java - 有条件地实例化不同的类

Question

假设我有以下类(class):

1) Animal类，基类

public abstract class Animal {
    public abstract void Hello();
}

2)Dog类，继承自Animal类

public class Dog extends Animal {
    @Override
    public void Hello() {
        System.out.println("I'm Dog");
    }
}

3) Wolf 类，继承自 Animal 类

public class Wolf extends Animal {
    @Override
    public void Hello() {
        System.out.println("I'm Wolf");
    }
}

在 Dog 类的构造函数或 newInstance 方法中，是否有办法在特定条件下(例如满月期间)将自身切换/变成 Wolf 类？因此，基本上，如果调用者执行如下操作，则变量“a”在满月期间将是 Wolf 对象，而不是 Dog 对象。

Animal a = (Animal) Class.forName("Dog").newInstance();

Answer 1

您可以使用 Animal 的包装器来委托(delegate)给您希望跟踪的实际 Animal；然后在委托(delegate)包装器中包含一个方法，用于根据您想要的 Animal 的类名交换委托(delegate)。这是一个示例类:

public abstract class Animal {

    public abstract String hello();

    static class Dog extends Animal {

        @Override
        public String hello() {
            return "I'm Dog";
        }
    }

    static class Wolf extends Animal {

        @Override
        public String hello() {
            return "I'm Wolf";
        }
    }

    static class AnimalWrapper extends Animal {

        private Animal delegate = new Dog();

        public void delegateTo(String className) throws Exception {
            this.delegate = (Animal) Class.forName(className).newInstance();
        }

        @Override
        public String hello() {
            return delegate.hello();
        }
    }

    public static void main(String[] args) throws Exception {
        AnimalWrapper animal = new AnimalWrapper();
        System.out.println("Animal says: " + animal.hello());
        animal.delegateTo(Wolf.class.getName());
        System.out.println("Animal says: " + animal.hello());
        animal.delegateTo(Dog.class.getName());
        System.out.println("Animal says: " + animal.hello());
    }
}

Complete code on GitHub