軟件設(shè)計(jì)方法-yuzhou

1、創(chuàng)建型設(shè)計(jì)模式,,工廠方法模式 (factory method pattern)單例模式（singleton pattern）抽象工廠模式 (abstract factory pattern)生成器模式 (builder pattern)原型模式（prototype pattern）,目錄,工廠方法模式,工廠方法模式實(shí)例：,采用new的if-else設(shè)計(jì),工廠方法模式,工廠方法模式實(shí)例(2)：,客戶(hù)端代碼,工廠方法模式實(shí)

2、例(3)：,8,單例模式,Singleton: a class that has only one instance,9,Singleton pattern,singleton: an object that is the only object of its typeensures that a class has at most one instanceprovides a global access point to that

3、 instancetakes responsibility of managing that instance away from the programmer (illegal to construct more instances)provide accessor method that allows users to see the (one and only) instancepossibly the most kn

4、own / popular design pattern! (this should tell you something),10,Restricting objects, continued,One way to avoid creating objects:use static methods insteadMath, System, JOptionPaneis this a good alternative choice?

5、 Why or why not?,Problem: lacks flexibilityExample: static methods can't be passed as an argument to a method, nor returnedProblem: cannot be extendedExample: static methods can't be subclassed and overridde

6、n like a singleton's could be,11,make constructor(s) private so that they can not be called from outsidedeclare a single static private instance of the classwrite a public getInstance() or similar method that all

7、ows access to the single instancepossibly protect / synchronize this method to ensure that it will work in a multi-threaded program,Implementing Singleton,12,Singleton sequence diagram,13,consider a singleton class Rand

8、omGenerator that generates random numberspublic class RandomGenerator { private static RandomGenerator gen = new RandomGenerator(); public static RandomGenerator getInstance() { return gen; }

9、 private RandomGenerator() {} ...}possible problem: always creates the instance, even if it isn't used,Singleton example,14,variation: don't create the instance until needed// Generates random numbers

10、.public class RandomGenerator { private static RandomGenerator gen = null; public static RandomGenerator getInstance() { if (gen == null) { gen = new RandomGenerator(); } re

11、turn gen; } ...}What could go wrong with this version?,Singleton example 2,15,variation: solve concurrency issue by locking// Generates random numbers.public class RandomGenerator { private static Rand

12、omGenerator gen = null; public static synchronized RandomGenerator getInstance() { if (gen == null) { gen = new RandomGenerator(); } return gen; } ...}Is anything wro

13、ng with this version?,Singleton example 3,16,variation: solve concurrency issue without unnecessary locking// Generates random numbers.public class RandomGenerator { private static RandomGenerator gen = null;

14、 public static RandomGenerator getInstance() { if (gen == null) { synchronized (RandomGenerator.class) { // must test again -- can you see why? // sometimes called tes

15、t-and-test-and-set (TTS) if (gen == null) { gen = new RandomGenerator(); } } } return gen; }},Singleton example 4,抽象工廠（Abstract Factory