If your constructor is similar to this one:
public Example(String example) {
if (example == null) {
throw new NullPointerException();
}
//do fun things with valid example here
}
Then, when you run this JUnit test you will get a green bar:
@Test(expected = NullPointerException.class)
public void constructorShouldThrowNullPointerException() {
Example example = new Example(null);
}