Is there a bug in java.util.Stack's Iterator?

Question

Today I was trying to push in java.util.Stack class and then use the Iterator to iterate (without using pop) through the items. I was expecting LIFO property but got surprised.

Here is the code that I was trying.

import java.util.*;
import java.util.Stack;

public class Main {
    public static void main(String[] args) {
        RobStack<Integer> rstack = new RobStack<Integer>(); // Correct Implementation
        Stack<Integer> jstack = new Stack<Integer>(); // Default Java Implementation
        rstack.push(0); jstack.push(0);
        rstack.push(1); jstack.push(1);
        rstack.push(2); jstack.push(2);
        rstack.push(3); jstack.push(3);

        System.out.print("Algo Stack: ");
        for (int i : rstack)
            System.out.print(i + " ");
        System.out.print("\nJava Stack: ");
        for (int i : jstack)
            System.out.print(i + " ");
    }

}

The output the above program is given below:

Algo Stack: 3 2 1 0 
Java Stack: 0 1 2 3 

In the above code jstack uses the default Java implementation and rstack uses the implementation provided by Robert Sedgewick for his Algorithm class. I found that Prof. Robert's implementation works fine but the java.util.Stack implementation fails.

Is it a bug or is it by design?

Answer 1

See Bug ID 4475301 : RFE: java.util.Stack.iterator() iterates the wrong way. This behavior is by (bad) design. Java's built-in Stack iterator methods are inherited from other classes, so they don't behave as you'd expect.