如何使方法返回类型通用? [英] How do I make the method return type generic?

问题描述

考虑这个例子(OOP 书中的典型例子):

Consider this example (typical in OOP books):

我有一个 Animal 类，其中每个 Animal 可以有很多朋友.
以及诸如 Dog、Duck、Mouse 等子类，它们添加了诸如 bark()、quack 之类的特定行为() 等

I have an Animal class, where each Animal can have many friends.
And subclasses like Dog, Duck, Mouse etc which add specific behavior like bark(), quack() etc.

这是 Animal 类:

public class Animal {
    private Map<String,Animal> friends = new HashMap<>();

    public void addFriend(String name, Animal animal){
        friends.put(name,animal);
    }

    public Animal callFriend(String name){
        return friends.get(name);
    }
}

这是一些包含大量类型转换的代码片段:

And here's some code snippet with lots of typecasting:

Mouse jerry = new Mouse();
jerry.addFriend("spike", new Dog());
jerry.addFriend("quacker", new Duck());

((Dog) jerry.callFriend("spike")).bark();
((Duck) jerry.callFriend("quacker")).quack();

有什么方法可以使用泛型作为返回类型来摆脱类型转换，以便我可以说

Is there any way I can use generics for the return type to get rid of the typecasting, so that I can say

jerry.callFriend("spike").bark();
jerry.callFriend("quacker").quack();

这是一些初始代码，返回类型作为从未使用过的参数传递给方法.

Here's some initial code with return type conveyed to the method as a parameter that's never used.

public<T extends Animal> T callFriend(String name, T unusedTypeObj){
    return (T)friends.get(name);        
}

有没有办法使用 instanceof 在没有额外参数的情况下在运行时找出返回类型?或者至少通过传递一个类型的类而不是一个虚拟实例.
我知道泛型用于编译时类型检查，但是否有解决方法?

Is there a way to figure out the return type at runtime without the extra parameter using instanceof? Or at least by passing a class of the type instead of a dummy instance.
I understand generics are for compile time type-checking, but is there a workaround for this?

推荐答案

你可以这样定义callFriend:

public <T extends Animal> T callFriend(String name, Class<T> type) {
    return type.cast(friends.get(name));
}

然后这样调用它:

jerry.callFriend("spike", Dog.class).bark();
jerry.callFriend("quacker", Duck.class).quack();

这段代码的好处是不会产生任何编译器警告.当然，这实际上只是从前通用时代开始的更新版本，并没有增加任何额外的安全性.

This code has the benefit of not generating any compiler warnings. Of course this is really just an updated version of casting from the pre-generic days and doesn't add any additional safety.

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