Unit Testing with Scenarios Helper Class

This is just a small example of a simple Helper class to use declarative parameterized tests with the given-when-then syntax.

This is a simple example of things one can do with simple domain specific languages (DSL) and/or Helper classes (e.g. emulate the Specification By Example).

This type of syntax can be found in some very sophisticated frameworks such as Spock and Cucumber BDD

JUnit DSL for BDD style

public class StringsTest {

    public void stringLength() {

                .given("john", "doe")
                .when(String::length) // function under test
                .expect(4, 3)

And here is the Scenarios class..

class Scenarios<T, R> {

    private List<T> parameters;
    private List<R> results;
    private Function<T, R> function;

    private Scenarios(T... parameters) {
        this.parameters = Arrays.asList(parameters);

    public static <T, R> Scenarios<T, R> given(T... parameters) {
        return new Scenarios<T, R>(parameters);

    public Scenarios<T, R> expect(R... results) {
        this.results = Arrays.asList(results);
        return this;

    public Scenarios<T, R> when(Function<T, R> mapFunction) {
        this.function = mapFunction;
        return this;

    public void build() {
        List<R> collect = parameters.stream().map(function).collect(Collectors.toList());
        assertEquals(results, collect);

JUnit also contains support for Parameters out-of-the-box.

Written on January 3, 2016