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?

  • As an aside, I just wanted to point out that not every Collection supports Cloneable. And since you know how you want to access the collection, it might make sense to create a collection that matches your access pattern. What's more, you can usually write simpler code by mapping null into an empty collection. So: this.coll = (coll == null ? new ArrayList() : new ArrayList(coll)); Apr 17, 2020 at 15:20

2 Answers 2


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;
  • Hmmf, well buggy tools aside, this human has no issue looking at 'expression-level control-flow' when it's well written like this. The if however needs to make up it's mind regarding use of this for the assignments. Jan 28, 2017 at 16:49
  • @CandiedOrange I once had a code golfing phase but that has passed. Now I try to put as little information on each line as possible. I like ternaries for data flow, but control flow (as in this case) is better expressed on a statement level. The above conditional has made up it's mind, with coll referring to the parameter, not the instance member which is assigned on the next line.
    – amon
    Jan 28, 2017 at 16:59
  • I do not test for the case of missing assignment, because I would have to test for all... seven or so parameters this way, but I currently test every other of those cases mentioned in the answer. Jan 28, 2017 at 17:57
  • @MichałZegan That's OK, since the test for assignment is implicit to all the other tests. I tend to include it for clarity, but it's superfluous, strictly speaking. Testing for null and for cloning is the only important part here.
    – amon
    Jan 28, 2017 at 19:08
  • but most parameters of this constructor are neither collections not arrays, and I also do not test for assignment for them :) Jan 28, 2017 at 19:13

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.

  • you know, it is a case where null and not null is ok, and the only reason I need to test anything is this one, that you cannot just this.col=col, as this will share the underlying array or collection, and is dangerous, so it needs a conditional clone. Jan 28, 2017 at 17:56

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