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Patterns from nanoGoogle I

Patterns from nanoGoogle I. Decorator Attach responsibilities dynamically Concrete class 'is-a' and 'has-a' decorator Avoid rewriting existing code, write new code BufferedReader and java.io classes From string, from web, from … Filter classes Boolean: accept/reject word

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Patterns from nanoGoogle I

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  1. Patterns from nanoGoogle I • Decorator • Attach responsibilities dynamically • Concrete class 'is-a' and 'has-a' decorator • Avoid rewriting existing code, write new code • BufferedReader and java.io classes • From string, from web, from … • Filter classes • Boolean: accept/reject word • Altering: remove punctuation, lowercase, …

  2. Decorator Details • Name: also-knowns-as Wrapper • Wrap existing object with more responsibilities • HFDP: tall, decaf, skim, latte • Forces: • Add responsibilities to objects without affecting other objects (and remove the responsibilities) • Extension by subclass impractical: class explosion or no access to parent class for subclassing

  3. Patterns from nanoGoogle II • Strategy • Change algorithm, policy without altering existing code, but by writing new code • Program to interface, not implementation • Algorithm varies independently from client • What to do when processing words • Count them, store them, dump them to disk • How to print results after processing words • XML, to file, standard output, …

  4. Strategy Details • Name: also known as Policy • Make algorithms/policies interchangeable • HFDP: how to quack, how to fly • Forces: • Re-use policies between contexts or change them at runtime • Context has-a policy, uses it, can change policy • Don't hardwire policy behavior into client

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