A good way to debug nullPointerException [closed]

Question

I am using eclipse and programing in java. Sometimes I come across a nullPointerException that will plague me for hours. Is there anyway to debug nullPointerExceptions better and figure out what variable values are and other things that would cause null Pointer Exceptions.

Answer 1

Look at the stack trace and read the line number where the NullPointerException is thrown. Almost always, the line has some method invocation like

x.getValue() 

If x is null, you get a NullPointerException. If the method at the top of the stack trace is not your code, trace the stack all the way back until you do reach your code. Very often in this case, you are passing null to a method that doesn't like null parameters. Find it and fix it.

Also, very often when you encounter a method that is not yours that is throwing a NullPointerException, read the documentation. For example, look at String.replace(CharSequence target, CharSequence replacement):

Throws:

NullPointerException - if target or replacement is null.

It doesn't get much clearer than that!

Here's an example:

Looking at line 4, we see foo.bar(). This implies that foo is null. That one is easy. Let's look at another example:

Tracing back to your code, we see that s.replace(target, replacement) is throwing. We should inspect target and replacement. Let's attach a debugger:

Aha! replacement is null. You can do the same thing in Eclipse. Set a breakpoint for when an exception is thrown, and use it to inspect the parameters to your method. I'm primitive here because I come from a school of thought where I genuinely believe everyone would be better off if they learned to do it the hard way first. Leaky abstractions and all that.