java 堆栈

import java.util.*;public class StackDemo {   static void showpush(Stack st, int a) {      st.push(new Integer(a));      System.out.println("push(" + a + ")");      System.out.println("stack: " + st);   }   static void showpop(Stack st) {      System.out.print("pop -> ");      Integer a = (Integer) st.pop();      System.out.println(a);      System.out.println("stack: " + st);   }   public static void main(String args[]) {      Stack st = new Stack();      System.out.println("stack: " + st);      showpush(st, 42);      showpush(st, 66);      showpush(st, 99);      showpop(st);      showpop(st);      showpop(st);      try {         showpop(st);      } catch (EmptyStackException e) {         System.out.println("empty stack");      }   }}