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 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 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
You could define callFriend
this way:
public T callFriend(String name, Class 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.
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