Parametrized Unit Tests (in Java)

While "creating" by hand parametrized test functionality is not extremely hard in Python (and in fact there are multiple people who did that, plus the "blessed" implementation which is going to be included in unittest2 some time soon, I hope), doing the same thing in Java is a nightmare.

Luckily enough, JUnit 4 offers the functionality out of the box.

In fact, I have to admit that it seems cleaner than Python alternative. Let us start with a Java quicksort (this time ugly, but not buggy, as far as my tests are correct).

package sorting.quicksort;

import java.util.List;

public class QuickSorter<E extends Comparable<E>> {
    List<E> lst;

    public synchronized void sort(List<E> lst) {
        this.lst = lst;
        sort(0, lst.size()-1);
    }

    private void sort(final int start, final int end) {
        if(start >= end) {
            return;
        }

        int pivot = partition(start, end);

        sort(start, pivot-1);
        sort(pivot+1, end);
    }

    private int partition(int start, int end) {
        int pivot = choosePivot(start, end);
        swap(pivot, start);

        int leftProcessed  = start, rightProcessed = end + 1;
        E pivotElement = lst.get(start);

        while(true) {
            do {
                leftProcessed++;
            } while(leftProcessed <= end &&
                lst.get(leftProcessed).compareTo(pivotElement) < 0);
            do {
                rightProcessed--;
            } while(lst.get(rightProcessed).compareTo(pivotElement) > 0);
            if(leftProcessed > rightProcessed) {
                break;
            }
            swap(leftProcessed, rightProcessed);
        }

        swap(start, rightProcessed);
        return rightProcessed;
    }

    private void swap(final int i, final int j) {
        E tmp = lst.get(i);
        lst.set(i, lst.get(j));
        lst.set(j, tmp);
    }

    private int choosePivot(final int left, final int right) {
        return left + (right - left)/2;
    }
}

The test case is:

package sorting.quicksort;

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.Parameterized;

import java.util.*;

@RunWith(value=Parameterized.class)
public class QuickSorterTest<E extends Comparable<E>> {
    private List<E> lst;
    private List<E> copyOfLst;
    private QuickSorter<E> sorter;

    public QuickSorterTest(List<E> lst) {
        this.lst = lst;
    }

    @Parameterized.Parameters
    public static Collection<Object[]> getParameters() {
        return Arrays.asList(
                new Object[]{ Arrays.asList(1, 2, 3, 4) },
                new Object[]{ Arrays.asList(1, 2, 4, 3) },
                new Object[]{ Arrays.asList(1, 3, 2, 4) },
                new Object[]{ Arrays.asList(1, 3, 2, 3) },
                new Object[]{ Arrays.asList(4, 2, 3, 1) },
                new Object[]{ Arrays.asList(1, 4, 3, 2) },
                new Object[]{ Arrays.asList(3, 2, 1, 4) },
                new Object[]{ Arrays.asList(3, 2, 2, 4) },
                new Object[]{ Arrays.asList(3, 1, 1, 3) },
                new Object[]{ Arrays.asList(1, 1, 1, 1) },
                new Object[]{ Arrays.asList(1, 2, 1, 1) },
                new Object[]{ Arrays.asList(1, 2, 3) },
                new Object[]{ Arrays.asList(1, 3, 2) },
                new Object[]{ Arrays.asList(2, 1, 3) },
                new Object[]{ Arrays.asList(2, 3, 1) },
                new Object[]{ Arrays.asList(3, 1, 2) },
                new Object[]{ Arrays.asList(3, 2, 1) },
                new Object[]{ Arrays.asList(3, 2, 3) },
                new Object[]{ Arrays.asList(3, 1, 1) },
                new Object[]{ Arrays.asList() },
                new Object[]{ Arrays.asList(1) },
                new Object[]{ Arrays.asList("hello", "cruel", "world") });
    }

    @Before
    public void setUp() {
        lst = new ArrayList<E>(lst);
        copyOfLst = new ArrayList<E>(lst);
        sorter = new QuickSorter<E>();
    }

    @Test
    public void testSort() throws Exception {
        sorter.sort(lst);
        Collections.sort(copyOfLst);
        Assert.assertEquals(copyOfLst, lst);
    }
}

Now, the idea is quite clear. The code to be written is ugly. The parametrized test case is annotated with @RunWith and the Parametrized.class. The test case must have a method annotated with @Parameterized.Parameters. This method return a collection of array of objects.

The idea is that for each element returned by that method a new test case object is constructed using the array of objects as actual arguments. In fact, the design is not extremely different from what we saw in Python.

In Python we had a generator yielding the tests. Here we have a static method which does quite the same job. Of course, here a Collection is returned. And Java syntax here is quite cumbersome. Luckily enough there is some flexibility with types so that I do not need to make different test case classes just because of that.

In the example we sort both arrays of integers and arrays of strings.

The setUp method and the test method do roughly the same thing they do in Python. And also the Java version uses a constructor (in fact, I'm led to believe that it is simply unavoidable).