JavaSE_坚持读源码_ArrayList对象_Java1.7

底层的数组对象

 /**
  * The array buffer into which the elements of the ArrayList are stored.
  * The capacity of the ArrayList is the length of this array buffer. Any
  * empty ArrayList with elementData == EMPTY_ELEMENTDATA will be expanded to
  * DEFAULT_CAPACITY when the first element is added.
  */
  //ArrayList底层的数组对象
 private transient Object[] elementData;

get(int index) 方法

     public E get(int index) {
     //先对角标进行校验，看看是否越界
         rangeCheck(index);

         return elementData(index);
     }

     //throws an ArrayIndexOutOfBoundsException if index is negative
     //如果index是负数，抛出ArrayIndexOutOfBoundsException
     private void rangeCheck(int index) {
         if (index >= size)
             throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
     }

     //真正的获取元素的方法，就是从底层的数据库获取index处的元素，返回值E是由ArrayList的泛型指定
     E elementData(int index) {
         return (E) elementData[index];
     }

set(int index, E element)

     public E set(int index, E element) {
     //先对角标进行校验，看看是否越界
         rangeCheck(index);

     //取出以前位于该指定位置上的元素
         E oldValue = elementData(index);
     //将数组位于index位置上的元素设置为element
         elementData[index] = element;
     //将以前位于该指定位置上的元素返回
         return oldValue;
     }

 isEmpty()

 public boolean isEmpty() {
     //如果ArrayList的size==0，那么即为Empty
         return size == 0;
     }

indexOf(Object o)

 public int indexOf(Object o) {
     //这里对传入的Object进行了一个是否为空的区分
     //不管是否为空，都去遍历底层的数组
     //如果在数组中得到与传入的Object相等的，则返回该元素在数组中的角标
     //否则，返回-1
         if (o == null) {
             for (int i = 0; i < size; i++)
                 if (elementData[i]==null)
                     return i;
         } else {
             for (int i = 0; i < size; i++)
                 if (o.equals(elementData[i]))
                     return i;
         }
         return -1;
     }