How do I make the method return type generic?

Question

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?

Answer 1

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.

Answer 2

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();