readme.md


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Property-based testing within the [JUnit](http://junit.org/) 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:

    @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);
        }
    }

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:

    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)

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`:

    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);
        }
    }