装饰模式(Decorator)

意图：动态的给一个对象添加一些额外的职责，就增加的功能来说，Decorator模式相比生成子类更为灵活。
别名：Wrapper
结构图如下：

Decorator维持一个指向Component对象的引用，并定义一个与Component接口一致的接口。
ConcreteDecorator则是负责向ConcreteComponent添加职责的类

Decorator将Operation()请求转发给它的Component对象，并有可能在转发请求前后执行一些附加的操作。

示例代码如下

public abstract class Component {
	public abstract void Operation();
}

public class Decorator extends Component {
	protected Component component;
	@Override
	public void Operation() {
		// TODO Auto-generated method stub
		if (component != null) {
			component.Operation();
		}
	}

	public void Decorate(Component component) {
		this.component = component;
	}
}

public class ConcreteComponent extends Component {

	@Override
	public void Operation() {
		// TODO Auto-generated method stub
		System.out.println("ConcreteComponent Operation");
	}

}

public class ConcreteDecoratorA extends Decorator {
	private String addedState;
	
	@Override
	public void Operation() {
		super.Operation();
		addedState = "Add a New State";
		System.out.println("ConcreteDecoratorA Decorate Operation");
	}
	
}

public class ConcreteDecoratorB extends Decorator {
	@Override
	public void Operation() {
		super.Operation();
		AddedBehavior();
		System.out.println("ConcreteDecoratorB Decorate operation");
	}
	
	private void AddedBehavior() {
		System.out.println("ConcreteDecoratorB private behavior");
	}
}

客户端使用

public class Client {
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		ConcreteComponent obj = new ConcreteComponent();
		ConcreteDecoratorA da = new ConcreteDecoratorA();
		ConcreteDecoratorB db = new ConcreteDecoratorB();
		
		System.out.println("Decorate obj");
		da.Decorate(obj);
		db.Decorate(da);
		
		db.Operation();
	}
}

运行结果

Decorate obj
ConcreteComponent Operation
ConcreteDecoratorA Decorate Operation
ConcreteDecoratorB private behavior
ConcreteDecoratorB Decorate operation

总结
1. 装饰模式符合开－闭原则，如果需要增加新的功能，只需要增加一个ConcreteDecoratorC等等，不需要修改其它的类，只要在客户端实例化一个ConcreteDecoratorC对象即可

ConcreteDecoratorC dc = new ConcreteDecoratorC();
dc.Decorate(db);

dc.Operation();

2. 调整ConcreteDecorator中Operation实现时父类Operation与子类操作的顺序，可以调整装饰的操作顺序

public class ConcreteDecoratorA extends Decorator {
	private String addedState;
	
	@Override
	public void Operation() {
		addedState = "Add a New State";
		System.out.println("ConcreteDecoratorA Decorate Operation");
                super.Operation();
	}
}

public class ConcreteDecoratorB extends Decorator {
	@Override
	public void Operation() {
		AddedBehavior();
		System.out.println("ConcreteDecoratorB Decorate operation");
		super.Operation();
	}
	
	private void AddedBehavior() {
		System.out.println("ConcreteDecoratorB private behavior");
	}
}

运行结果

Decorate obj
ConcreteDecoratorB private behavior
ConcreteDecoratorB Decorate operation
ConcreteDecoratorA Decorate Operation
ConcreteComponent Operation