GenStack ArrayList

import java.util.*;
 
public class GenStackAL<T> {
    ArrayList<T> stack;
	GenStackAL(ArrayList<T> stack){
		this.stack=stack;
	}
	void push(T a){
        stack.add(a);
	}
	T pop(){
		if(stack.size()==0){
			return null;
		}
		else{
            T i= stack.remove(stack.size()-1);
			return i;
		}
		
	}
	void display(){
		if(stack.size()==0){
			System.out.println("Stack is empty");
		}
		else{
			for(int i=stack.size()-1;i>=0;i--){
				System.out.println(stack.get(i));
			}
		}
	}
	
	public static void main(String args[]){
		Scanner sc= new Scanner(System.in);
		ArrayList<Integer> a= new ArrayList<>();
		GenStackAL<Integer> s1= new GenStackAL<>(a);
		
		int op=0;
		Integer ele;
		System.out.println("1.push\n2.pop\n3.Display\n4.Exit");
		while(op!=4){
			
			System.out.print("Enter option: ");
			op=sc.nextInt();
			switch(op){
				case 1:
					System.out.println("enter element to push: ");
					ele = sc.nextInt();
					s1.push(ele);
					break;
				case 2:
					ele=s1.pop();
					if(ele==null){
						System.out.println("Stack is empty");
					}
					else{
						System.out.println("Poped element:"+ele);
					}
					break;
				case 3:
					s1.display();
					break;
				case 4:
					return;
				default :
					System.out.println("Enter valid option");
					
			}
		}
	}
}