CS 520/620 Advanced Software Engineering Spring February 02, 2016

Transcription

1 CS 520/620 Advanced Software Engineering Spring 2016 February 02, 2016

2 Logistics Project proposals Elevator pitches on Thursday Deadline for creating groups: 02/03/2016, 11:55pm Deadline for submitting slides: 02/04/2016, 8:00am 2 minutes (~2 slides, a single pdf). Slides should clearly indicate: 520 or 620 project (e.g., 520: My awesome project). the current group members. Time to discuss ideas and (re-)group after presentations.

3 Recap Design patterns What is a design pattern? Categories of design patterns Structural patterns Creational patterns Behavioral patterns Two design problems & potential solutions Complex View -> composite pattern Input streams -> decorator pattern

4 Recap Design patterns What is a design pattern? Categories of design patterns Structural patterns Creational patterns Behavioral patterns Two design problems & potential solutions Complex View -> composite pattern Input streams -> decorator pattern

5 Today More on design patterns Decorator pattern revisited Behavioral patterns Template method Strategy

6 Decorator vs. composite 1 Component + operation() 1..n Decorator CompA Composite + operation() + operation() + operation() + addcomp(c:component) + removecomp(c:component)

7 Decorator pattern revisited InputStream is = new FileInputStream(); int b; while((b=is.read())!= -1) { // do something } InputStream Suppose the client needs 1 byte at a time FileInputStream

8 Decorator pattern revisited InputStream is = new FileInputStream(); int b; while((b=is.read())!= -1) { // do something } InputStream FileInputStream

9 Decorator pattern revisited InputStream is = new FileInputStream(); int b; while((b=is.read())!= -1) { // do something } InputStream Only half the truth FileInputStream

10 Decorator pattern revisited InputStream is = new FileInputStream(); int b; while((b=is.read())!= -1) { // do something } InputStream + read(buf:byte[]):int Only half the truth FileInputStream + read(buf:byte[]):int

11 Decorator pattern revisited InputStream is = new FileInputStream(); int b; while((b=is.read())!= -1) { // do something } InputStream + read(buf:byte[]):int FileInputStream Problem: filesystem IO is expensive + read(buf:byte[]):int

12 Decorator pattern revisited InputStream is = new FileInputStream(); int b; while((b=is.read())!= -1) { // do something } InputStream + read(buf:byte[]):int FileInputStream + read(buf:byte[]):int Problem: filesystem IO is expensive Solution: use a buffer! Why not simply implement the buffering in the client or subclass?

13 Decorator pattern revisited InputStream is = new BufferedInputStream( new FileInputStream()); int b; while((b=is.read())!= -1) { // do something } InputStream + read(buf:byte[]):int 1 FileInputStream + read(buf:byte[]):int BufferedInputStream - buffer:byte[] + read():byte + read(buf:byte[]):int

14 Find the median in an array of doubles Examples: median([1, 2, 3, 4, 5]) = 3 median([1, 2, 3, 4]) = 2.5 Algorithm: Input: array of length n Output: median

15 Find the median in an array of doubles Examples: median([1, 2, 3, 4, 5]) = 3 median([1, 2, 3, 4]) = 2.5 Algorithm: Input: array of length n Output: median 1. Sort array 2. if n is odd return ((n+1)/2)th element otherwise return arithmetic mean of (n/2)th element and ((n/2)+1)th element

16 One possible solution: template method AbstractStats + median(a:double[]):double + sort(a:double[]) The template method (median) implements the algorithm but leaves the sorting of the array open. MyStats + sort(a:double[]) The concrete subclass only needs to implement the actual sorting.

17 Another possible solution: strategy Stats + median(a:double[]):double 1 Sorter + sort(array:double[]) MyStats - sortstrategy:sorter HeapSort QuickSort + median(a:double[]):double MyStats delegates the sorting of the array to a strategy, which can be configured and changed at runtime.

18 Template method vs. strategy Two solutions to the same problem Template method Behavior selected at compile time. Don t call us, we ll call you. Template method is usually final. Strategy Behavior selected at runtime. Composition over inheritance.