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

1. E push(E item) 把项压入堆栈顶部。
2. E pop() 移除堆栈顶部的对象，并作为此函数的值返回该对象。
3. E peek() 查看堆栈顶部的对象，但不从堆栈中移除它。
4. boolean empty() 测试堆栈是否为空。
5. 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：

1. public class Stack<E> extends Vector<E> {
2. /**

• * 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() {
• E obj;
• int len = 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() {
• int len = 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：
    • • ------isEmpty
      • true
      • ------push
      • iterator:1;2;3;
      • ------pop
      • 3 --栈顶是数组最后一个
      • iterator:1;2;
      • ------peek
      • 2 --pop取后删掉，peek只取不删
      • iterator:1;2;
      • ------search
      • 1 --以1为基数，即栈顶为1
      • 2 --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）