How do I make the method return type generic?

Consider this example (typical in OOP books):

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.

Here's the Animal class:

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);
    }
}

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();

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();

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);        
}

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?

---

You could define callFriend this way:

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

Then call it as such:

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

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.

---

No. The compiler can't know what type jerry.callFriend("spike") would return. Also, your implementation just hides the cast in the method without any additional type safety. Consider this:

jerry.addFriend("quaker", new Duck());
jerry.callFriend("quaker", /* unused */ new Dog()); // dies with illegal cast

In this specific case, creating an abstract talk() method and overriding it appropriately in the subclasses would serve you much better:

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

jerry.callFriend("spike").talk();
jerry.callFriend("quacker").talk();

---

You could implement it like this:

@SuppressWarnings("unchecked")
public <T extends Animal> T callFriend(String name) {
    return (T)friends.get(name);
}

(Yes, this is legal code; see Java Generics: Generic type defined as return type only.)

The return type will be inferred from the caller. However, note the @SuppressWarnings annotation: that tells you that this code isn't typesafe . You have to verify it yourself, or you could get ClassCastExceptions at runtime.

Unfortunately, the way you're using it (without assigning the return value to a temporary variable), the only way to make the compiler happy is to call it like this:

jerry.<Dog>callFriend("spike").bark();

While this may be a little nicer than casting, you are probably better off giving the Animal class an abstract talk() method, as David Schmitt said.

链接地址: http://www.djcxy.com/p/50196.html

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