Writing JUnit 4 parameterized tests with varargs data

One of the many excellent new features of JUnit 4 is parameterized tests. These are data-driven tests where a test case runs repeatedly against a collection of test data, as if each run were its own test case.

A standard example:

@RunWith(Parameterized.class)
public class ExampleDataDrivenTest {
    public static final class Data {
        private final String name;

        public Data(final String name) {
            this.name = name;
        }

        @Override
        public String toString() {
            return name;
        }
    }

    private final List<Data> data;

    public ExampleDataDrivenTest(final Data datum1, final Data datum2) {
        System.out.println("ExampleDataDrivenTest");

        this.data = asList(datum1, datum2);
    }

    @Test
    public void dumpData() {
        for (final Data datum : data)
            System.out.println("datum = " + datum);
    }

    @Parameters
    public static Collection<Data[]> data() {
        final Data[][] data = new Data[][]{
                {new Data("apple"), new Data("core")}};

        return asList(data);
    }
}

This produces:

ExampleDataDrivenTest
datum = apple
datum = core

This works great for a fixed number of test data, but I want to test a variable number of data. The change is straight-forward once you recall that the varargs Java language feature is implemented as an array:

@RunWith(Parameterized.class)
public class ExampleDataDrivenTest {
    public static final class Data {
        private final String name;

        public Data(final String name) {
            this.name = name;
        }

        @Override
        public String toString() {
            return name;
        }
    }

    private final List<Data> data;

    public ExampleDataDrivenTest(final Data... data) {
        System.out.println("ExampleDataDrivenTest");

        this.data = asList(data);
    }

    @Test
    public void dumpData() {
        for (final Data datum : data)
            System.out.println("datum = " + datum);
    }

    @Parameters
    public static Collection<Data[][]> data() {
        final Data[][][] data = new Data[][][]{
                {{new Data("apple"), new Data("core")}}};

        return asList(data);
    }
}

Reflect on the constructor and you see:

public ExampleDataDrivenTest(ExampleDataDrivenTest$Data[])

This makes the change more obvious.

I can now write my annotated parameters as:

    @Parameters
    public static Collection<Data[][]> data() {
        final Data[][][] data = new Data[][][]{{{ /* no data */ }},
                {{new Data("apple"), new Data("core")}},
                {{new Data("baltimore")}}};

        return asList(data);
    }

To produce:

ExampleDataDrivenTest
ExampleDataDrivenTest
datum = apple
datum = core
ExampleDataDrivenTest
datum = baltimore