Is it okay to throw NullPointerException programmatically?

java exception-handling nullpointerexception throw
stratwine picture stratwine · Jul 23, 2010 · Viewed 82.9k times · Source

When there is a post-condition, that return value of a method must not be null, what can be done?

I could do

assert returnValue != null : "Not acceptable null value";

but assertions could be turned off!

So is it okay to do

if(returnValue==null)
      {
           throw new NullPointerException("return value is null at method AAA");
      }

?

Or is it better to use a user-defined exception (like NullReturnValueException ) for such a condition?

Answer

Roman picture Roman · Jul 24, 2010

I would recommend you never throw NullPointerException by yourself.

The main reason not to do this, as Thorbjørn Ravn Andersen says in a comment below, is that you don't wan't to mix 'real, bad NPEs' with NPEs thrown intentionally.

So, until you're confident that you're able to recognize 'valid' NPE, I'd recommend to use IllegalArgumentException when you want to tell to your API user that null is not a valid argument value. Your method's behavior when illegal null-parameter passed should be documented.

Another (more modern imho) option is to use @NotNull annotation near the argument. Here is an article about using @NotNull annotation.

As I mentioned earlier, there can also be cases, when throwing NPE will not be confusing either to you or to your teammates: NPE cause should be clear and recognizable.

For instance, if you use some library with preconditions module, like Guava, then I find using checkNotNull()-like methods is a preferable way to deal with illegally-passed nulls.

checkNotNull(arg, msg) throws NPE, but from the stacktrace it's quite clear, that it was produced by Preconditions.checkNotNull() and thus it's not an unknown bug but rather expected behavior.

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