What is null time exception error how to resolve it

Answer 1

I am guessing you mean null pointer exception?
If that is the case then it simply means you declared an object of a certain type and attempted to use the delcared variable before actually pointing it at a certain object.

Answer 2

Let me expand on Ousmane's answer by providing a code example.
Only declaring an object would look like the following:
Student s1;

Declaring and initializing an object (declaring and pointing it):
Student s1 = new Student();

Here's an article from Oracle on the subject (https://docs.oracle.com/javase/tutorial/java/javaOO/objectcreation.html)