Writing a single unit test for multiple implementations of an interface

Question

I have an interface List whose implementations include Singly Linked List, Doubly, Circular etc. The unit tests I wrote for Singly should do good for most of Do

Answer 1

I'd probably avoid JUnit's parameterized tests (which IMHO are pretty clumsily implemented), and just make an abstract List test class which could be inherited by tests implementations:

public abstract class ListTestBase<T extends List> {

    private T instance;

    protected abstract T createInstance();

    @Before 
    public void setUp() {
        instance = createInstance();
    }

    @Test
    public void testOneThing(){ /* ... */ }

    @Test
    public void testAnotherThing(){ /* ... */ }

}

The different implementations then get their own concrete classes:

class SinglyLinkedListTest extends ListTestBase<SinglyLinkedList> {

    @Override
    protected SinglyLinkedList createInstance(){ 
        return new SinglyLinkedList(); 
    }

}

class DoublyLinkedListTest extends ListTestBase<DoublyLinkedList> {

    @Override
    protected DoublyLinkedList createInstance(){ 
        return new DoublyLinkedList(); 
    }

}

The nice thing about doing it this way (instead of making one test class which tests all implementations) is that if there are some specific corner cases you'd like to test with one implementation, you can just add more tests to the specific test subclass.