Combining design patterns

Design patterns can be combined in numerous interesting ways to create super-patterns aka OO programs. A great example of this being done well is JUnit.

Singleton + X

The singleton can be combined with a wide variety of patterns to create more specialised behaviour.

Proxy

By using a singleton on certain parts of a proxy a specific effect can be generated.

• Real Subject - Adding a singleton here means that any number of proxys can access this one class. This creates a specialised behaviour where different proxies can access a central object in different ways without being concerned that the right object is being called.
• Proxy - A singleton here makes sure that only a single proxy can be used to call any subject.
• Subject - A simple singleton here breaks the proxy pattern as no subclasses are allowed. A complex singleton, i.e. one that allows a controlled number of instances could be very powerful.