8.

Overview

When an interface implementation doesn't satisfy all necessary invariants, you may have encountered the difficult-to-diagnose Fictitious Implementation bug pattern. We look at a double threat prevention method—assertions and unit tests.

Java interfaces are a great tool—they provide many of the advantages of multiple inheritance without all of the problems. By specifying an interface for all the services that a client expects to use, you make it possible to plug in various implementations of that interface as necessary.

Unfortunately, the only parts of an interface specification that can be expressed in code are method signatures. There may very well be other invariants that are expected to hold for any implementation, but the Java language provides no facility to check them. When an implementation of an interface fails to satisfy some of these unchecked invariants, we have an instance of the Fictitious Implementation bug pattern.

We can summarize this bug pattern as follows:

• Pattern: Fictitious Implementation.

• Symptoms: A client class that works with a specified interface breaks when a certain implementation of that interface is used.

• Cause: The interface includes many intended invariants that aren't satisfied by the implementation.

• Cures and Preventions: Fix the implementation to include those invariants. Make the invariants explicit in the documentation of the interface, if they aren't already.