转载自  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() {  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）