StackAL.java

import java.util.*;

public class StackAL {
  public static class Stack {
    static ArrayList<Integer> list = new ArrayList<>();

    public boolean isEmpty() {
      return list.size() == 0;
    }

    public void push(int data) {
      list.add(data);
    }

    public int pop() {
      if (isEmpty()) {
        return -1;
      }
      int top = list.get(list.size() - 1);
      list.remove(list.size() - 1);
      return top;
    }

    public int peek() {
      if (isEmpty()) {
        return -1;
      }

      return list.get(list.size() - 1);
    }

  }

  public static void main(String args[]) {
    Stack s = new Stack();
    s.push(1);
    s.push(2);
    s.push(3);
    s.push(4);

    while (!s.isEmpty()) {
      System.out.println(s.peek());
      s.pop();
    }
  }
}