Question

I am new to Java and getting an error for the following case:

markerObj = null;
markerObj.sections = RowData[1];

But, after removing the first line, it works fine.
What is the issue?

Answer 1

Variable names are only references. They point to a space in memory (in the heap) which contains an object.

When you do MyObj myObject = new MyObj();, it does two things: create the object in memory, and point the myObject reference to it.
So when you do myObject.sections, it follows the reference, and check the sections part of your object in memory.

Then when you do myObject = null, you basically destroy the link between your reference and your object in memory. Hence when doing myObject.sections, Java cannot follow the reference anymore because it’s not pointing to anything. So it throws a NullPointerException

Note that the object itself is not destroyed by myObject = null, only the reference is nullified. The Garbage Collector then detects that your object is unreachable (if there’s no other reference to it) and reclaims the memory, destroying the object.