Styjun

I created a stack data structure from scratch using a linked list. It seems to push() properly because I can use the display method using first.data, and it displays the list. But when I use peek() I get a NoSuchElementException because there is an if statement if (first == null), but first should not be null.

I am not sure how to tackle this differently.

public class StackNew implements Stack{ //Stack is an interface given by my professor

private int size;

 private class Node 
 public Object data;
 public Node next;
 public Node(Object data, Node next) 
 this.data = data;
 this.next = next;
 
 

private Node first = null;

public void push(Object newElement) 
 first = new Node(newElement, first);
 size++;


public Object peek()
 if (first == null) 
 throw new NoSuchElementException();
 
 return first.data;

public void display() 
 Node previous = null;
 while(first != null) 
 System.out.println((first.data));
 previous = first;
 first = first.next;
 

public static void main(String args[]) 
 StackNew stack = new StackNew();
 stack.push("java");
 stack.push(1);
 stack.push("code");

 stack.display();
 System.out.println(stack.isEmpty());
 stack.peek();

When I try to use the peek() method, the first node is null, when it should contain "code".

In your code you would notice that you are getting the error only when you call display method.

This is because in your implementation, you are overwriting the first to first.next which ultimately becomes null in the last iteration of the while loop. So you need a new pointer that will keep track of the current element and that will progress to the next element without impacting the first pointer.

So you need to correct the display method to:

 public void display() 
 Node current = first;
 while(current != null) 
 System.out.println((current.data));
 current = current.next;
 
 

Here a new pointer current will keep track of the current element and in the while loop will consecutively print the elements.

Some other issues in your code (if you are trying to extend java.util.Stack):

1. You are trying to implement the Stack class, but in fact you should extend it.


2. In order to use isEmpty() from the Stack class you need to increment the elementCount++; in the push() method instead of size. As isEmpty() checks elementCount inherited from the Stack class.


3. 
   

   The return type of push should not be void instead it should return the argument. (as per the Java doc of the Stack class).

   
   
   

   @Override
   public Object push(Object newElement) 
    first = new Node(newElement, first);
    elementCount++;
    return newElement;