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Hammock - Mock Objects for Java ME


In the last decade, unit-testing with the xUnit family of test frameworks has become popular; in particular Test-Driven Development (TDD) has been embraced by companies and, even more importantly, by programmers as a means to improve the quality of code. For J2ME there are several derivatives of JUnit (for example, JMUnit, J2MEUnit and MoMEUnit). The goal of unit testing is that each class should be tested in isolation from other classes. However, in practice, this can be difficult since classes need to collaborate to do anything useful. A popular method to work around this dependency problem is to use mock objects. A mock object stands in for a real object that a class under test collaborates with. The test code specifies how a mock object should respond when a class under test calls methods on the mock object. By using a mock object, rather than a real object, we can simulate scenarios that may be difficult to configure for a real object or test code where a collaborating class hasn't yet been written yet. The ability to write tests before all collaboratoring classes have been written works well with TDD since it enables tests to be written before the code to be tested. Additionally, testing with mock objects encourages "programming to an interface" rather than to an implementation.
For more information visit the mock objects website or Wikipedia.


Mock objects can be used effectively when
  • The real object is difficult to configure.
  • The real object has behavior that is difficult to replicate (such as a network error).
  • The real object is a GUI component.
  • The real object doesn't yet exist.

Many Java ME applications can benefit from the use of mock objects for testing since typical mobile applications have a large number of user interfaces and make use of several network protocols (such as HTTP, SMS, bluetooth, etc). Hammock is a mock object framework for Java ME that provides mock objects for many collaborating classes that are used in a typical application. Mocks are provided for most of the classes in the microedition UI and I/O packages as well as for the optional SMS, OBEX, PDA and location APIs.
Hammock includes a utility, HammockMaker, that generates the source code of classes and interfaces that need to be mocked. HammockMaker can be run from the command line or, more usefully, as an Ant task. Since HammockMaker is an Ant task it can be incorporated into a build, test and deploy script. A sample application, ObjectStreams, includes a detailed guide showing how to test effectively using Hammock.
An Eclipse plug-in for HammockMaker provides a fast way to create test doubles.
The current stable version of Hammock is version 2.1.

Getting Hammock

Hammock can be downloaded from Sourceforge.
The distribution includes
  • A user guide.
  • A reference guide.
  • A guide to using spy objects rather than mock objects.
  • The source code.
  • JARs for MIDP 1.0 and MIDP 2.0 (both CLDC 1.0 and CLDC 1.1).
  • Detailed examples showing how to test using mock objects.

The Hammock user guide, Hammockmaker user guide, spy objects guide, reference guide and JavaDocs are also available online.