大B:“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(ShoppingCat)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。”
小A:“一般會有哪些情況哩?”
大B:“比如:1、可能對所有的兒童類圖書實行每本一元的折扣;2、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;3、剩餘的圖書沒有折扣。4、還會有新的打折策略。由於有這樣複雜的折扣算法,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算法
*/
publicabstractclassBook……{
privatedoubleprice;
privateStringname;
publicStringgetName()……{
returnname;
}
publicvoidsetName(Stringname)……{
this.name=name;
}
publicdoublegetPrice()……{
returnprice;
}
publicvoidsetPrice(doubleprice)……{
this.price=price;
}
publicabstractdoublegetSalePrice();
}
publicclassCsBookextendsBook……{
publicCsBook(Stringname,doubleprice)
{
this.setName(name);
this.setPrice(price);
}
publicdoublegetSalePrice()
……{
returnthis.getPrice()*0.93;
}
}
publicclassChildrenBookextendsBook……{
publicChildrenBook(Stringname,doubleprice)……{
this.setName(name);
this.setPrice(price);
}
publicdoublegetSalePrice()……{
returnthis.getPrice()-1;
}
}
publicclassClient……{
publicstaticvoidmain(Stringargs[])
……{
BookcsBook1=newCsBook(“Thinkinjava”,45);
BookchildrenBook1=newChildrenBook(“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算法,即使打折算法相同。所以codereuse不好
方案二:
//把打折策略代碼提到父類來實現codereuse
publicabstractclassBook……{
privatedoubleprice;
privateStringname;
publicStringgetName()……{
returnname;
}
publicvoidsetName(Stringname)……{
this.name=name;
}
publicdoublegetPrice()……{
returnprice;
}
publicvoidsetPrice(doubleprice)……{
this.price=price;
}
//銷售價格
publicstaticdoubletoSalePrice(Bookbook)
……{
if(bookinstanceofChildrenBook)
……{
returnbook.getPrice()-1;
}
elseif(bookinstanceofCsBook)
……{
returnbook.getPrice()*0.93;
}
return0;
}
}
publicclassClient……{
publicstaticvoidmain(Stringargs[])
……{
BookcsBook1=newCsBook(“Thinkinjava”,45);
BookchildrenBook1=newChildrenBook(“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代碼。
方案三:策略模式
codereuse時最好用合成(HAS-A)而不用(IS-A),更加靈活。
publicabstractclassBook……{
privatedoubleprice;
privateStringname;
privateDiscountStrategydiscountStrategy;//折扣策略
publicStringgetName()……{
returnname;
}
publicvoidsetName(Stringname)……{
this.name=name;
}
publicdoublegetPrice()……{
returnprice;
}
publicvoidsetPrice(doubleprice)……{
this.price=price;
}
publicDiscountStrategygetDiscountStrategy()……{
returndiscountStrategy;
}