How to add test coverage to a private constructor?

Question

This is the code:

package com.XXX;
public final class Foo {
  private Foo() {
    // intentionally empty
  }
  public static int bar() {
    return 1;
  }
}

This is the test:

package com.XXX;
public FooTest {
  @Test 
  void testValidatesThatBarWorks() {
    int result = Foo.bar();
    assertEquals(1, result);
  }
  @Test(expected = java.lang.IllegalAccessException.class)
  void testValidatesThatClassFooIsNotInstantiable() {
    Class cls = Class.forName("com.XXX.Foo");
    cls.newInstance(); // exception here
  }
}

Works fine, the class is tested. But Cobertura says that there is zero code coverage of the private constructor of the class. How can we add test coverage to such a private constructor?

Answer 1

I don't entirely agree with Jon Skeet. I think that if you can get an easy win to give you coverage and eliminate the noise in your coverage report, then you should do it. Either tell your coverage tool to ignore the constructor, or put the idealism aside and write the following test and be done with it:

@Test
public void testConstructorIsPrivate() throws NoSuchMethodException, IllegalAccessException, InvocationTargetException, InstantiationException {
  Constructor<Foo> constructor = Foo.class.getDeclaredConstructor();
  assertTrue(Modifier.isPrivate(constructor.getModifiers()));
  constructor.setAccessible(true);
  constructor.newInstance();
}