裝飾模式: Decorator Pattern

概括: 動態地給一個物件新增一些額外的職責。單從增加功能來說，用裝飾模式比簡單使用子類繼承的方式來的靈活。

關鍵字: Has-a，

重點: 每個裝飾類都要實現定義好的實現功能的純虛裝飾函式。各個裝飾類都has-a一個擁有基本功能的類，通過對介面的實現，給has-a關係的基礎功能增加新的功能

類圖:

待裝飾的基類:

// The Coffee Interface defines the functionality of Coffee implemented by decorator
public interface Coffee {
    public
 
 double getCost(); // returns the cost of the coffee
    public String getIngredients(); // returns the ingredients of the coffee
}
 
// implementation of a simple coffee without any extra ingredients
public class SimpleCoffee implements Coffee {
    public double getCost() {
        return 1;
    }
 
    public
 
 String getIngredients() {
        return "Coffee";
    }
}

----------------------

虛裝飾類:

// abstract decorator class - note that it implements Coffee interface
abstract public class CoffeeDecorator implements Coffee {
    protectedfinal Coffee decoratedCoffee; // has-a一個待裝飾的基類
    protected String ingredientSeparator =
 
 ", ";
 
    public CoffeeDecorator(Coffee decoratedCoffee) {
        this.decoratedCoffee = decoratedCoffee;
    }
 
    public double getCost() { // implementing methods of the interface
        return decoratedCoffee.getCost();
    }
 
    public String getIngredients() {
        return decoratedCoffee.getIngredients();
    }
}

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實際增加功能的裝飾類,即提供給使用者使用的裝飾類：

// Decorator Milk that mixes milk with coffee
// note it extends CoffeeDecorator
public class Milk extends CoffeeDecorator {
    public Milk(Coffee decoratedCoffee) {
        super(decoratedCoffee);
    }
 
    public double getCost() { // overriding methods defined in the abstract superclass
        return super.getCost() + 0.5;
    }
 
    public String getIngredients() {
        return super.getIngredients() + ingredientSeparator + "Milk";
    }
}

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實際使用者子類:

public class Main
{
    public static void main(String[] args)
    {
        Coffee c = new SimpleCoffee();
        System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());
 
        c = new Milk(c);
        System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());
        
        ...

    }
}