Can Java's NullPointerException be changed to report on which variable was null? [duplicate]

Answer 1

No. NullPointerException does not always have to be caused by a variable/identifier being null. E.g. throw new NullPointerException() and throw null. Though it is often the case, it is not strictly required.

In most cases, it is fairly obvious what has caused the NPE. If not, then you may have too much going on in one line of code.

Consider this use case:

foo.doWork(bar1, bar2, bar3);

Here it is obvious that foo is `null.

Another case:

foo.doWork(bar.get(), bar2.get())

Here it could be foo, bar, or bar2.

The point is, that armed with this information and a breakpoint, it should be obvious what was null. If worse comes to worse, a static code analyzer like FindBugs could also give you some hints.

Answer 2

No, the debug information in the class file does not contain enough information to allow this.

You can, however, improve on the situation. There are two things that can cause a NPE to be thrown:

• A . dereferencing a variable, like foo.bar().
• A [ indexing an array, like args[0].

If you write your code so there is only one of these on a given code line, there is simply no doubt about which one caused the NPE. It will introduce a lot of temporary variables but then you have more information readily available when debugging.