How to write Junit for Interface default methods

Question

Please help in writing Junit for the interface default method.

public interface ABC {
    default List getSrc(DEF def, XYZ xyz) t         


        

Answer 1

If you're using Mockito, the simplest way to unit-test a default (AKA "defender") method is to make a spy¹ using the interface class literal². The default method can then be invoked on the returned spy instance as normal. The following example demonstrates:

import org.junit.Test;

import static org.junit.Assert.assertFalse;
import static org.junit.Assert.assertTrue;
import static org.mockito.Mockito.spy;

interface OddInterface {
    // does not need any unit tests because there is no default implementation
    boolean doSomethingOdd(int i);

    // this should have unit tests because it has a default implementation
    default boolean isOdd(int i) {
        return i % 2 == 1;
    }
}

public class OddInterfaceTest {
    OddInterface cut = spy(OddInterface.class);

    @Test
    public void two_IS_NOT_odd() {
        assertFalse(cut.isOdd(2));
    }

    @Test
    public void three_IS_odd() {
        assertTrue(cut.isOdd(3));
    }
}

^{(tested with Java 8 and mockito-2.24.5)}

^{1People often warn using a spy can be indicative of a code or test smell, but testing a default method is a perfect example of when using a spy is a good idea.}

^{2As of the time of this writing (2019), the signature of spy which accepts a class literal is marked as @Incubating, but has been around since mockito-1.10.12 which was released in 2014. Furthermore, support for default methods in Mockito has been around since mockito-2.1.0 which was released in 2016. It seems like a safe bet that this method will continue to work in future versions of Mockito.}