Question

I have 2 java program located seperately
One in c:\test and the other in c:\test\new

I can compile both of it without any error \javac

But when i try to execute the file \java
it shows the error like this

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at ButtonFrame.makeButton(ButtonTest3.java:42)
    at ButtonFrame.<init>(ButtonTest3.java:29)
    at ButtonTest$1.run(ButtonTest.java:17)
    at java.awt.event.InvocationEvent.dispatch(Unknown Source)
    at java.awt.EventQueue.dispatchEvent(Unknown Source)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
    at java.awt.EventDispatchThread.run(Unknown Source)

i put this in my classpath

CLASSPATH value- C:\test;C:\test\new

but if i change the order of the value in CLASSPATH to this

CLASSPATH value- C:\test\new;C:\test

the error is simply gone

Why?? this could happening
Only the order matters?

Answer 1

You’ve a class with the same name in the both folders. In C:\test there’s a version of the ButtonTest3 class which contains a programming bug causing this NullPointerException. In C:\test\new there’s a different version of the ButtonTest3 class which doesn’t contain this bug, or probably there’s a ButtonTest class which does entirely different things than the one in C:\test.

Cleanup your classpath. It’s not good to have duplicate different versioned classes with the same signature in the classpath. If your intent is that new is supposed to be a package identifier, then you need to leave it away from the classpath. However, such a package name would have resulted in a compilation error, so that can’t be it.

---

As to the bug, a NullPointerException is relatively trivial to naildown and fix. First look at the first line of the stacktrace:

at ButtonFrame.makeButton(ButtonTest3.java:42)

It’s telling that it has occurred in line 42 of ButtonTest3 class, inside the makeButton() method. Now go to line 42 of ButtonTest3.java, it’ll look something like:

someObject.doSomething();

Look there where a dot operator . is been used to invoke a method or access a field of some object. The NullPointerException means that someObject is null at the particular moment. There is no instance!

It’s an easy fix: just ensure that it is not null at the moment you’re invoking/accessing it:

someObject = new SomeObject();
// ...
someObject.doSomething();