import java.util.*;

public class CollectionStackQueue {

public static void main(String[] args) {

Stack<String> stack = new Stack<>();

stack.push("A");

stack.push("B");

stack.push("C");

stack.push("D");

System.out.println(stack);

String out = stack.pop();

System.out.println(out);

String last = stack.peek();

System.out.println(last);

System.out.println(stack);

Queue<String> queue = new LinkedList<>();

queue.offer("A");

queue.offer("B");

queue.offer("C");

queue.offer("D");

System.out.println(queue);

String outQ = queue.poll();

System.out.println(outQ);

String frontQ = queue.peek();

System.out.println(frontQ);

System.out.println(queue);

Set<String> set = new HashSet<>();

set.add(null);

System.out.println(set);

}

}