行為型模式--策略模式(Strategy)
阿新 • • 發佈：2019-01-04
一：定義:
Strategy:Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
二：引入
假設現在要設計一個販賣各類書籍的電子商務網站的購物車（Shopping Cat）系統。一個最簡單的情況就是把所有貨品的單價乘上數量，但是實際情況肯定比這要複雜。
比如:
- 本網站可能對所有的兒童類圖書實行每本一元的折扣；
- 對計算機類圖書提供每本7％的促銷折扣，而對電子類圖書有3％的折扣；
- 對其餘的圖書沒有折扣。
- 未來可能還會有新的打折策略。
由於有這樣複雜的折扣演算法，使得價格計算問題需要系統地解決。
方案一：業務邏輯放在各具體子類
/*
*各子類實現銷售價格演算法
*/publicabstractclass Book { privatedouble price; private String name; 
public String getName() {
return name;
} publicvoid setName(String name) { } publicdouble getPrice() { return price; } publicvoid setPrice(double price) { this.price = price; } publicabstractdouble getSalePrice() ; }
publicclass CsBook extends Book{ public CsBook(String name,double price) { this this.setPrice(price); } publicdouble getSalePrice() { returnthis.getPrice()*0.93; }}publicclass ChildrenBook extends Book { public ChildrenBook(String name, double price) { this.setName(name); this.setPrice(price); } publicdouble getSalePrice() { returnthis.getPrice() -1; }}publicclass Client { publicstaticvoid main(String args[]) { Book csBook1=new CsBook("Think in java",45); Book childrenBook1=new ChildrenBook("Hello",20); System.out.println(csBook1.getName()+":"+csBook1.getSalePrice()); System.out.println(childrenBook1.getName()+":"+childrenBook1.getSalePrice()); }}
問題：每個子類必須都各自實現打折演算法，即使打折演算法相同。所以code reuse不好
方案二：
//把打折策略程式碼提到父類來實現code reusepublicabstractclass Book { privatedouble price; private String name; public String getName() { return name; } publicvoid setName(String name) { this.name = name; } publicdouble getPrice() { return price; } publicvoid setPrice(double price) { this.price = price; }// 銷售價格publicstaticdouble toSalePrice(Book book) { if (book instanceof ChildrenBook) { return book.getPrice()-1; } elseif (book instanceof CsBook) { return book.getPrice()*0.93; } return0; }}publicclass Client { publicstaticvoid main(String args[]) { Book csBook1=new CsBook("Think in java",45); Book childrenBook1=new ChildrenBook("Hello",20); System.out.println(csBook1.getName()+":"+Book.toSalePrice(csBook1)); System.out.println(childrenBook1.getName()+":"+Book.toSalePrice(childrenBook1)); }}
toSalePrice方法是比較容易change的地方，如果策略複雜用if判斷比較亂，並且策略修改或增加時需改變原始碼。
方案三：策略模式
code reuse時最好用合成(HAS-A)而不用(IS-A)，更加靈活。
publicabstractclass Book { privatedouble price; private String name; private DiscountStrategy discountStrategy;//折扣策略 public String getName() { return name; } publicvoid setName(String name) { this.name = name; } publicdouble getPrice() { return price; } publicvoid setPrice(double price) { this.price = price; } public DiscountStrategy getDiscountStrategy() { return discountStrategy; } publicvoid setDiscountStrategy(DiscountStrategy discountStrategy) { this.discountStrategy = discountStrategy; } publicdouble getSalePrice() { return discountStrategy.getSalePrice(price); } }publicclass CsBook extends Book{ public CsBook(String name,double price) { this.setName(name); this.setPrice(price); }}publicabstractclass DiscountStrategy { publicabstractdouble getSalePrice(double orgPrice) ;}/* * 按折扣率打折 */publicclass PercentDiscountStrategy extends DiscountStrategy { privatedouble percent;//折扣率public PercentDiscountStrategy(double percent) { this.percent=percent; } publicdouble getPercent() { return percent;