22) Null Object pattern

类别：

 

问题：

 

方案：

 

 

 

示例：

 

public class NullObjectPatternDemo {
    public static void main(String[] args) {

        Animal animal;

        animal = getAnimal("cat");
        animal.makeSound();

        animal = getAnimal("dog");
        animal.makeSound();

        animal = getAnimal("duck");
        animal.makeSound();
    }

    public static Animal getAnimal(String type) {
        Animal animal;
        switch (type) {
        case "dog": {
            animal = new Dog();
            break;
        }
        case "cat": {
            animal = new Cat();
            break;
        }
        default: {
            animal = new NullAnimal();
            break;
        }
        }
        return animal;
    }

}

interface Animal {
    void makeSound();
}

class Dog implements Animal {
    public void makeSound() {
        System.out.println("woof!");
    }
}

class Cat implements Animal {
    public void makeSound() {
        System.out.println("meowww..");
    }
}

class NullAnimal implements Animal {
    public void makeSound() {
        System.out.println("...");
    }
}

 

 

meowww..
woof!
...

 

应用：

 

不足：(

 

优化：）