Iterable adapter - Ways to define characteristics

The current development version of Quickcheck for Java contains new Iterable adapter. This adapter lets you convert a Generator implementation into an Iterable. This Iteraterable can then be used in Java for loops. This makes definitions of tests much shorter as no inner class has to be used to define a characteristic.

When I started porting Quickcheck from Haskell I thought that there will be some closure-like facility in the Java language in 2 years. (After all more and more developers seemed to get exposed to the concept of functions and higher order functions. Most recent languages have some form of language construct to build new control structures in libraries.) So I did not bother myself too much with the usability problems that arise from the use of inner classes. In the meantime time it is obvious that Java will not have closures soon and maybe never will. So as long as we are using Java we have to get around this.

Quickcheck has now three different ways to express a characteristic: inner classes, annotations and for-loop expressions. Each of these solutions has its advantages and disadvantages. The example shows a characteristic defined for all three approaches.

public class IterableExample {

  public void classic() {
    forAll(PrimitiveGenerators.integers(), new AbstractCharacteristic<Integer>() {

      protected void doSpecify(Integer any) throws Throwable {
        assertEquals(any * 2, any + any);

  @ForAll(generatorMethod = "net.java.quickcheck.generator.PrimitiveGenerators.integers")
  public void annotation(Integer any) {
    assertEquals(any * 2, any + any);

  public void iterable() {
    for (Integer any : Iterables.toIterable(PrimitiveGenerators.integers())) {
      assertEquals(any * 2, any + any);

Inner classes are the standard implementation and fine if one can ignore the boilerplate code that is used to express a characteristic as an inner class.
Annotations are not type-safe (and therefore refactoring safe), they cannot be navigated in IDEs and the generator definition and use is separated.
The Iterable adapter is type-safe and has concise definition of the characteristic but it lacks all features of the Quickcheck runner. So the Iterator<T> created by the Iterable<T> adapter will produce a fixed number of values. As the Java for loop construct is used the context information for the test run is missing. Setup and tear down have to be called explicitly.


Defining the Collection.add contract in Quickcheck for Java

The Collection add method does define a contract that is expressed in javadoc.
It states:
  • A collection implementation can throw an exception. The collection will stay unchanged and contains() will return false.
  • The add method return value indicates if the collection changed as a result of the call. The collection will contain the element (based on equals()) but only if the collection changed the very instance will be part of the collection.
This textual form can be expressed as a Quickcheck characteristic.

public void collectionAdd() {
 for (Pair<Integer, List<Integer>> any : somePairs(integers(), lists(integers()))) {
  Integer element = any.getFirst();
  Collection<Integer> collection = any.getSecond();

  boolean changedCollection = false;
  boolean exceptionThrown = false;
  try {
   changedCollection = collection.add(element);
  } catch (Exception e) {
   exceptionThrown = true;
  assertTrue(collection.contains(element) != exceptionThrown);
  assertTrue(changedCollection == containsInstance(collection,

private void assertException(Exception e) {
 assertTrue(e instanceof UnsupportedOperationException
   || e instanceof ClassCastException
   || e instanceof IllegalArgumentException
   || e instanceof IllegalStateException);

private boolean containsInstance(Collection<?> collection, Object element) {
 for (Object e : collection) if (e == element) return true;
 return false;

So in this way a specification can be provided of an interface some third party has to implement. It's then up to the implementer to provide a generator for the implementation besides the working code.

This has obviously some overlap with contract-driven development and the assertions facility. One interesting reuse of the characteristic could be to take an implementation that states it fulfills a given contract and monitor the behaviour of the implementation at run time (i.e. not while test time) and to report violations of the contract.