Unit testing with JUnit+Mockito vs. Spock. Have a look how Spock usage is concise than traditional JUnit side.

EvenOddPickGame.kt

class EvenOddPickGame(private val randomIntGen : RandomValueGenerator<Integer>) {
    fun pick(times: Int): List<String> {
        return ArrayList<String>().apply {
            for (i in 0 until times) {
                val rolled = randomIntGen.nextRandom(lowerBound = 1, upperBound = 2) % 2
                if (rolled == 0) {
                    add("EVEN")
                } else {
                    add("ODD")
                }
            }
        }
    }
}

RandomValueGenerator.kt

interface RandomValueGenerator<T> {
    fun nextRandom(lowerBound: T? = null, upperBound: T? = null): T
}

class RandomIntGenerator : RandomValueGenerator<Int> {
    override fun nextRandom(lowerBound: Int?, upperBound: Int?): Int {
        return with(Random()) {
            (lowerBound ?: 0) + if (upperBound == null) {
                nextInt()
            } else {
                nextInt(upperBound)
            }
        }
    }
}

Z_1_EvenOddPickGameTest.java

// JUnit + Mockito
@Test
public void testPick() {
    // setup (given)
    final RandomValueGenerator<Integer> mockRandomIntGen = mock(RandomValueGenerator.class);
    final EvenOddPickGame sut = new EvenOddPickGame(mockRandomIntGen);
    doAnswer(new Answer<Integer>() {
        private int count = 0;

        public Integer answer(final InvocationOnMock invocation) {
            count++;
            switch (count) {
                case 1:
                    return 4;
                case 2:
                    return 9;
                default:
                    throw new UnsupportedOperationException("Out of specification")
            }
        }
    }).when(mockRandomIntGen).nextRandom(anyInt(), anyInt());

    // replay (then)
    final List<String> expected = new ArrayList<>(Arrays.listOf("EVEN", "ODD"))
    final List<String> actual = sut.pick(2);

    // verify (expect)
    assertListEquals(actual, expected)
}

Z_2_EvenOddPickGameSpec.groovy

// Spock
def "Verify roll mod 2 yields correct #result"() {
    given:
    final mockRandomIntGen = Mock(RandomValueGenerator);
    final sut = new EvenOddPickGame(mockRandomIntGen);
    mockRandomIntGen.nextRandom(_, _) >>> [ roll1, roll2 ]

    expect:
    sut.pick(2) == result

    where:
    roll1 || roll2 || result
    4     || 9     || [ "EVEN", "ODD" ]
}