stack implementation using linked list in java

 stack implementation using linked list in java

public class linkstack {
    class Node{
        int data;
        Node next;
        Node(int data){
            this.data=data;
            this.next=null;
        }
    }
    Node head=null;
    int size;
    public void push(int data){
        Node newnode=new Node(data);
        size++;
        if (head==null){
            head=newnode;
        }
        else{
            newnode.next=head;
            head=newnode;
        }
    }
    public void pop(){
        if (head==null){
            System.out.println("underflow");
        }
        else{
            size--;
            head=head.next;
        }
    }
    public void display(){
        Node temp=head;
        if (temp==null){
            System.out.println("empty");
        }
        while(temp!=null){
            System.out.println(temp.data);
            temp=temp.next;
        }
    }
    public int size(){
        return size;
    }
    public void peek(){
        System.out.println(head.data);
    }
    public static void main(String[] args) {
        linkstack lss=new linkstack();
        lss.push(10);
        lss.push(20);
        lss.push(30);
        lss.display();
        System.out.println(lss.size);
        lss.peek();
        System.out.println("**********");
        lss.pop();
        lss.display();
        System.out.println(lss.size);
        lss.peek();
    }
}