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The Stack class implements a Last In, First Out (LIFO) stack.  It extends Vector, and so is a kind of List.

Object
    AbstractCollection (implements Collection)
        AbstractList (implements List)
            AbstractSequentialList
                LinkedList
            ArrayList
            Vector
                Stack
        AbstractSet (implements Set)
            HashSet
            TreeSet (implements SortedSet)
    AbstractMap (implements Map)
        HashMap
        TreeMap (implements SortedMap)
        WeakHashMap
    Dictionary
        Hashtable (implements Map)
            Properties

Stack adds the following methods beyond what its super-classes provide:

package java.util;

public class Stack extends Vector 
{
    // Constructs an empty stack
    public Stack() {}
    // Pushes an item onto the stack
    public Object push(Object item) {...)
    // Pops the top item off the stack
    public synchronized Object pop() {...}
    // Peeks at the top item on the stack, without removing it.
    public synchronized Object peek() {...}
    // Returns whether the stack is empty.
    public boolean empty() {...}
    // Returns the 1-based position where an object is on the stack.
    public synchronized int search(Object o) {...}
}

Here's an example of the use of a Stack:

package examples;

import java.util.Stack;

public class StackExample
{
  public static void main(String[] args)
  {
    Stack stack = new Stack();
    stack.push("Mary");
    stack.push("Frank");
    stack.push("Joe");
    stack.push("Sylvia");
    stack.push("Vanessa");

    while ( !stack.empty() )
    {
      String name = (String) stack.pop(); // Note required cast
      System.out.println(name);
    }
  }
}

This program outputs the following:

Vanessa
Sylvia
Joe
Frank
Mary

which is in the exact reverse order of the elements pushed onto the stack.

Note that I've used the empty method, which is in the JDK 1.1 version of Stack.  If I had used the isEmpty method, this would have restricted the class to working only under JDK1.2 and beyond.

This page was last modified on 02 October, 2007