JUnit: testing helper class with only static methods

There are several solutions:

1. You can add a public constructor and call it from a test. While it doesn't make sense, it also doesn't hurt (much).

2. Create a dummy static instance (you can call the private constructor here). Ugly but you can give the field a name to communicate your intent (JUST_TO_SILENCE_COBERTURA is a good name).

3. You can let your test extend the helper class. That will intrinsically call the default constructor but your helper class can't be final anymore.

I suggest the last approach especially because the class can't be final anymore. If a consumer of your code wants to add another helper method, they can now extend the existing class and receive one handle to get to all helper methods. This creates a coupling of the helper methods which communicates the intent (these belong together) - which is impossible if the helper class is final

If you want to prevent users to accidentally instantiate the helper class, make it abstract instead of using a hidden constructor.

No.

Unless you call the private constructor explicitly (which would be bad code) you won't be able to cover those lines.

Obtaining 100% coverage in all cases it's good, but there are some cases in which this is not possible. Of course if you have a class that is never instantiated, Cobertura will get this as a not complete test coverage, because those lines of code are actually in the class, but they're not tested.

Fact is you'll never call a private constructor (I'm assuming you've hidden the constructor by making it private), so I wouldn't bother. Test should be about getting what you're expecting, and while I agree 100% coverage is good, in some cases (like this) this isn't useful.

Have a look at 100% Code Coverage as well.

If you absolutely need to achieve 100% code coverage - the merits of that can be debated elsewhere :) - you can achieve it using reflection in your tests. As habit, when I implement a static-only utility class, I add in a private constructor to ensure that instances of the class can't be created. For example:

/** 
 * Constructs a new MyUtilities.
 * @throws InstantiationException
 */
private MyUtilities() throws InstantiationException
{
    throw new InstantiationException("Instances of this type are forbidden.");
}

Then your test might look something like this:

@Test
public void Test_Constructor_Throws_Exception() throws IllegalAccessException, InstantiationException {
    final Class<?> cls = MyUtilties.class;
    final Constructor<?> c = cls.getDeclaredConstructors()[0];
    c.setAccessible(true);

    Throwable targetException = null;
    try {
        c.newInstance((Object[])null);
    } catch (InvocationTargetException ite) {
        targetException = ite.getTargetException();
    }

    assertNotNull(targetException);
    assertEquals(targetException.getClass(), InstantiationException.class);
}

Basically, what you're doing here is getting the class by name, finding the constructors on that class type, setting it to public (the setAccessible call), calling the constructor with no arguments, and then ensuring that the target exception that is thrown is an InstantiationException.

Anyway, as you said, the 100% code coverage requirement here is kind of a pain, but it sounds like it's out of your hands, so there's little you can do about it. I've actually used approaches similar to the above in my own code, and I did find it beneficial, but not from a testing perspective. Rather, it just helped me learn a little bit more about reflection than I knew before :)