设计模式之装饰者模式

什么是装饰者模式

装饰者模式动态地将责任附加到对象上。若要扩展功能，装饰者提供了比继承更有弹性的替代方案。这就是面向对象设计中常提到的优先使用组合而不是继承。

UML图

图中抽象类Component是被装饰者基类，ConcreteComponent是将要动态地加上新行为的对象，它扩展了Component。Decorator是装饰者共同实现的接口或者是抽象类。ConcreteDecoratorA和ConcreteDecoratorB都是实现或扩展了Decorator的装饰者，它们都有一个Component的私有实例变量，实现了抽象类的方法，并且可以加上新的方法从而实现扩展功能。

代码实现

Component

/** * Created by yrs on 2017/7/2. */public abstract class Component {    public abstract void say();}

ConcreteComponent

/** * Created by yrs on 2017/7/2. */public class ConcreteComponent extends Component {    @Override    public void say() {        System.out.println("this is ConcreteComponent.");    }}

Decorator

/** * Created by yrs on 2017/7/2. */public abstract class Decorator extends Component {}

ConcreteDecoratorA

/** * Created by yrs on 2017/7/2. */public class ConcreteDecoratorA extends Decorator {    private Component component;    public ConcreteDecoratorA(Component component) {        this.component = component;    }    @Override    public void say() {        preSay();        component.say();        sufSay();    }    public void preSay() {        System.out.println("Welcome everyone!");    }    public void sufSay() {        System.out.println("Thank you!");    }}

ConcreteDecoratorB

/** * Created by yrs on 2017/7/2. */public class ConcreteDecoratorB extends Decorator {    private Component component;    public ConcreteDecoratorB(Component component) {        this.component = component;    }    @Override    public void say() {        component.say();        sayHelp();    }    public void sayHelp() {        System.out.println("please help me!");    }}

Test

/** * Created by yrs on 2017/7/2. */public class Test {    public static void main(String [] args) {        //用ConcreteDecoratorA类装饰ConcreteComponent对象        component = new ConcreteDecoratorA(component);         //用ConcreteDecoratorB类装饰ConcreteComponent对象        component = new ConcreteDecoratorB(component);        component.say();    }}

装饰者模式的重点是装饰者有一个被装饰者的实例变量

Java IO中的装饰者模式

代码下载：https://github.com/ByrsH/Design-Patterns/tree/master/Design%20Patterns/src/main/java/com/yrs/decorator