Example based Testing

Testing division

public class MyTest {

  @Test
  public void testDivision() {
    assertEquals("Integer division", 5, 15 / 3);
    assertEquals("Double division", 0.5, 1.0 / 2);
  }

  @Test(expected = ArithmeticException.class)  
  public void testDivisionException() {
    // should fail
    final int i = 1 / 0;
  }

}

JUnit Theories

@RunWith(Theories.class)
public class MyTheoryTest {

    @DataPoints
    public static int[] data = { 1, 2, 3, 4, 5, 6 };

    @Theory
    public void divisionIsTheReversionOfMultiplication(int a, int b) {
        assertEquals(a, a * b / b);
    }

}

Generators

@RunWith(Theories.class)
public class MyGeneratorTest {

    @Theory
    public void divisionIsTheReversionOfMultiplication(
            @ForAll int a,
            @ForAll @InRange(min = "1") int b) {

        assertEquals(a, a * (b / b));
    }

    @Theory
    public void concatenationLength(
            @ForAll String s1, 
            @ForAll String s2) {

        assertEquals(s1.length() + s2.length(), (s1 + s2).length());
    }
}

Math to the rescue

\begin{align} \frac{a}{b} &= \frac{a}{b}\tag{1} \\ \\ a &= \frac{a}{b} \cdot b\tag{2} \end{align}

Property based Testing

• Link to John Hughes' paper

Properties (or Invariants)

• Identity Test / Roundtrip
• Idempotency
• Reference Implementation

Generators

Libraries

• Haskell: Quickcheck
• Java: junit-quickcheck
• Javascript: JSVerify
• Scala: ScalaCheck
• Clojure: test.check
• Erlang: Erlang QuickCheck
• ...

JUnit Theories