#+TITLE: java-check

Property-based testing within the [[http://junit.org/][JUnit]] test framework.

Using ~java-check~ you can write ~@Property~ tests. These tests will
have any parameters randomly generated (using generators marked with
~@DataSource~). This allows you to write tests which assert general
properties rather than specific cases.

As an example, let's test the ~Collections.reverse~ function. One
simple property of ~Collections.reverse~ is that reversing a list
twice results in the original list (ie. ~reverse(reverse(x)) = x)~).

Let's write a property for this:

#+BEGIN_SRC java
  @RunWith(Properties.class)
  public class ExampleTest {
      @DataSource
      public static Generator<List<Integer>> integerList = listOf(integer());

      @Property
      public void reverseIsInvolution(List<Integer> list) {
          List<Integer> reversed = new ArrayList<>(list);
          Collections.reverse(reversed);
  //        reverse(reversed);

          assertEquals(list, reversed);
      }
  }
#+END_SRC

Oh no! The test above is incorrect! I've accidentally commented out
the second ~reverse(reversed)~ line, which means I'm trying to test
whether ~reverse(x) = x~. That means this test should fail:

#+BEGIN_EXAMPLE
  au.id.zancanaro.javacheck.junit.PropertyError: reverseIsInvolution([0, -1])
          Seed: 1433134487916

  Expected :[0, -1]
  Actual   :[-1, 0]
    <Click to see difference>

          at au.id.zancanaro.javacheck.junit.Properties$GenerativeTester.evaluate(Properties.java:217)
          ...  (extra stack trace entries have been removed for clarity)
  Caused by: java.lang.AssertionError: expected:<[0, -1]> but was:<[-1, 0]>
          at org.junit.Assert.fail(Assert.java:88)
          ...  (extra stack trace entries have been removed for clarity)
#+END_EXAMPLE

The test has failed, and it's managed to reduce the failing case to
the smallest possible failing case: a two-element list with both
elements being different.

It's easy for us to fix the test, and here is the complete file for
~ExampleTest.java~:

#+BEGIN_SRC java
  package com.example;

  import au.id.zancanaro.javacheck.Generator;
  import au.id.zancanaro.javacheck.annotations.DataSource;
  import au.id.zancanaro.javacheck.annotations.Property;
  import au.id.zancanaro.javacheck.junit.Properties;
  import org.junit.runner.RunWith;

  import java.util.ArrayList;
  import java.util.List;

  import static au.id.zancanaro.javacheck.Generators.integer;
  import static au.id.zancanaro.javacheck.Generators.listOf;
  import static java.util.Collections.reverse;
  import static org.junit.Assert.assertEquals;

  @RunWith(Properties.class)
  public class ExampleTest {
      @DataSource
      public static Generator<List<Integer>> integerList = listOf(integer());

      @Property
      public void reverseIsInvolution(List<Integer> list) {
          List<Integer> reversed = new ArrayList<>(list);
          reverse(reversed);
          reverse(reversed);

          assertEquals(list, reversed);
      }
  }

#+END_SRC