Unit tests for constructors

Question

If I make a class, that in the constructor accepts an array or collection, but this array or collection is optional (the parameter can be null), and then I store that collection in the object in question, in the constructor i need something like:

this.coll = coll == null ? null : coll.clone();

Should I unit-test such kind of code? like check if it works when passing null or not?

Answer 1

Technically, what you should test depends on your test goals. But in general, you should try to test everything that can go wrong. Here:

this.coll = coll == null ? null : coll.clone();

• You might have forgotten to use the coll parameter, i.e. the above statement is missing. This can be checked by a test that depends on the coll provided to the constructor.

• You might have forgotten to clone the parameter:

  this.coll = coll;
  

  This can be checked by a test that modifies the parameter coll and compares the result with the object-owned coll. They should be different after the modification.

• You might have forgotten to handle the null case:

  this.coll = coll.clone();
  

  This can be checked by a test that omits this parameter/provides a null value. Usually we'd expect an exception, here a null is OK.

• Another thing that could go wrong is that you create a null object or default instance when a null is encountered, e.g.:

  this.coll = coll == null ? new Collection() : coll.clone();
  

  If your code is correct, you want to ensure that an actual null here, not a default instance.

That's already four test cases that would be sensible for this simple line of code. Using a code coverage tool can help to detect uncovered cases in your code, in particular if you also look at branch coverage. Some tools have problems with expression-level control-flow (?:, &&, ||) so it's better (and more readable for humans, too!) to use statement-level conditionals:

if (coll != null) {
    coll = coll.clone();
}
this.coll = coll;

Answer 2

If it should throw an exception when you pass the constructor a null it's good to have a test that only turns green when passing null produces an exception.

Pretty much any behavior you expect out of a class is a candidate for unit testing. This is true for constructors as well.