Program 21.3: A generic Queue class

Stacks are last-in, first-out, LIFO for short. Another common data structure is the queue. The queue is like a stack except that elements are put in at the bottom of the list and come off the top of the list. Itıs first-in, first out or FIFO. There is no built-in queue data structure in java but itıs easy to implement one as a subclass of Vector. Program 21.3 is one such implementation.

import java.util.Vector;
import java.util.EmptyStackException;

public class Queue extends Vector {
  public void add(Object o) {

  public Object remove() {
    Object o  = peek();
    return o;


  public Object peek() {

    if (size() == 0) throw new EmptyStackException();
    return elementAt(0);

  public boolean empty() {
    return size() == 0;


Copyright 1996 Elliotte Rusty Harold
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