mockito.AdditionalAnswers.returnsFirstArg（）用于重载方法

Question

I have a problem with returnsFirstArg for overloaded methods, where the firstArg is not nessessarily the same type as the return type.

1. I want both methods to return the string value of the first argument.
2. Even better they always return the value of key

Is there anyway to achieve this and get these Tests green?

This is a minimum NOT working example, in real live Translator is more complex and can't be modified.

package test;

import static org.hamcrest.Matchers.is;
import static org.mockito.Mockito.mock;
import static org.hamcrest.MatcherAssert.assertThat;
import static org.mockito.AdditionalAnswers.returnsFirstArg;

import java.util.Locale;

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.runners.MockitoJUnitRunner;

@RunWith(MockitoJUnitRunner.class)
public class TranslatorTest {

    private class Translator {

        String getText(final String key) {
            return "translated " + key;
        }

        String getText(final Locale locale, final String key) {
            return "translated " + key + " for locale " + locale;
        }
    }

    @Test
    public void test_withoutToString() throws Exception {
        final String key = "com.stackoverflow.translator.label";
        Translator translator = mock(Translator.class,  returnsFirstArg());

        assertThat(translator.getText(key), is(key));

        final Locale locale = new Locale("en_GB"); // java.lang.ClassCastException: java.util.Locale cannot be cast to java.lang.String
        assertThat(translator.getText(locale, key), is(key));
    }

    @Test
    public void test_withToString() throws Exception {
        final String key = "com.stackoverflow.translator.label";
        Translator translator = mock(Translator.class,  returnsFirstArg().toString());

        assertThat(translator.getText(key), is(key));
        /*
         * java.lang.AssertionError:
         *  Expected: is "com.stackoverflow.translator.label"
         *  but: was null
         */

        final Locale locale = new Locale("en_GB");
        assertThat(translator.getText(locale, key), is(key));
    }
}

Answer 1

I wouldn't bend over backwards to try to define some generic default behavior when creating the mock object. You have two different methods, and could just specify two different behaviors explicitly:

@Before
public void setUp() {
    translator = mock(Translator.class);
    when(translator.getText(any(), any()))
        .thenAnswer(AdditionalAnswers.returnsSecondArg());
    when(translator.getText(any()))
        .thenAnswer(AdditionalAnswers.returnsFirstArg());
}

EDIT:
Addressing the added/clarified requirement in the comments could be problematic, unless you have a good way to recognize argument you want to return.

Assuming you can use some straight-forward logic, like returning the first string argument, you could implement your own Answer :

Answer<Object> returnsFirstString =
        invocationOnMock -> Arrays.stream(invocationOnMock.getArguments())
                .filter(String.class::isInstance)
                .findFirst()
                .orElse(null);

translator = mock(Translator.class, returnsFirstString);

This can, of course, be refined with some different logic like applying to getText methods only, etc.