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Mastering Design Patterns: Strategy, Context, and Decorator Patterns

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  1. Design Pattern Aside • Why you should read extensively and often • What are the downsides? • What are the upsides? • TANSFAAFL • How to understand what you're reading? • Practice what you're reading about • Read more • … http://www.cs.duke.edu/news/threads/2008/fall.pdf

  2. Strategy Design Pattern (aka Policy) • Algorithm varies independently of client that uses algorithm • Need context to use algorithm, these may be coupled • Quiz taking "algorithms" we've discussed • Should there be sub-classes of quiz for … • Buzzfeed quizzes or Date/Matching "quiz" • Multiple correct vs. Single correct https://en.wikipedia.org/wiki/Strategy_pattern

  3. Context and Strategy (from GOF) • Context has-a strategy • Clients interact with the Context, not Strategy • Context uses strategy • Quiz compared to QuizTaker • What's the difference? What about whether questions should be shuffled? Adaptable question generation?

  4. What about sorting in java.util publicclass Sorter { publicvoid sort(ArrayList<String> list, Comparator<String> comp) { Collections.sort(list); Collections.sort(list,Comparator.reverseOrder()); Collections.sort(list,comp); } }

  5. Strategy Benefits • Open/Closed Principle • Program is open to extension without being modified • How to create/use .zip, or .tgz, or .rar • What are the differences here? What about adding new compression methods? • How to use encryption/cipher algorithms? • https://docs.oracle.com/javase/7/docs/technotes/guides/security/crypto/CryptoSpec.html#Cipher

  6. Prerequisites for Compsci 101

  7. Decorator and Strategy in Java • First let's look at some examples • http://www.programcreek.com/java-api-examples/index.php?api=javax.crypto.CipherInputStream • Let's look at what's common across several examples • How to create CipherInputStream? • How to use the InputStream created?

  8. Decorator Pattern • Add functionality to objects • Configure input stream at runtime • Rectangle, BorderedRect, ShadowedRect, … • The Decorator typically is-a and has-a • Is an input stream, has an input stream • Forwards requests to the has-a object • Behaves like because also is-a https://en.wikipedia.org/wiki/Decorator_pattern

  9. Decorator Pattern • Typically is a light-weight class, not too many "guts" • In latter case, prefer Strategy pattern! • If you need to add many "adornments", e.g., borders, shading, rounded, etc. • Perhaps the borders/adornments are a Strategy for drawing, not Bordered(Rounded(...) but ... • TANSTAAFL • How do you make choices when designing?

  10. From GOF book on Decorator • Functionality composed of simple pieces, don't need to anticipate everything • Think buffered readers, cipher input streams • Lots of little objects that all look alike • Easy to use if you understand how to create and compose, but hard to learn and debug There are always trade-offs. Understanding options is harder than knowing only one way

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