IllegalArgumentException or NullPointerException for a null parameter? [closed]

Answer 1

The accepted practice if to use the IllegalArgumentException( String message ) to declare a parameter to be invalid and give as much detail as possible... So to say that a parameters was found to be null while exception non-null, you would do something like this:

if( variable == null )
    throw new IllegalArgumentException("The object 'variable' cannot be null");

You have virtually no reason to implicitly use the "NullPointerException". The NullPointerException is an exception thrown by the Java Virtual Machine when you try to execute code on null reference (Like toString()).

Answer 2

You should throw an IllegalArgumentException, as it will make it obvious to the programmer that he has done something invalid. Developers are so used to seeing NPE thrown by the VM, that any programmer would not immediately realize his error, and would start looking around randomly, or worse, blame your code for being 'buggy'.

Answer 3

If it's a setter method and null is being passed to it, I think it would make more sense to throw an IllegalArgumentException. A NullPointerException seems to make more sense in the case where you're attempting to actually use the null.

So, if you're using it and it's null, NullPointer. If it's being passed in and it's null, IllegalArgument.

Answer 4

As a subjective question this should be closed, but as it's still open:

This is part of the internal policy used at my previous place of employment and it worked really well. This is all from memory so I can't remember the exact wording. It's worth noting that they did not use checked exceptions, but that is beyond the scope of the question. The unchecked exceptions they did use fell into 3 main categories.

NullPointerException: Do not throw intentionally. NPEs are to be thrown only by the VM when dereferencing a null reference. All possible effort is to be made to ensure that these are never thrown. @Nullable and @NotNull should be used in conjunction with code analysis tools to find these errors.

IllegalArgumentException: Thrown when an argument to a function does not conform to the public documentation, such that the error can be identified and described in terms of the arguments passed in. The OP's situation would fall into this category.

IllegalStateException: Thrown when a function is called and its arguments are either unexpected at the time they are passed or incompatible with the state of the object the method is a member of.

For example, there were two internal versions of the IndexOutOfBoundsException used in things that had a length. One a sub-class of IllegalStateException, used if the index was larger than the length. The other a subclass of IllegalArgumentException, used if the index was negative. This was because you could add more items to the object and the argument would be valid, while a negative number is never valid.

As I said, this system works really well, and it took someone to explain why the distinction is there: "Depending on the type of error it is quite straightforward for you to figure out what to do. Even if you can't actually figure out what went wrong you can figure out where to catch that error and create additional debugging information."

NullPointerException: Handle the Null case or put in an assertion so that the NPE is not thrown. If you put in an assertion is just one of the other two types. If possible, continue debugging as if the assertion was there in the first place.

IllegalArgumentException: you have something wrong at your call site. If the values being passed in are from another function, find out why you are receiving an incorrect value. If you are passing in one of your arguments propagate the error checks up the call stack until you find the function that is not returning what you expect.

IllegalStateException: You have not called your functions in the correct order. If you are using one of your arguments, check them and throw an IllegalArgumentException describing the issue. You can then propagate the cheeks up against the stack until you find the issue.

Anyway, his point was that you can only copy the IllegalArgumentAssertions up the stack. There is no way for you to propagate the IllegalStateExceptions or NullPointerExceptions up the stack because they had something to do with your function.

Answer 5

NullPointerException thrown when attempting to access an object with a reference variable whose current value is null.

IllegalArgumentException thrown when a method receives an argument formatted differently than the method expects.

Answer 6

Couldn't agree more with what's being said. Fail early, fail fast. Pretty good Exception mantra.

The question about which Exception to throw is mostly a matter of personal taste. In my mind IllegalArgumentException seems more specific than using a NPE since it's telling me that the problem was with an argument I passed to the method and not with a value that may have been generated while performing the method.

My 2 Cents