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Lecture 27: Façade Pattern

Computer Science 313 – Advanced Programming Topics. Lecture 27: Façade Pattern. “In a Nutshell”. Commonly used English expression Means “in a few words” or “briefly” Summarizing idea with many details left out. “In a Nutshell”.

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Lecture 27: Façade Pattern

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  1. Computer Science 313 – Advanced Programming Topics Lecture 27:Façade Pattern

  2. “In a Nutshell” • Commonly used English expression • Means “in a few words” or “briefly” • Summarizing idea with many details left out

  3. “In a Nutshell” I could be bounded in a nutshell, and count my selfe a King of infinite space. • Commonly used English expression • Means“in a few words”or“briefly” • Summarizing idea with many details left out

  4. “In a Nutshell” • Usage can be traced back to 1608 • Even before Hamlet used it • Meaning has not changed in 400 years • Also became a verb in that time • “To nutshell” popularized by Mark Twain • Any questions about what it means?

  5. “In a Nutshell” • Usage can be traced back to 1608 • Even before Hamlet used it • Meaning has not changed in 400 years • Also became a verb in that time • “To nutshell” popularized by Mark Twain • Any questions about what it means? • Will someone explain this to O’Reilly!

  6. Big Picture for Façade • Provide simple interface to low-level functions • Create single class defining simple methods • May combine functionality of multiple instances • Façade makes client easier to write & use • Packages tricky parts into simple method(s) • Allows client to avoid working with many instances • Nothing should change in lower level classes • Using the façade complex subsystem hidden • But still usable without going through façade

  7. Real-Life is Hard • Real life is full of complex ideas & concepts • Filling out your tax forms every April • Rules for holding in (American) football • Explaining why the sky is blue • Appeal of soccer to billions worldwide

  8. Real-Life is Hard • Often rely on simplicities to understand these • Filling out your tax forms every April“Politician got bribe contribution to create deduction” • Rules for holding in (American) football “Stay within the shoulder pads & don’t get caught” • Explaining why the sky is blue“Wavelengths not scattered by the atmosphere” • Appeal of soccer to billions worldwide“Excuse to drink beer & occasionally riot”

  9. Real-Life Façade • Compiler is (real-life)example of façade • Typically we think of compiler as single system • Many complex systems combine to make compiler • Code divided into tokens by the parser • Lexigraphical analyzer computes tokens’ meanings • Analysis improved & simplified by optimizer • Code generator lowers & outputs result • Even so, many of these systems also façades: • Loop analyzer, SSA transform, peepholerin optimizer

  10. Façades On Computers

  11. Façade We Regularly Use • Usually do all interactions via simple interface • javac foo.java compiles class named foo • But can also access components directly • final& volatile provide details to optimizer • Can get instance of javax.tools.JavaCompiler

  12. Façade We Regularly Use • Compilation easy because of this façade • Façade simplifies & allows messing with internals • All skill levels helped using the compiler • Beginner javacfoo.java • Expert javac –g:nonefoo.java • Stupid javac –nowarn –Xprintfoo.java

  13. Façade versus Adapter • Façade and Adapter patterns very similar • Enable simplifying code to be used by client • Both patterns effectively hide subsystem details • 1 or more instances behind exposed interface • Idealized UML class diagram may/may notdiffer • “What is the difference?” should be asked

  14. Intent Crucial to Usage • Adapter makes existing code use interface • Original implementation hidden by pattern • Lets client code think class written for interface • Adapts existing code so it can work in other places • Façade used to create interface to use system • Original implementation hidden by pattern • Lets client write code as if design has simple interface • Provides a false front for use in another system

  15. Clients of Façade & Adapter • With either pattern within client code: No difference in usage No difference in design

  16. Using Façade & Adapter Façade Adapter Yams \ Sweet Potatoes

  17. Using Façade & Adapter Façade Adapter Yams Sweet Potatoes \ Sweet Potatoes Yams

  18. Kissing Cousins of Patterns • Adapter & Façadealso close to Decorator • Structural patterns that help simplify client code • Within each pattern, hide implementation details • Workings invisible to those outside pattern • Decorator has some important differences • Decorations invisible; adapter & façade not • Decorator requires specific class relationships • Interface unchanged by decorator pattern

  19. For Next Class • Lab #6 due in one week • Do not leave labs to last second • Read pages 265 – 275 for Friday • What is the Principle of Least Knowledge? • How much of class already follows this principle? • What is the Law of Demeter? • Can we quantify this principle?

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