Re: Question re testing constructor

Daniel Pitts <>
Tue, 06 Dec 2011 09:49:21 -0800
On 12/6/11 7:14 AM, Novice wrote:

I'm writing JUnit test cases for a constructor but have hit a bit of a
snag. I hope someone here can help.

I've satisfied myself via googling that it makes sense to test constructors
if they do something that might fail or if they can throw exceptions. The
constructor in question can throw exceptions so I'm trying to write some
test cases for it. I've got cases that cause each of the exceptions to be
thrown but I'm having a bit of a problem with what seemed like the easiest
test of all: existence.

It seems to me - correct me if I'm wrong - that I can easily test to make
sure the constructor actually built SOMETHING by testing the object against
null. That won't prove it created exactly the right object but it will
prove that something got created. Testing the objects OTHER methods will
verify that the correct object got created.

If that is right, then it seems that this should test the existence of the
object well enough:

      Set<String> testValues = new HashSet<String>();
      for (String key : testValues) {
      HexColor hexColor = new HexColor(key);
      if (hexColor == null) {
      assertTrue("The HexColor has been created for input value,
" + key + ", but has been found to be null.", false);

Unfortunately, the assertTrue() statement gets flagged by the compiler as
being dead code. Am I right in assuming that it is essentially looking at
the instantiation of the HexColor class and reasoning that it will
inevitably create SOMETHING so that hexColor can't possibly be null,
therefore the assertTrue() can't ever be executed?

If so, I don't have a problem with that but it leaves me a bit baffled
about how to test that the constructor created something when I gave it
good input values.

Can someone enlighten me on a better way to test this aspect of the
constructor? Or can I simply assume that the constructor worked as long as
it didn't actually throw an exception and omit any existence tests?

Tests work best if you think about pre-conditions and post-conditions.

Test 1:
Pre condition: I pass these objects to a constructor...
Expected post conditions: I have an object with these state/behavior.

Test 2:
Pre condition: I pass these other objects to a constructor...
Expected post condition: Constructor throws NullPointerException.


So, if your constructor is non-trivial, you should be able to interact
with the object to verify the constructor did its job.

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