Software Engineers Notebook

Software Engineering notes to myself that you may find useful too

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18 September 2018

Compound Patterns

by Martyn Butterworth

Compound patterns class diagram

A lot of patterns work together very well to form compound patterns. Shown is a UML class diagram that implements various design patterns to form a compound pattern. Here are some notes describing how and why this design was arrived at.

  1. Polymorphism was introduced by the use of the Quackable interface. This makes a number of design patterns available to us.
  2. The decorator pattern is used to allow us to track the number of times the method quack() is called. Using the decorator pattern means we don’t have to modify Duck and DuckCall
    • QuackDecorator implements Quackable so it can be referenced as a Quackable type
    • QuackDecorator has a Quackable instance variable so it can be passed a Quackable type
    • QuackDecorator is the first object in the chain, so it’s quack() method is called first. This allows it to update a counter variable, then call the quack() of its Quackable instance variable
  3. To use the Goose class, the adapter pattern is utilised so that we can convert the call to Goose’s honk() method to Quackables quack() method. Note that GooseAdapter implements Quackable so we can refer to all our objects as the same Quackable type.
  4. A factory pattern method is used to create our objects, so that we separate object creation from object use.