[OO 設計模式] Prototype Patterns : 原型模式 - 從物件建立另一個可定製物件且不需要知道建立細節

前言 :
轉載自 這裡
有些物件若以標準的方式建立實例，或者是設定至某個狀態需要複雜的運算及昂貴的資源，則您可以考慮直接以某個物件作為原型，在需要個別該物件時，複製原型並傳回。

請注意，在這邊Cloneable並非指Java中的java.lang.Cloneable，而是指支援原型複製的物件，必須實作之公開協定。
不 同的語言可能提供不同程度支援之物件複製技術，以Java而言，java.lang.Object本身即定義有clone()方法，因此所有的物件基本上 皆具自我複製之能力，不過真正要讓物件支援複製，則物件必須實作 java.lang.Cloneable 這個標示介面（Tag interface）

範例代碼 :
下面這個範例示範了，如何使用Java實作Prototype模式（建議您參考：How to avoid traps and correctly override methods from java.lang.Object）：

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1. package ch09.car;  
2.   
3. import java.util.*;  
4.   
5. class Wheel implements Cloneable{  
6.     public Wheel(){  
7.         System.out.println("Wheel constructor is called!");  
8.     };  
9.      // 也許還有一些複雜的設定  
10.     protected Object clone() throws CloneNotSupportedException {   
11.         return super.clone();   
12.     }   
13. }  
14.   
15. class Car implements Cloneable{  
16.     private String name="Unknown";  
17.       
18.     public Car(){  
19.         System.out.println("Car constructor is called!");  
20.     }  
21.     public Car(String n) {  
22.         name = n;  
23.         System.out.printf("Car:%s constructor is called!\n", n);  
24.     }  
25.     // 也許還有一些複雜的設定  
26.     private Wheel[] wheels = {new Wheel(), new Wheel(), new Wheel(), new Wheel()};  
27.       
28.     protected Object clone() throws CloneNotSupportedException {  
29.         Car copy = (Car) super.clone();  
30.         copy.wheels = (Wheel[]) this.wheels.clone();  
31.         for(int i = 0; i < this.wheels.length; i++) {  
32.             copy.wheels[i] = (Wheel) this.wheels[i].clone();  
33.         }  
34.         return copy;   
35.     }  
36.       
37.     public String getName(){  
38.         return name;  
39.     }  
40. }  
41.   
42. class Cars {  
43.     private Map prototypes = new HashMap();  
44.     void addPrototype(String brand, Car prototype) {  
45.         prototypes.put(brand, prototype);  
46.     }  
47.     Car getPrototype(String brand) throws CloneNotSupportedException {  
48.         return (Car) prototypes.get(brand).clone();  
49.     }  
50. }  
51.   
52. public class Main {  
53.     public static void main(String[] args) throws Exception {  
54.         Car bmw = new Car("BMW");  
55.         // 作一些 BMW 複雜的初始、設定、有的沒的  
56.         Car benz = new Car("BENZ");  
57.         // 作一些 BENZ 複雜的初始、設定、有的沒的  
58.         Cars cars = new Cars();  
59.         cars.addPrototype("BMW", bmw);  
60.         cars.addPrototype("BENS", benz);  
61.         // 取得 BMW 原型複製, Constructor 不會被呼叫.  
62.         System.out.println("\t>>>Clone Car Type:BMW");  
63.         Car bmwPrototype = cars.getPrototype("BMW");  
64.         System.out.printf("Prototype Car(%s)\n", bmwPrototype.getName());  
65.     }  
66. }  

執行結果 :

Wheel constructor is called!
Wheel constructor is called!
Wheel constructor is called!
Wheel constructor is called!
Car:BMW constructor is called!
Wheel constructor is called!
Wheel constructor is called!
Wheel constructor is called!
Wheel constructor is called!
Car:BENZ constructor is called!
>>>Clone Car Type:BMW <執行Clone 時,物件的Constructor 不會被執行 >
Prototype Car(BMW)

補充說明 :
* Wiki : Prototype pattern

The prototype pattern is a creational design pattern used in software development when the type of objects to create is determined by a prototypical instance, which is cloned to produce new objects. This pattern is used to:
- avoid subclasses of an object creator in the client application, like the abstract factory pattern does.
- avoid the inherent cost of creating a new object in the standard way (e.g., using the 'new' keyword) when it is prohibitively expensive for a given application.

This message was edited 8 times. Last update was at 17/05/2010 13:32:35