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

As suggested in the answer, create implementation class for the interface and test it, for an example I modified getSrc method in your ABC interface, as below:

import java.util.ArrayList;
import java.util.List;

public interface ABC {

    default List getSrc(DEF def, XYZ xyz) {
        final List defaultList = new ArrayList<>();
        defaultList.add("default");
        defaultList.add("another-default");
        return defaultList;
    }
}

Created an implementation class for the same, optionally you can create another method calling super method and write @Test for both, as I does:

import java.util.List;

public class ABCImpl implements ABC {

    public List getSrcImpl(DEF def, XYZ xyz) {
        final List list = getSrc(def, xyz);
        list.add("implementation");
        return list;
    }
}

Corresponding Test class for the implementation is as follows:

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.empty;
import static org.hamcrest.Matchers.is;
import static org.hamcrest.Matchers.not;
import static org.hamcrest.Matchers.contains;
import java.util.Collection;
import java.util.List;

import org.junit.Before;
import org.junit.Test;

public class ABCImplTest {

    private ABCImpl abcImpl;

    @Before
    public void setup() {
        abcImpl = new ABCImpl();
    }

    @Test
    public void testGetSrc() throws Exception {
        List result = abcImpl.getSrc(new DEF(), new XYZ());
        assertThat((Collection) result, is(not(empty())));
        assertThat(result, contains("default", "another-default"));
    }


    @Test
    public void testABCImplGetSrc() throws Exception {
        List result = abcImpl.getSrcImpl(new DEF(), new XYZ());
        assertThat((Collection) result, is(not(empty())));
        assertThat(result, contains("default", "another-default", "implementation"));
    }
}