Stack.java

public class Stack {
    int size = 20;
    Integer[] stack = new Integer[size];
    int top = -1;

    public static void main(String[] args){
        Stack stack = new Stack();

        System.out.println("Pushing 10...");
        stack.Push(10);

        System.out.println("Pushing 20...");
        stack.Push(20);

        System.out.println("Peeking top...");
        Integer peeked = stack.Peek();
        System.out.println("Top element is: " + peeked);

        System.out.println("Popping...");
        stack.Pop();

        System.out.println("Peeking again...");
        peeked = stack.Peek();
        System.out.println("Top element is now: " + peeked);

        System.out.println("Popping again...");
        stack.Pop();

        System.out.println("Attempting one more pop (should trigger underflow)...");
        stack.Pop();

    }
    private void Push(int data){
        if(top >= size - 1){
            System.out.println("OverFlow error. You cannot Push into a full stack");
            return;
        }
        top++;
        stack[top] = data;
    }
    private void Pop(){
        if(top <= -1){
            System.out.println("UnderFlow Error. You cannot Pop from an empty list");
            return;
        }
        System.out.println(stack[top] + " has been removed from the stack");
        stack[top] = null;
        top--;
    }
    private Integer Peek(){
        if(top <= -1){
            System.out.println("Stack is empty");
            return null;
        }
        return stack[top];
    }
}