Re: NullPointerException, IllegalArgumentException, or AssertionError for null constructor argument

Lew <>
Fri, 28 Dec 2007 22:15:22 -0500
Owen Jacobson wrote:

On Dec 28, 2:11 pm, Daniel Pitts
<> wrote:

I have a constructor that takes a String argument. I'd like to throw an
exception if the constructor is invoked with a null argument, but I'm
not sure which instruction I should use.
NullPointerException is technically accurate, since it is a null
pointer, but it is also an IllegalArgumentException. I think that
IllegalArgumentException is more specific, so I'll probably go with
that, but wanted opinions.

The third option is AssertionError. I could just use assert arg!=null,
and that could be enough. This is for a personal project, so it doesn't
  *really* matter, but at the same time its good practice for me to
think about these sort of things :-)


Thoughts: private members get assertions if anything at all for
parameter checking (as well as other invariants); public members get
explicit tests if it matters that parameters meet specific
requirements. I tend to leave NPE to the java runtime to throw and
use IllegalArgumentException for things caught in explicit tests.

Asserts can, after all, be turned off.

Throwing AssertionError explicitly seems like the Wrong Thing. :)

It is. It is completely contrary to the purpose of the 'assert' mechanism,
and it's an Error, not an Exception. The 'assert' mechanism is not meant for
runtime checks of data, but for test-time checks of algorithmic correctness.

The great third phase of programming I don't see mentioned much is test-time.
  Everyone talks about compile-time and run-time, but 'assert' is a leader in
test-time functionality.

Again, its purpose is to check the algorithm, not the data.


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