Announcements. John Jannotti (cs32) Design Patterns Feb 13, / 1

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1 Announcements We ll code review Stars on Thursday. Volunteer your code by ing me. Lab this week covers Ajax/Javascript. Interactive UIs. No lab (or lab hours) next week. Submit a group project idea on Piazza by Friday. Autocorrect Gear-up 6:30pm today here. (Metcalf) Science Center s Learning Exchange program is looking for student facilitators to teach this semester! LE helps middle school students develop their coding and math skills by using Scratch and Javascript to create their own games. As a facilitator, you ll work one-on-one with students, guiding them in completing their projects and introducing them to coding. It s a low-time commitment and a great way to make a difference off College Hill. If interested, please fill out this form: John Jannotti (cs32) Design Patterns Feb 13, / 1

2 Design Patterns John Jannotti /course/cs0320/www/lectures/ Feb 13, 2018 John Jannotti (cs32) Design Patterns Feb 13, / 1

3 Project Ideas League / tournament scheduler (round-robin, double elimination, matched strength) Anti-clickbait (or fake news) as a service and a browser plugin to use it. Analyze, find anomolies in large document dumps. Wait-in-line app (need to make it interesting texting, history) Good Go, Chess, Scrabble, Crossword, Poker, Bridge players Real-time Strategy Game (Ikariam, Call of War) Desktop Tower Defense (Mix up a classic.) Teleconferencing, burner numbers, other voice app (w/ Twilio) John Jannotti (cs32) Design Patterns Feb 13, / 1

4 (Bad) Project Ideas Anything with network effects. (Not enough time.) Anything that requires specific data (that you don t have). Real startup ideas (If simplistic implementation) Try to look outside the market of Any Brown Student. Find a niche. Don t just build a C.R.U.D app! John Jannotti (cs32) Design Patterns Feb 13, / 1

5 Design Patterns Designers (of all kinds) work by recycling ideas. Experience provides more & better ideas. Knowing what ideas will work when. And how to combine them into something new Patterns are a way of naming these ideas. The basic ideas themselves. The information for how & when to use them. John Jannotti (cs32) Design Patterns Feb 13, / 1

6 Patterns at multiple levels Code patterns for-loop over array indices Test a condition and acquire a lock atomically. Data structure patterns Interface X, AbstractX class. Provide iterator as inner class, Itr Design patterns Organizing classes, methods, interfaces. Architectural patterns Plugins, Thread-per-request, Client-Server, Peer-to-peer. John Jannotti (cs32) Design Patterns Feb 13, / 1

7 What is a Design Pattern? Problem to be addressed What the pattern is trying to do Motivation for using the pattern Exactly what the pattern does for you Conditions When the pattern can be applied Strengths & Weaknesses of the pattern Implementation What are the classes and methods? How are the classes related? How are the methods implemented? John Jannotti (cs32) Design Patterns Feb 13, / 1

8 Caveats These are solutions looking for problems. Understand & analyze the problem first. Then consider suitable patterns. The pattern s problem description is vital. Experience will (eventually) tell what works. Beware of added/excess complexity. A simple rule of thumb: Introduce a pattern (or, any complexity) when your code needs it, not in anticipation of needing it. YAGNI. But don t use that as an excuse. Often, general code is easier and clearer!. Think about a SIZE constant, or n-dimensional vs 3-dimensions. John Jannotti (cs32) Design Patterns Feb 13, / 1

9 Standard Patterns Design Patterns, Gamma et al. And lots of other books Anti-patterns (code smells ) Patterns can be classified by use Factory patterns Delegation patterns Structural patterns Control patterns John Jannotti (cs32) Design Patterns Feb 13, / 1

10 Factory Patterns How to create objects Without using new directly. Why? Various forms Factory class - Builders, hides multiple implementation classes Factory interface - callers provide factory to library Factory method - good for Fly-weight (below) Interesting variants Prototype: copy a sample object (But Java s clone() has problems.) Singleton: allow only one instantiation (Some recommend an enum.) Fly-weight: shared use of immutable instances (maybe copy-on-write) John Jannotti (cs32) Design Patterns Feb 13, / 1

11 Sidebar: The need for Multimaps You may have code that does this: 1 Map<Clerk, L i s t <Sale >> l e d g e r = new HashMap<>(); 2 3 p u b l i c v o i d makesale ( C l e r k c l e r k, S a l e s a l e ) { 4 L i s t <Sale > s a l e s = l e d g e r. get ( c l e r k ) ; 5 i f ( s a l e s == n u l l ) { 6 s a l e s = new A r r a y L i s t <Sale >(); 7 l e d g e r. put ( c l e r k, s a l e s ) ; 8 } 9 s a l e s. add ( s a l e ) ; 0 } John Jannotti (cs32) Design Patterns Feb 13, / 1

12 With Multimaps, you can say 1 Multimap<Clerk, Sale > l e d g e r = 2 new ArrayListMultimap <>(); 3 4 p u b l i c v o i d makesale ( C l e r k c l e r k, S a l e s a l e ) { 5 l e d g e r. put ( c l e r k, s a l e ) ; 6 } There s Multiset, too. Boy, that s handy when I need to count how often something (words, bigrams?) appears in a dataset (corpus?). John Jannotti (cs32) Design Patterns Feb 13, / 1

13 Factory use in Guava s Multimaps Clients can choose iteration order / overhead. ArrayListMultimap, HashMultimap, LinkedHashMultimap... Clients can use new or Multimaps.newXXX() Factory methods here are mostly historic, aesthetic. Immutable versions for safety ImmutableListMultimap, ImmutableSetMultimap Builder pattern is required. (two slides forward) Some are not even public classes Multimaps.newListMultimap() Multimaps.newSortedSetMultimap() Factory methods hide details. And these Factory methods take Factory objects! John Jannotti (cs32) Design Patterns Feb 13, / 1

14 Builder Pattern Avoids confusing construction for many fielded objects. 1 gc = new G r a p h i c s C o n t e x t ( 1, 30, n u l l, 2 C o l o r. RED, C o l o r. Black, 3... ) ; vs 1 gc = new G r a p h i c s C o n t e x t. B u i l d e r ( ) 2. l i n e T h i c k n e s s ( 1 ) 3. o p a c i t y (30) 4. f o r e g r o u n d ( C o l o r.red) 5. background ( C o l o r. BLACK) 6. b u i l d ( ) ; John Jannotti (cs32) Design Patterns Feb 13, / 1

15 Builder Pattern Avoids confusing construction for many fielded objects. 1 gc = new G r a p h i c s C o n t e x t ( 1, 30, n u l l, 2 C o l o r. RED, C o l o r. Black, 3... ) ; vs 1 gc = new G r a p h i c s C o n t e x t. B u i l d e r ( ) 2. l i n e T h i c k n e s s ( 1 ) 3. o p a c i t y (30) 4. f o r e g r o u n d ( C o l o r.red) 5. background ( C o l o r. BLACK) 6. b u i l d ( ) ; Make Builder a static inner class that calls the outer class s private constructor. John Jannotti (cs32) Design Patterns Feb 13, / 1

16 Builder Pattern for Collections Why must ImmutableMap offer a Builder class? 1 Map<S t r i n g, I n t e g e r > p o p u l a t i o n = 2 new ImmutableMap. B u i l d e r <S t r i n g, I n t e g e r >() 3. put ( AL, ) 4. put ( AK, ) put ( WY, ) 7. b u i l d ( ) ; John Jannotti (cs32) Design Patterns Feb 13, / 1

17 Builder Pattern for Collections Why must ImmutableMap offer a Builder class? 1 Map<S t r i n g, I n t e g e r > p o p u l a t i o n = 2 new ImmutableMap. B u i l d e r <S t r i n g, I n t e g e r >() 3. put ( AL, ) 4. put ( AK, ) put ( WY, ) 7. b u i l d ( ) ; You can t build an immutable map up bit by bit! (And you can t make a type safe varargs constructor that alternates types.) John Jannotti (cs32) Design Patterns Feb 13, / 1

18 Delegation Patterns Separate implementation and interface Multiple possible implementations Might want to reuse existing implementation Implementation might change later, be remote, be complex... Solutions Proxy real object is remote, protected, created on demand... Early hint: This is great for objects in databases. Object might be complex: Facade Translate an interface: Adapter Adding dynamic functionality: Decorator John Jannotti (cs32) Design Patterns Feb 13, / 1

19 ForwardingMultimap See ForwardingMultimap.java. It forwards every single method to a designated Multimap. 2 p u b l i c v o i d c l e a r ( ) { 3 d e l e g a t e ( ). c l e a r ( ) ; 4 } 6 p u b l i c M u l t i s e t <K> keys ( ) { 7 r e t u r n d e l e g a t e ( ). k eys ( ) ; 8 } John Jannotti (cs32) Design Patterns Feb 13, / 1

20 Decorator allows more flexibility than hierarchy 1 c l a s s LoggingMultimap<K, V> 2 e x t e n d s ForwardingMultimap<K, V> { 3 p r i v a t e Multimap<K, V> i n n e r ; 4 LoggingMultimap ( Multimap<K, V> mmap) { 5 i n n e r = mmap; 6 } p u b l i c v o i d d e l e g a t e ( ) { r e t u r n i n n e r ; } p u b l i c v o i d c l e a r ( ) { 9 s u p e r. c l e a r ( ) ; 0 System. out. p r i n t l n ( C l e a r e d a multimap ) ; 1 } 2 } You can replace a Multimap mm with new LoggingMultimap(mm) and carry on. And it works for any Multimap. (Guava provides at least 10 different implementations!) John Jannotti (cs32) Design Patterns Feb 13, / 1

21 Other Patterns Structural patterns Composite Group several of something into one of those things. Command Great for undo. Control patterns Iterator expose elements, but not container. Strategy embed algorithm in a class. Template algorithm with virtual hooks, a skeleton. Visitor apply operations to an object graph. Algorithmic patterns Observer publish-subscribe. Momento save/restore state. John Jannotti (cs32) Design Patterns Feb 13, / 1

22 Composite Pattern for Delivery Areas 1 a b s t r a c t p u b l i c c l a s s Area { 2 a b s t r a c t p u b l i c b o o l e a n c o n t a i n s ( LatLong x ) ; 3 } 4 p u b l i c c l a s s J o i n t A r e a e x t e n d s Area { 5 p r i v a t e L i s t <Area> a r e a s = new A r r a y L i s t <>(); 6 p u b l i c J o i n t A r e a ( Area... as ) { 7 f o r ( Area a : as ) 8 a r e a s. add ( a ) ; 9 } 0 p u b l i c boolean c o n t a i n s ( LatLong x ) { 1 f o r ( Area a r e a : a r e a s ) 2 i f ( a r e a. c o n t a i n s ( x ) ) 3 r e t u r n t r u e ; 4 r e t u r n f a l s e ; 5 } 6 } John Jannotti (cs32) Design Patterns Feb 13, / 1

23 Strategy in Autocorrect Your program will need to Create suggestions using a few different techniques. Rank those suggestions a couple different ways. We suggest using the Strategy pattern. Generator Ranker John Jannotti (cs32) Design Patterns Feb 13, / 1

24 Correcter.java 1 2 p u b l i c c l a s s C o r r e c t e r { 3 C o r r e c t e r ( G e n e r a t o r g, Ranker r ) {... } ; 4 S t r i n g s u g g e s t ( S t r i n g p r e v i o u s, S t r i n g l a s t ) { 5 L i s t <S t r i n g > s u g g e s t i o n s = g... ( p r e v i o u s, l a s t ) ; 6 C o l l e c t i o n s. s o r t ( s u g g e s t i o n s, r... ) ; 7 } 8 } Generator and Ranker are interfaces for Strategies abstract classes if you have a good reason How can you turn on LED and word splitting at the same time? How will you make the Comparator? Maybe the Corpus belongs in the Ranker? Maybe you need to return a Suggestion/Bigram instead of a String? Maybe there s an existing interface Ranker can implement? John Jannotti (cs32) Design Patterns Feb 13, / 1

25 Using Patterns When you run into a problem The solution isn t obvious But the problem feels common Might be encoded as a design pattern Awareness of patterns helps you notice alternatives For describing a design Experienced programmers have seen the same patterns Stating a pattern simplifies description Don t get it wrong! John Jannotti (cs32) Design Patterns Feb 13, / 1