapacity;

      // minCapacity is usually close to size, so this is a win:

      elementData = Arrays.copyOf(elementData, newCapacity);  //将老数组拷贝一份到新数组

    }

 }