Queue By Using Stack

Question

I have implemented a queue by using a stack data structure. How could I improve it?

package _amazonAskedQuestions.dataStructures;
import java.util.Iterator;
import java.util.Stack;
public class Queue_Stack<T> implements Iterable<T>{
 Stack<T> stack = new Stack<>();
 public void enqueue(T item){
 Stack<Integer> temp = new Stack<Integer>();
 while(!stack.empty()){
 temp.push((Integer) stack.pop());
 }
 stack.push(item);
 while(!temp.empty()){
 stack.push((T) temp.pop());
 }
 }
 public T dequeue(){
 return stack.pop();
 }
 @Override
 public Iterator<T> iterator() {
 return (Iterator<T>) stack.lastElement();
 }
 public static void main(String args[]){
 Queue_Stack<Integer> qs = new Queue_Stack<>();
 qs.enqueue(9);
 qs.enqueue(8);
 qs.enqueue(1);
 qs.enqueue(3);
 qs.enqueue(4);
 qs.enqueue(5);
 System.out.println( qs.dequeue());
 }
}

Answer 1

Your Queue_Stack only works with Integer. I tried using String and got a ClassCastException.

I made some modifications to your code. I added a test to make sure you don't get an error if you dequeue more than you enqueue. I used the underlying iterator to get rid of the temp Stack.

import java.util.Iterator;
import java.util.Optional;
import java.util.Stack;
public class Queue_Stack<T> implements Iterable<T> {
 Stack<T> stack = new Stack<>();
 public void enqueue(T item) {
 stack.push(item);
 }
 public Optional<T> dequeue() {
 if (!stack.empty()) {
 T item = stack.iterator().next();
 stack.remove(0);
 return Optional.of(item);
 } else {
 return Optional.empty();
 }
 }
 @Override
 public Iterator<T> iterator() {
 return (Iterator<T>) stack.iterator();
 }
 public static void main(String args[]) {
 Queue_Stack<String> qs = new Queue_Stack<>();
 qs.enqueue("zeta");
 qs.enqueue("alpha");
 qs.enqueue("beta");
 qs.enqueue("gamma");
 Iterator<String> iter = qs.iterator();
 while (iter.hasNext()) {
 System.out.println(iter.next());
 }
 System.out.println(qs.dequeue());
 System.out.println(qs.dequeue());
 System.out.println(qs.dequeue());
 System.out.println(qs.dequeue());
 System.out.println(qs.dequeue());
 }
}

Answer 2

Gilbert Le Blanc's answer already covered all the aspects of your question, I'm adding just one consideration about the Stack class from documentation:

A more complete and consistent set of LIFO stack operations is provided by the Deque interface and its implementations, which should be used in preference to this class. 

So instead of a Stack instance stack in your code you should consider to use :

Deque<T> stack = new ArrayDeque<>();