Stack - Data Structure using Java(Without using library)

Stack.java

/**
  * A implementation of Stack using Java Generic Programming
  * @author Al- Imran Ahmed
  */

public class Stack<T>{
  
  //Top node of the stack 
  private Node top = null;
  
  /**
    * InnerClass to represent the data elements as in stack
    */
  class Node{
    T item;
    Node next;
  }
  
  /**
    * Remove the top element of the stack and return the value of that element
    * @return T
    * public T pop(){ return null; } ;stub
    */ 
  public T pop(){
    if(isEmpty()){
      return null;
    }
    T item = top.item;
    top = top.next;
    return item;
  } 
  
  /**
    * Insert an element to the stack
    * @param T s
    * public void push(T s){ return null; }  ;sbub
    */  
  public void push(T s){
    Node oldtop = top;
    top = new Node();
    top.item = s;
    top.next = oldtop;
  }
 
  /**
    * Check whether the stack is empty or not
    * @return boolean
    * public boolean isEmpty(){ return false; } ;stub
    */ 
  public boolean isEmpty(){ 
    return top == null;
  }
} 

Client.java

/**
  * Client to Testing Queue.java
  * @author Al- Imran Ahmed
  * java Client < input.txt
  */

import java.util.Scanner;
public class StackClient{
  //Client method 
  public static void main(String[] CHAND){
    
    //Create an instance of stack 
    Stack<String> stack = new Stack<String>();
    Scanner input = new Scanner(System.in);
    String s;
    System.out.println("Output: ");
    while(input.hasNext()){
      s = input.next();
      if(s.equals("-")){
        System.out.print(stack.pop()+" ");
      }else{
        stack.push(s);
      }
    }
    System.out.println();
  }
}