In a Nutshell.

[Kotlin, FOSS, Developer's Life.]

Small scopes and SRP rocks, isn’t it?

We discussing recent post by “OOP” lover Yegor Bugayenko.

My thoughts that it’s not about FP or OOP, objects or functions, it’s all about SRP. Like we have two initialization blocks in one method, and then we test results of these blocks. We can move this blocks in lambdas/functions/classes, but in general it’s just two functions that produces a and b and then we compare a and b. After working with Kotlin I’ll write this test like:

void testIntStream() {
    final long seed = System.currentTimeMillis();

        run(() -> {
            final Random r1 = new Random(seed);
            final int[] a = new int[SIZE];
            for (int i = 0; i < SIZE; i++) {
                a[i] = r1.nextInt();
            return a;
        run(() -> {
            final Random r2 = new Random(seed);
            return r2.ints().limit(SIZE).toArray();

Just like in Yegor’s solution my test have only one statement: assertEquals.

My approach much more clean, and for every new test I don’t write any boilerplate to the glory of “OOP” God.

Just like Yegor’s solution, my approach have same benefits over original test:

  • Reusability — I can easy extract lambda from test, and reuse between number of test.
  • Brevity — It’s much less code with in place lambdas.
  • Readability
  • Immutability

In post Yegor’s approach seems good, but it doesn’t scale to real applications.

run function for the reference:

public static <R> R run(Producer<R> producer) {
    return producer.produce();

Kotlin version

So also I’d like to add Kotlin example for this test, it’s much more readable even than Java version with lambdas (no surprise here)

@Test fun testIntStream() {
        val seed = System.currentTimeMillis()

            expected = {
                val r1 = Random(seed)
                IntArray(SIZE).also { array ->
                    for (i in 0..SIZE - 1) {
                        array[i] = r1.nextInt()
            actual = {
                val r2 = Random(seed)

I am using JUnit 5 for this test, and was expecting that JUnit 5 has methods that accepts lambdas for assert, but unfortunately – there are no such signature. So I added it:

fun assertArrayEquals(expected: () -> IntArray, actual: () -> IntArray) {
    Assertions.assertArrayEquals(expected.invoke(), actual.invoke())

I think will be nice to have support library for JUnit 5 with such methods.

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