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Java
import java.util.*;
public class StudentMain
{
    public static void main(String args[])
    {
        Sll ab= new Sll();
        ab.create();
        ab.display();

    }
}
class Student
{
    String name;
    int roll;
    double marks;
    void setStudentDetails()
    {
        Scanner sc=new Scanner (System.in);
        System.out.println("Enter the name of Student");
        name=sc.nextLine();
        System.out.println("Enter the roll number");
        roll=sc.nextInt();
        System.out.println("Enter the marks" );
        marks=sc.nextDouble();
    }

    void displayStudent()
    {
        System.out.println("******************************");
        System.out.println("Name : - " + name);
        System.out.println("Roll : - " + roll);
        System.out.println("Marks :- " + marks);
        System.out.println("******************************");
    }
}
class Nodess
{
    Student stud;
    Nodess link;
}
class Sll
{
    Nodess start=new Nodess();
    Scanner sc=new Scanner(System.in);
    
    void create()
    {
        Nodess obj=new Nodess();
        obj.stud.setStudentDetails();
        obj.link=null;

        if(start==null)
        {
            start=obj;
        }
        else
        {
            Nodess temp=new Nodess();
            temp=start;
            while(temp.link!=null)
            {
                temp=temp.link;
            }
            temp.link=obj;
        }
    }

    void display()
    {
        Nodess temp = new Nodess();
        temp = start;
        if(temp!=null)
        {
            System.out.println("The Linked List Is :");
            while(temp!=null)
            {
                temp.stud.displayStudent();
                temp=temp.link;
            }
            System.out.println();
        }
    }
}

I am getting the error
java.lang.NullPointerException
	at Sll.create(StudentMain.java:50)
	at StudentMain.main(StudentMain.java:7)


What I have tried:

Tried to change the code. Checked instance variables . I please request someone to help me out. This is creating a lot of anxiety

Error is in the Class Student
Posted
Updated 5-Jul-21 1:25am
v2

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 yesterday's 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 the debugger will help you here. Run your program in the debugger and when it fails, it 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, the debugger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

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!
   
You are accessing a field before you have assigned a value to it.
Java
Nodess obj = new Nodess();     // Here, obj.stud has not been initialized, and will be null.
obj.stud.setStudentDetails();  // Boom!
You need to assign a value to it before accessing it:
Java
Nodess obj = new Nodess();
obj.stud = new Student();
obj.stud.setStudentDetails();
   
Comments
CPallini 5-Jul-21 7:36am
   
5.

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