The ArrayDeque Class in Java

import java.util.*;

public class ArrayDQ {
    public static void main(String[] args) {
        ArrayDeque<String> ad = new ArrayDeque<String>();
        ad.push("a");
        ad.push("b");
        ad.push("c");
        ad.push("d");
        while (ad.peek() != null) {
            System.out.print(ad.pop() + " ");
        }
        System.out.println();
    }
}