Object-Oriented Design. NCCU Fall Lecture 4-2: Design Patterns-I Oct

Transcription

1 NCCU Fall 2005 Lecture 4-2: Design Patterns-I Oct

2 Topics One form of Class-to-Class Coupling Factory Pattern and Singleton pattern Abstract Factory Pattern 2

3 Problem: Constructors create objects Constructors make objects. Only constructors can make objects. When you call a constructor of a class, you will get an instance of that class. public ClassC { ClassA ana = new ClassA( ); This class name needs to appear everywhere you need the constructor It has to be changed everywhere it appears if you decide to substitute a different class One form of Class-to-Class Coupling 3

4 Need a Flexible Object Creation Scheme Sometimes you want more flexibility than that You may want to guarantee that you can never have more than one object of a given class You may want to create an object only if you don t already have an equivalent object You may want to create an object without being sure exactly what kind of object you want The key insight is that, although only constructors make objects, you don t have to call constructors directly you can call a method that calls the constructor for you Several creational Design Patterns are based on this observation 4

5 Singleton A Singleton is a class that can have only one instance You may want just one instance of a null object, which you use in many places You may want to create just one AudioStream, so you can only play one tune at a time class Singleton { private static Singleton instance = new Singleton(); // don t let Java give you a default public constructor private Singleton() { Singleton getinstance() { return instance;... Client code: Singleton sa= Singleton.getInstance() ; 5

6 public class Singleton { protected Singleton() { Another Style //... static private Singleton _instance = null; static public Singleton instance() { if (null == _instance) { _instance = new Singleton(); return _instance; 6

7 The Factory Method Design Pattern Suppose you write a class that works with several different kinds of objects You can do this if the classes all have a common interface You may want to be able to create objects, without being dependent on the kind of object A factory methods can create instances of different classes, depending (say) on its parameters Example: Image createimage (String ext) { if (ext.equals("gif")) return new GIFImage(); if (ext.equals("jpg")) return new JPEGImage();... 7

8 Factory pattern: sample code public class Parent { protected String first; protected String second; public class Child1 extends Parent { public class Child2 extends Parent { public class ParentFactory { public Parent getparent(string argument) // Factory method { if (argument.equals( )) return new Child1(argument); else return new Child2(argument); 8

9 Creating Account Objects in the Banking System Example Account selectedaccount = new Account(accountID);?? Add a createaccount(string) method to the Bank class. Client: Bank.createAccount( Savings ).calculateinterest(); abstract calculateinterest() calculateinterest() calculateinterest() calculateinterest() 9

10 Design Exercise A B A1 A2 B1 B2 getthreshold() getthreshold() 10

11 Abstract Factory Pattern 11

12 Factories ( ) new product object Client constructs a new factory object by calling the constructor for the factory class Client asks the factory object to construct product objects the objects the client really needs by calling a non-constructor method (usually a method called create) 12

13 Implementing the Abstract Factory Pattern For every product interface, there is a corresponding factory interface It has one method: create For every product class that implements the product interface, there is a corresponding factory class that implements the factory interface It has one method: create 13

14 Complex Numbers In Math, we have at least two different representations for Complex numbers. re = 4 this = (4,3) (Real, Imaginative) (Magnitude, Angel) im = 3 this = (4,3) mag = 5 ang = arctan(3/4) Each one has its advantages and weakness. 14

15 IComplex Interface <<interface>> IComplex + complex + set (re: double, im: double) + getre ( ): double + getim ( ): double + clear ( ) + tostring ( ): String + equals (x: Object): boolean + clone ( ): Object Complex_2 Complex_1 15

16 Example: Complex Numbers <<interface>> IComplexFactory + create ( ): IComplex <<interface>> IComplex ComplexFactory_1 + create ( ): IComplex Complex_1 ComplexFactory_2 + create ( ): IComplex Complex_2 16

17 Client Code Original client code using Complex_1: IComplex c1 = new Complex_1 ();... IComplex c2 = new Complex_1 (); Client code using abstract factory pattern: IComplexFactory complexfactory = new ComplexFactory_1 ();... IComplex c1 = complexfactory.create ();... IComplex c2 = complexfactory.create (); 17

18 Factory Interface Code package complex; public interface IComplexFactory { public IComplex create (); 18

19 package complex; Factory Class Code public class ComplexFactory_1 implements IComplexFactory { public IComplex create () { return (IComplex) new Complex_1 (); 19

20 Clearer Figurgre Family: ProductA and ProductB One factory method For each class 20

21 The Bank Factory Provide an interface for creating families of related or dependent objects without specifying their concrete classes Factory Methods 21

22 The MegaBankFactory *Only one instance of a ConcreteFactory (Singleton) per product family 22

23 Code Snippet public abstract class BankFactory { abstract public Customer createcustomer (String loginname, String password); abstract public Account createaccount(string AccountID); abstract public Bank createbank(); // other methods and data members Capture family creation in a class containting a factory method for each class in the family. 23

24 public class MegaBankFactory extends BankFactory { public Customer createcustomer(string loginname, String password) { // MegaBank specific code goes here public Account createaccount(string AccountID) { // MegaBank specific code goes here public Bank createbank() { // MegaBank specific code goes here // other methods and data members 24

25 Client Code // first create an appropriate factory BankFactory factory = new MegaBankFactory(); // somehow, get the login and password info String loginname = getloginname(); String password = getpassword(); // ask the factory for a Customer object Customer currentcustomer = factory.createcustomer(loginname,password); // remember that currentcustomer might be null // if the credentials are invalid 25