Unable to understand NullPointerException in Java program

I am a beginner in Java and I am facing a NullPointerException in my program. I am not able to understand why this error is happening.
My Goal:
I want to create a simple Student object and print the student name.
Problem:
When I run the program, I get the following error:
Exception in thread "main" java.lang.NullPointerException
   at Main.main(Main.java:6)
My Code:
class Student {    String name;    void display() {        System.out.println(name.length());    } }
public class Main {    public static void main(String[] args) {        Student s = new Student();        s.display();    } }

Work the problem backwards. Main() calls s.display(), has 's' been initialized? yes. So then look  in the display() method itself, it calls name.length(), has 'name' been initialized?

I have the concept that every class should have fields, methods, and constructors (well, at least one of each of those). I think you wouldn't have this problem if  you had done that.

Campbell Ritchie wrote:I have the concept that every class should have fields, methods, and constructors (well, at least one of each of those).

Plenty of cases where that isn't so. But while there are cases where a class might define none of those, typically a class should define at least one.

You should also look more carefully at the exact error message.  If you're using a modern version of Java, you should see something like this:
Exception in thread "main" java.lang.NullPointerException: Cannot invoke "String.length()" because "this.name" is null at Student.display(Student.java:8) at Main.main(Main.java:8)
In particular: Cannot invoke "String.length()" because "this.name" is null.  That's pretty specific, and helpful.

Mike Simmons wrote:. . . the exact error message. . . .

I tried it and got more or less the same on JShell (Java25):-jshell> Main.main(); |  Exception java.lang.NullPointerException: Cannot invoke "String.length()" because "this.name" is null |        at Student.display (#1:5) |        at Main.main (#2:5) |        at (#3:1) I hadn't noticed that error message before, and agree, it is much more helpful than the error messages I used to see. The line numbers, which have always been in the stack trace, are also helpful. They don't tell you where the error occurred, but they do tell you where it was detected.

The more informative NullPointerException messages came as part of JDK 14: JEP 358: Helpful NullPointerExceptions

A NullPointerException occurs here because while you created the Student object, the name variable inside it is still empty (null), and you cannot call .length() on something that doesn't exist.

The term "null pointer exception" is a bit of a misnomer in Java. Java does not have pointers, only object references. The term "null pointer" comes from Java's origins in C, which were discarded while the language was still being designed.

A more accurate term would be "NullReferenceException" or "NullObjectException".

Or, like in C#, “NullMarkerException”. But the name was created so long ago that we are stuck with it.

Campbell Ritchie wrote:Or, like in C#, “NullMarkerException”. But the name was created so long ago that we are stuck with it.

C# did not exist when Java was created. The whole "#" language environment is, in fact, largely in response to the fact that Microsoft couldn't hijack Java via J++.

A more direct connection comes from C++, which made object references a key part of the ecosystem. But since object references in C++ are basically a shortcut to pointers, a null object reference in C++ is literally a null pointer value.