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I am getting nullpointerexception in my code..don't know whats wrong in that code
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Java
I am getting Null pointer exception...don't know whats wrong in that code...how to resolve it.

What I have tried:

Java
import java.util.Scanner;

class Node
{
	private int data;
	private Node next;
	

	
	public Node(int d , Node n)
	{
		data = d;
		next = n;
	}
	
	public void setData(int d)
	{
		data = d;
	}
	
	public void setNext(Node n)
	{
		next = n;
	}
	
	public int getData()
	{
		return(data);
	}
	
	public Node getNext()
	{
		return(next);
	}
}

class Linkedlist
{
	private int size;
	private Node start;
	
	public Linkedlist()
	{
		size = 0;
		start = null;
	}
	
	public void insetAtfirst(int val)
	{
		Node n = new Node(val,start);
		
		n.setData(val);
		n.setNext(start);
		
		start = n;
		size++;
	}
	
	public void viewlist()
	{
		Node t;
		
		t = start;
		
		for(int i=1;i<=size;i++)
		{
			System.out.println(" " + t.getData());
			t = t.getNext();
		}
	}
	
	public void deleteAltNode()
	{
		
		Node t1;
		
		t1 = start;
		
		for(int i=1;i<=size;i++)
		{
			System.out.println(" " + t1.getData());
			t1 = t1.getNext().getNext();
			
		}
		
		
		size--;
		
	
		
		
	}
}



public class Delete_alternate_node 
{

	
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);

		Linkedlist l = new Linkedlist();
		
		System.out.println("Enter the no of node ");
		int k = sc.nextInt();
		
		for(int i=1;i<=k;i++)
		{
			System.out.println("Enter the elements for linkedlist");
			int val = sc.nextInt();
			l.insetAtfirst(val);
		}
		
		System.out.println();
		l.viewlist();
		
		System.out.println();
		l.deleteAltNode();
		
	
		
	}

}
Posted
Updated 21-Nov-18 11:26am
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3 solutions

The logic of the deleteAltNode method is wrong. Try
Java
import java.util.Scanner;

class Node
{
  private int data;
  private Node next;

  public Node(int d , Node n)
  {
    data = d;
    next = n;
  }

  public void setData(int d)
  {
    data = d;
  }

  public void setNext(Node n)
  {
    next = n;
  }

  public int getData()
  {
    return(data);
  }

  public Node getNext()
  {
    return(next);
  }
}

class Linkedlist
{
  private int size;
  private Node start;

  public Linkedlist()
  {
    size = 0;
    start = null;
  }

  public void insertAtfirst(int val)
  {
    Node n = new Node(val,start);

    n.setData(val);
    n.setNext(start);

    start = n;
    size++;
  }

  public void viewlist()
  {
    Node t;

    t = start;

    while ( t != null )
    {
      System.out.print(" " + t.getData());
      t = t.getNext();
    }
    System.out.println();
  }

  public void deleteAltNode()
  {
    Node node;
    node = start;
    while ( node != null )
    {
      Node next = node.getNext();
      if ( next != null)
      {
        node.setNext (next.getNext());
        --size;
      }
      node = node.getNext();
    }
  }
}

public class Delete_alternate_node
{
  public static void main(String[] args)
  {
    Scanner sc = new Scanner(System.in);

    Linkedlist l = new Linkedlist();

    System.out.println("Enter the no of node ");
    int k = sc.nextInt();

    for(int i=1;i<=k;i++)
    {
      System.out.println("Enter the elements for linkedlist");
      int val = sc.nextInt();
      l.insertAtfirst(val);
    }
    System.out.print("list before alternate nodes deletion: ");
    l.viewlist();

    System.out.print("list after alternate nodes deletion: ");
    l.deleteAltNode();

    l.viewlist();
  }
}
 
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Posted
Comments
Akash Tawade 22-Nov-18 7:54am    
Perfect solution Thank you very much @CPallini
CPallini 22-Nov-18 8:37am    
You are welcome.
This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and your IDE will help you here. Run your program in the debugger and when it fails, the debugger will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, execution will stop before the error, and let you examine what is going on by stepping through the code looking at your values. Exactly how you use the debugger will depend on the IDE you are using, but a quick google for the name of your IDE plus "debugger tutorial" should get you the info you need.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!
 
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Posted
Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

http://docs.oracle.com/javase/7/docs/technotes/tools/windows/jdb.html[^]
https://www.jetbrains.com/idea/help/debugging-your-first-java-application.html[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.
 
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