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