Best way to test a method's outcome with JUnit and Mockito

I am trying to test the method:

@Override
    public boolean test(Apple apple) {
        if (apple == null) {
            return false;
        }
        return "green".equals(apple.getColor());
}

My first guess was to test it in the following way:

package io.warthog.designpatterns.behaviorparameterization.impl;

import io.warthog.designpatterns.behaviorparameterization.Apple; import org.junit.Test;

import static org.mockito.Mockito.when;

public class AppleGreenColorPredicateTest {

    private AppleGreenColorPredicate classUnderTest = new AppleGreenColorPredicate();

    @Test
    public void test_WithGreenApple_ReturnsTrue() {
        Apple apple = new Apple();
        apple.setColor("green");
        when(classUnderTest.test(apple)).thenReturn(true);
    }

}

But it gave me the error message:

org.mockito.exceptions.misusing.MissingMethodInvocationException: when() requires an argument which has to be 'a method call on a mock'. For example: when(mock.getArticles()).thenReturn(articles);

So I ended up doing:

package io.warthog.designpatterns.behaviorparameterization.impl;

import io.warthog.designpatterns.behaviorparameterization.Apple; import org.junit.Assert; import org.junit.Test;

public class AppleGreenColorPredicateTest {

    private AppleGreenColorPredicate classUnderTest = new AppleGreenColorPredicate();

    @Test
    public void test_WithGreenApple_ReturnsTrue() {
        Apple apple = new Apple();
        apple.setColor("green");
        Assert.assertEquals(true, classUnderTest.test(apple));
    }

}

The question here is, when would you recommend using the Mockinto.when() approach and when the Assert.equals().

Any help will be greatly appreciated!

I'd follow the XP rule of doing the simplest thing that could possibly work. You don't need to mock anything, so just use the objects directly.

private AppleGreenColorPredicate classUnderTest;

@Before
public void setUp() {
    classUnderTest = new AppleGreenColorPredicate();
}

@Test
public void nullTest() {
    assertFalse(classUnderTest.test(null));
}

@Test
public void green() {
    Apple a = new Apple();
    a.setColor("green");
    assertTrue(classUnderTest.test(a));
}

@Test
public void notGreen() {
    Apple a = new Apple();
    a.setColor("red");
    assertFalse(classUnderTest.test(a));
}

In your mockito code, you seem to be mocking what you're testing, i.e., you won't discover any problems in your code because your test only calls the mocks. That gives a false sense of security. Only mock when you have to. Mock the dependencies of the class under test, not the actual class under test.