Generic Method in Non-Generic Class

Question

I am trying to use a generic method so I don't have to repeat code. I have tried:

private Listener createListenerAdapter(Class<T> clazz)
{ 
   // do something
}

(clazz being important because class is reserved).

But Netbeans complains that : "Cannot find symbol class T".

I am going to be passing in a few different classes that have the same methods on them. Where am I supposed to define T?

Solution 1

Declare the method as:

private <T> Listener createListenerAdapter(Class<T> clazz)

See the Java Tutorials for more information.

Edit: If T isn't related to the return type you could also just use a wildcard:

private Listener createListenerAdapter(Class<?> clazz)

Edit 1: If clazz is meant to represent a type of Listener, you could define bounds to restrict the caller (to avoid casts and potential runtime exceptions):

private <L extends Listener> L createListenerAdapter(Class<L> clazz)

Or with the wildcard:

private Listener createListenerAdapter(Class<? extends Listener> clazz)

But that depends on how clazz is used in the body.

Solution 2

Generic declarations can also be made at method-level by parametrizing them like this:

private <T> Listener createListenerAdapter(Class<T> clazz)
{ 
   // do something
}

Comments

[Admin]

clazz is a GUI widget in GWT.

[Paul Bellora]

Okay, so probably Class<? extends Widget> is the best fit.

[Admin]

Is there a way to call a method on clazz inside createListenerAdapter() that corresponds to a method inside the class? So if I pass in ButtonWidget I want to be able to call clazz.setEnable(true). All widgets have the same method call. I am passing in ButtonWidget as createListenerAdapter(myWidget.getClass()); clazz.setEnable(true) does not work because it thinks I am asking for a method on Class<?>

[Paul Bellora]

setEnable is an instance method right? You would need to use clazz to instantiate a new Widget and then call the method on it.

[Admin]

Yes, it is an instance method. I have several classes that derive from the same superclass, but is there a way to tell which subclass was passed in? I tried Class<? extends MySuperClass> but that doesn't give me the subclass. There are several different subclasses.

[Paul Bellora]

Well the idea behind polymorphism is that you shouldn't have to know. If they share a common interface/superclass you can just treat them all like that and not worry about their implementation details.

[Paul Bellora]

let us continue this discussion in chat