Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

装饰模式可以，可以动态地为一个对象(注意是对象，不是类)添加功能，而不是通过继承来扩展整个类。

LCD- 最低共同点

/**
 * Lowest Common Denominator
 */
public interface LCD {
    void doIt();
}

核心被包装类

/**
 * 被包装的核心类实现LCD
 */
public class Core implements LCD {
    @Override
    public void doIt() {
        System.out.println("Core - I am closed for modification.");
    }
}

装饰器基类

/**
 * 抽象装饰器也实现LCD("is a LCD")，并且"has a LCD"
 */
public abstract class AbstractDecorator implements LCD {
    // has a LCD
    private LCD core;

    public AbstractDecorator(LCD inner) {
        core = inner;
    }

    public void doIt() {
        if (core != null) {
            core.doIt();
        }
    }
}

注意装饰器基类不仅是一个LCD，同时拥有一个LCD。

具体装饰器类

public class DecoratorA extends AbstractDecorator {
    public DecoratorA(LCD inner) {
        super(inner);
    }

    public void doIt() {
        super.doIt();
        doA();
    }

    private void doA() {
        System.out.println("A");
    }
}

public class DecoratorB extends AbstractDecorator {
    public DecoratorB(LCD inner) {
        super(inner);
    }

    public void doIt() {
        super.doIt();
        doB();
    }

    private void doB() {
        System.out.println("B");
    }
}

public class DecoratorC extends AbstractDecorator {
    public DecoratorC(LCD inner) {
        super(inner);
    }

    public void doIt() {
        super.doIt();
        doC();
    }

    private void doC() {
        System.out.println("C");
    }
}

执行

public class Test {
    public static void main(String[] args) {
        DecoratorA a = new DecoratorA(new Core());
        a.doIt();
        System.out.println("-----------------------------------");

        DecoratorB b = new DecoratorB(a);
        b.doIt();
        System.out.println("-----------------------------------");

        // 一层一层的包装
        // 不需要去继承Core类，就可以为其添加额外的功能
        DecoratorC c = new DecoratorC(new DecoratorB(new DecoratorA(new Core())));
        c.doIt();
    }
}

类图

decorator.png

参考：

• decorator
• proxy vs decorator