引数を使ってスタブの返値を決める

Gender.java

public enum Gender {
    MALE, FEMALE;
}

GenderDetector.java

public interface GenderDetector {
    Gender detect(String name);
}

Honorific.java

public class Honorific {
    GenderDetector detector;

    Honorific(GenderDetector detector) {
        this.detector = detector;
    }

    public String makeFor(String name) {
        Gender gender = this.detector.detect(name);
        if (gender == Gender.MALE) {
            return "くん";
        } else if (gender == Gender.FEMALE) {
            return "ちゃん";
        } else {
            return null;
        }
    }
}

MockitoSample.java

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.runners.MockitoJUnitRunner;

import java.util.HashMap;
import java.util.Map;

import static org.mockito.Matchers.anyString;
import static org.mockito.Mockito.doAnswer;

@RunWith(MockitoJUnitRunner.class)
public class MockitoSample {
    private static final Map<String, Gender> GENDER_MAP =
            new HashMap<String, Gender>() {{
                put("大槻ケンヂ", Gender.MALE);
                put("荻野由佳", Gender.FEMALE);
                put("ピーター", null);
            }};

    @Mock
    private GenderDetector genderDetectorMock;

    private Honorific tested;

    @Before
    public void setUp() {
        tested = new Honorific(genderDetectorMock);
    }

    @Test
    public void 引数を使って返値を返す() throws Exception {
        doAnswer(invocation ->
                GENDER_MAP.get(invocation.getArguments()[0]))
                .when(genderDetectorMock).detect(anyString());

        Assert.assertEquals("くん", tested.makeFor("大槻ケンヂ"));
        Assert.assertEquals("ちゃん", tested.makeFor("荻野由佳"));
        Assert.assertEquals(null, tested.makeFor("ピーター"));
    }
}