How to write junit tests for interfaces?

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Contrary to the much-voted-up answer that @dlev gave, it can sometimes be very useful/needful to write a test like you’re suggesting. The public API of a class, as expressed through its interface, is the most important thing to test. That being said, I would use neither of the approaches you mentioned, but a Parameterized test instead, where the parameters are the implementations to be tested:

@RunWith(Parameterized.class)
public class InterfaceTesting {
    public MyInterface myInterface;

    public InterfaceTesting(MyInterface myInterface) {
        this.myInterface = myInterface;
    }

    @Test
    public final void testMyMethod_True() {
        assertTrue(myInterface.myMethod(true));
    }

    @Test
    public final void testMyMethod_False() {
        assertFalse(myInterface.myMethod(false));
    }

    @Parameterized.Parameters
    public static Collection<Object[]> instancesToTest() {
        return Arrays.asList(
                    new Object[]{new MyClass1()},
                    new Object[]{new MyClass2()}
        );
    }
}

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