Decorator pattern

1. 定义，来自wiki（http://en.wikipedia.org/wiki/Decorator_pattern）

The decorator pattern can be used to extend (decorate) the functionality of a certain object statically, or in some cases at run-time, independently of other instances of the same class, provided some groundwork is done at design time. This is achieved by designing a new decoratorclass that wraps the original class. This wrapping could be achieved by the following sequence of steps:

1. Subclass the original "Component" class into a "Decorator" class (see UML diagram);
2. In the Decorator class, add a Component pointer as a field;
3. Pass a Component to the Decorator constructor to initialize the Component pointer;
4. In the Decorator class, redirect all "Component" methods to the "Component" pointer; and
5. In the ConcreteDecorator class, override any Component method(s) whose behavior needs to be modified.

2. 实例，来自wiki（http://en.wikipedia.org/wiki/Decorator_pattern）

// The abstract Coffee class defines the functionality of Coffee implemented by decorator
public abstract class Coffee {
    public abstract double getCost(); // Returns the cost of the coffee
    public abstract String getIngredients(); // Returns the ingredients of the coffee
}
 
// Extension of a simple coffee without any extra ingredients
public class SimpleCoffee extends Coffee {
    public double getCost() {
        return 1;
    }
 
    public String getIngredients() {
        return "Coffee";
    }
}

// Abstract decorator class - note that it extends Coffee abstract class
public abstract class CoffeeDecorator extends Coffee {
    protected final Coffee decoratedCoffee;
    protected String ingredientSeparator = ", ";
 
    public CoffeeDecorator (Coffee decoratedCoffee) {
        this.decoratedCoffee = decoratedCoffee;
    }
 
    public double getCost() { // Implementing methods of the abstract class
        return decoratedCoffee.getCost();
    }
 
    public String getIngredients() {
        return decoratedCoffee.getIngredients();
    }
}

// Decorator Milk that mixes milk with coffee.
// Note it extends CoffeeDecorator.
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";
    }
}
 
// Decorator Whip that mixes whip with coffee.
// Note it extends CoffeeDecorator.
class Whip extends CoffeeDecorator {
    public Whip (Coffee decoratedCoffee) {
        super(decoratedCoffee);
    }
 
    public double getCost() {
        return super.getCost() + 0.7;
    }
 
    public String getIngredients() {
        return super.getIngredients() + ingredientSeparator + "Whip";
    }
}
 
// Decorator Sprinkles that mixes sprinkles with coffee.
// Note it extends CoffeeDecorator.
class Sprinkles extends CoffeeDecorator {
    public Sprinkles (Coffee decoratedCoffee) {
        super(decoratedCoffee);
    }
 
    public double getCost() {
        return super.getCost() + 0.2;
    }
 
    public String getIngredients() {
        return super.getIngredients() + ingredientSeparator + "Sprinkles";
    }
}

测试类

public class Main {
 
    public static final 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());
 
    c = new Sprinkles(c);
    System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());
 
    c = new Whip(c);
    System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());
 
    // Note that you can also stack more than one decorator of the same type
    c = new Sprinkles(c);
    System.out.println("Cost: " + c.getCost() + "; Ingredients: " + c.getIngredients());
    }
 
}

输出结果

Cost: 1.0; Ingredients: Coffee
Cost: 1.5; Ingredients: Coffee, Milk
Cost: 1.7; Ingredients: Coffee, Milk, Sprinkles
Cost: 2.4; Ingredients: Coffee, Milk, Sprinkles, Whip
Cost: 2.6; Ingredients: Coffee, Milk, Sprinkles, Whip, Sprinkles

3. 优缺点（http://tianli.blog.51cto.com/190322/35287/）

Decorator模式有以下的优缺点：

1.      比静态继承更灵活 与对象的静态继承相比，Decorator模式提供了更加灵活的向对象添加职责的方式，可以使用添加和分离的方法，用装饰在运行时刻增加和删除职责。使用继承机制增加职责需要创建一个新的子            类，如果需要为原来所有的子类都添加功能的话，每个子类都需要重写，增加系统的复杂度，此外可以为一个特定的Component类提供多个Decorator，这种混合匹配是适用继承很难做到的。

2.      避免在层次结构高层的类有太多的特征，Decorator模式提供了一种“即用即付”的方法来添加职责，他并不试图在一个复杂的可订制的类中支持所有可预见的特征，相反可以定义一个简单的类，并且用Decorator            类给他逐渐的添加功能，可以从简单的部件组合出复杂的功能。

3.      Decorator 与它的Component不一样 Decorator是一个透明的包装，如果我们从对象标识的观点出发，一个被装饰了的组件与这个组件是有差别的，因此使用装饰时不应该以来对象标识。

4.      产生许多小对象，采用Decorator模式进行系统设计往往会产生许多看上去类似的小对象，这些对象仅仅在他们相互连接的方式上有所不同。