Test callback logic

Suppose we need to write unit tests for the Service class. The Dao class interacts with database, so it needs to be mocked.

class Service{
    
    Dao dao;
    
    Service(Dao dao){
        this.dao = dao;
    }
    
    public void doStuff(String s) {
        Callback<String> callback = i -> {
            System.out.println("Hello "+i);  
        };
        dao.doStuff(s, callback);
    }
    
}

class Dao{
    public void doStuff(String s, Callback<String> callback) {
        callback.apply(s);
        //Some DB operation
    }
    
}

interface Callback<T>{
    void apply(T t);
}

The question is how to test the callback logic (the System.out.println() line). If we simply write test case as below, it doesn't print out 'Hello John'.

@ExtendWith(MockitoExtension.class)
@RunWith(JUnitPlatform.class)
public class CallbackTest {

    @Mock Dao dao;
    
    @Test
    void test() {
        Service service = new Service(dao);
        service.doStuff("John");
    }

}

Well, we can mock dao.doStuff() by using doAnswer() method. It allows us to define the method body. What we do here is only invoking callback.apply(s) while skipping the database operations.

@Test
void test() {
    Service service = new Service(dao);
    doAnswer(i -> {
        Callback<String> callback = i.getArgument(1);
        callback.apply(i.getArgument(0));
        return null;
    }).when(dao).doStuff(anyString(), any());
    service.doStuff("John");
}