java1.7集合源码阅读: Stack

Stack类也是List接口的一种实现，也是一个有着非常长历史的实现，从jdk1.0开始就有了这个实现。

Stack是一种基于后进先出队列的实现(last-in-first-out (LIFO)),实际上jdk也提供了有关队列的其他实现，这里就先看看Stack的实现：

类定义：

public class Stack<E> extends Vector<E> { //从类定义看，Stack是线程安全的
.....
}

看看Stack提供的一些CRUD方法:

    /**
      * Pushes an item onto the top of this stack. This has exactly
      * the same effect as:
      * <blockquote><pre>
      * addElement(item)</pre></blockquote>
      *
      * @param   item   the item to be pushed onto this stack.
      * @return  the <code>item</code> argument.
      * @see     java.util.Vector#addElement
      */
     public E push(E item) {  //  push方法是在队尾增加一个元素
         addElement(item);

         return item;
     }

     /**
      * Adds the specified component to the end of this vector,
      * increasing its size by one. The capacity of this vector is
      * increased if its size becomes greater than its capacity.
      *
      * <p>This method is identical in functionality to the
      * {@link #add(Object) add(E)}
      * method (which is part of the {@link List} interface).
      *
      * @param   obj   the component to be added
      */
     public synchronized void addElement(E obj) {  //Vector中的方法
         modCount++;
         ensureCapacityHelper(elementCount + 1);
         elementData[elementCount++] = obj;
     }

重点注意一下pop方法：

     /**
      * Removes the object at the top of this stack and returns that
      * object as the value of this function.
      *
      * @return  The object at the top of this stack (the last item
      *          of the <tt>Vector</tt> object).
      * @throws  EmptyStackException  if this stack is empty.
      */
     public synchronized E pop() {  //pop方法是获取并删除队尾的元素
         E       obj;
         int     len = size();

         obj = peek();  //见后续peek()方法
         removeElementAt(len - 1);

         return obj;
     }
  /**
      * Deletes the component at the specified index. Each component in
      * this vector with an index greater or equal to the specified
      * {@code index} is shifted downward to have an index one
      * smaller than the value it had previously. The size of this vector
      * is decreased by {@code 1}.
      *
      * <p>The index must be a value greater than or equal to {@code 0}
      * and less than the current size of the vector.
      *
      * <p>This method is identical in functionality to the {@link #remove(int)}
      * method (which is part of the {@link List} interface).  Note that the
      * {@code remove} method returns the old value that was stored at the
      * specified position.
      *
      * @param      index   the index of the object to remove
      * @throws ArrayIndexOutOfBoundsException if the index is out of range
      *         ({@code index < 0 || index >= size()})
      */
     public synchronized void removeElementAt(int index) {  //Vector中的方法，删除指定index元素
         modCount++;
         if (index >= elementCount) {
             throw new ArrayIndexOutOfBoundsException(index + " >= " +
                                                      elementCount);
         }
         else if (index < 0) {
             throw new ArrayIndexOutOfBoundsException(index);
         }
         int j = elementCount - index - 1;
         if (j > 0) {
             System.arraycopy(elementData, index + 1, elementData, index, j);
         }
         elementCount--;
         elementData[elementCount] = null;
     }

Stack 的peek方法，只是获取元素，但并不会去做删除处理，当队列中没有元素的时候，会报EmptyStackException异常：

    public synchronized E peek() {
        int     len = size();

        if (len == 0)
            throw new EmptyStackException();
        return elementAt(len - 1);
    }

Stack 比较简单，大部分实现都在Vector中，可参考：java1.7集合源码阅读: Vector