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.
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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.
Facade
This is often a logical approach as it makes obtaining the correct object to access the complex system underneath straightforward.
Patterns Using Inheritance
- Superclass - Out of all of the subclasses there may only be one instance.
- Subclasses - Allows the inheritance hierarchy to have controlled constraints. See the attached image.
Note: the class vs object Encapsulation boundary becomes very important here.
Composite + X
Thoughts?
| Design patterns | |
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Creational: Abstract Factory | Builder | Factory Method | Prototype | Singleton | |
