java.util.Stack类简介

Stack是一个后进先出（last in first out，LIFO）的堆栈，在Vector类的基础上扩展5个方法而来

Deque（双端队列）比起Stack具有更好的完整性和一致性，应该被优先使用

 E push(E item)           把项压入堆栈顶部。  E pop()           移除堆栈顶部的对象，并作为此函数的值返回该对象。  E peek()           查看堆栈顶部的对象，但不从堆栈中移除它。  boolean empty()           测试堆栈是否为空。   int search(Object o)           返回对象在堆栈中的位置，以 1 为基数。

Stack本身通过扩展Vector而来，而Vector本身是一个可增长的对象数组（ a growable array of objects）那么这个数组的哪里作为Stack的栈顶，哪里作为Stack的栈底？

答案只能从源代码中寻找，jdk1.6：

public class Stack<E> extends Vector<E> {    /**     * Creates an empty Stack.     */    public Stack() {    }    /**     * 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) {addElement(item);return item;    }    /**     * 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).     * @exception  EmptyStackException  if this stack is empty.     */    public synchronized E pop() {Eobj;intlen = size();obj = peek();removeElementAt(len - 1);return obj;    }    /**     * Looks at the object at the top of this stack without removing it     * from the stack.     *     * @return     the object at the top of this stack (the last item     *             of the <tt>Vector</tt> object).     * @exception  EmptyStackException  if this stack is empty.     */    public synchronized E peek() {intlen = size();if (len == 0)    throw new EmptyStackException();return elementAt(len - 1);    }    /**     * Tests if this stack is empty.     *     * @return  <code>true</code> if and only if this stack contains     *          no items; <code>false</code> otherwise.     */    public boolean empty() {return size() == 0;    }    /**     * Returns the 1-based position where an object is on this stack.     * If the object <tt>o</tt> occurs as an item in this stack, this     * method returns the distance from the top of the stack of the     * occurrence nearest the top of the stack; the topmost item on the     * stack is considered to be at distance <tt>1</tt>. The <tt>equals</tt>     * method is used to compare <tt>o</tt> to the     * items in this stack.     *     * @param   o   the desired object.     * @return  the 1-based position from the top of the stack where     *          the object is located; the return value <code>-1</code>     *          indicates that the object is not on the stack.     */    public synchronized int search(Object o) {int i = lastIndexOf(o);if (i >= 0) {    return size() - i;}return -1;    }    /** use serialVersionUID from JDK 1.0.2 for interoperability */    private static final long serialVersionUID = 1224463164541339165L;}

通过peek()方法注释The object at the top of this stack (the last item of the Vector object，可以发现数组（Vector）的最后一位即为Stack的栈顶

pop、peek以及search方法本身进行了同步

push方法调用了父类的addElement方法

empty方法调用了父类的size方法

Vector类为线程安全类

综上，Stack类为线程安全类(多个方法调用而产生的数据不一致问题属于原子性问题的范畴)

public class Test {public static void main(String[] args) {Stack<String> s = new Stack<String>();System.out.println("------isEmpty");System.out.println(s.isEmpty());System.out.println("------push");s.push("1");s.push("2");s.push("3");Test.it(s);System.out.println("------pop");String str = s.pop();System.out.println(str);Test.it(s);System.out.println("------peek");str = s.peek();System.out.println(str);Test.it(s);System.out.println("------search");int i = s.search("2");System.out.println(i);i = s.search("1");System.out.println(i);i = s.search("none");System.out.println(i);}public static void it(Stack<String> s){System.out.print("iterator:");Iterator<String> it = s.iterator();while(it.hasNext()){System.out.print(it.next()+";");}System.out.print("\n");}}

结果：

------isEmptytrue          ------pushiterator:1;2;3;  ------pop3--栈顶是数组最后一个iterator:1;2;------peek2--pop取后删掉，peek只取不删iterator:1;2;------search    1--以1为基数，即栈顶为12--和栈顶见的距离为2-1=1-1--不存在于栈中

Stack并不要求其中保存数据的唯一性，当Stack中有多个相同的item时，调用search方法，只返回与查找对象equal并且离栈顶最近的item与栈顶间距离（见源码中search方法说明）