10.
OO原則:好萊塢原則——別找(調用)我,我會(調用)找你。
OO模式:模板方法模式——在一個方法中定義一個算法的骨架,而將一些步驟延遲到子類中。模板方法使得子類可以在不改變算法結構的情況下,重新定義算法中的某些步驟。
例子:沖泡咖啡和茶的準備工作
public abstract class CaffeineBeverage {
final void prepareRecipe() {
boilWater();
brew();
pourInCup();
addCondiments();
}
abstract void brew();
abstract void addCondiments();
void boilWater() {
System.out.println("Boiling water");
}
void pourInCup() {
System.out.println("Pouring into cup");
}
}
public class Coffee extends CaffeineBeverage {
public void brew() {
System.out.println("Dripping Coffee through filter");
}
public void addCondiments() {
System.out.println("Adding Sugar and Milk");
}
}
public class Tea extends CaffeineBeverage { public void brew() { System.out.println("Steeping the tea"); } public void addCondiments() { System.out.println("Adding Lemon"); } }
以下是上述類的關係圖:
從上面我們可以看到,prepareRecipe()就是我們的模板方法,而brew()和addCondiments()需要子類提供方法實現。
public abstract class CaffeineBeverageWithHook {
final void prepareRecipe() {
boilWater();
brew();
pourInCup();
if (customerWantsCondiments()) {
addCondiments();
}
}
abstract void brew();
abstract void addCondiments();
void boilWater() {
System.out.println("Boiling water");
}
void pourInCup() {
System.out.println("Pouring into cup");
}
boolean customerWantsCondiments() {
return true;
}
}
import java.io.*;
public class CoffeeWithHook extends CaffeineBeverageWithHook {
public void brew() {
System.out.println("Dripping Coffee through filter");
}
public void addCondiments() {
System.out.println("Adding Sugar and Milk");
}
public boolean customerWantsCondiments() {
String answer = getUserInput();
if (answer.toLowerCase().startsWith("y")) {
return true;
} else {
return false;
}
}
private String getUserInput() {
String answer = null;
System.out.print("Would you like milk and sugar with your coffee (y/n)? ");
BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
try {
answer = in.readLine();
} catch (IOException ioe) {
System.err.println("IO error trying to read your answer");
}
if (answer == null) {
return "no";
}
return answer;
}
}
在此比較一下策略模式、模板方法模式、工廠方法模式:
策略模式:封裝可互換的行爲,然後使用委託來決定要採用哪一個行爲;(用組合實現)
模板方法模式:子類決定如何是想算法中的步驟;(用繼承實現)
工廠方法模式:由子類決定實例化那個具體類。(模板方法模式的一個特殊版本)