Using Java generics in interfaces

Question

I'd like to create an interface that has the following methods:

public interface MyInterface {
  public Class<X> getClass();
  public void handle(X message);
}

Where X has to be the same type in both methods. The reason I need to do this is I'm getting messages that are blobs that need to be casted into the appropriate class (using getClass) and then I'd like to force implementers to implement a handle method that takes the appropriate class (instead of assuming there is one and using reflection to call it).

It seems like the only way to do this is to add a type param to the interface like so:

public interface MyInterface<T> {
...
}

but that is less than ideal because of cascading dependencies that would require specifying that type in various other classes. Any suggestions? Thanks!

+1 for interesting as a general question, though having a getClass method on your interface smells of implementation leak. — Perception
– Perception, Commented May 1, 2012 at 10:51
MyInterface.java:2: getClass() in MyInterface cannot override getClass() in java.lang.Object; overridden method is final public Class<X> getClass (); ^ — user unknown
– user unknown, Commented May 1, 2012 at 10:52
I don't see how your approach can be useful. Generics are purely a compiler artifact, so there's no sense in calling getClass to find the runtime type and then pushing that type into the static type parameter. — Marko Topolnik
– Marko Topolnik, Commented May 1, 2012 at 11:23

Cratylus · Accepted Answer · 2012-05-01 10:48:36Z

2

Where X has to be the same type in both methods

You can not enforce 2 methods of an interface to depend one another just by the definition.
getClass and handle will always be 2 separate methods.
What you perhaps should be doing is have 1 method, the handle and pass as a parameter a type of the class (could be something as simple as an Enum or any indicator) and delegate to a Factory to create the appropriate class to be used inside the handle method.
So handle method would (eventually) get the appropriate class to work with.

answered May 1, 2012 at 10:48

Cratylus

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beerbajay · Accepted Answer · 2012-05-01 10:55:47Z

2

You can specify generic type parameters for methods, like so:

public <X> void handle(X message);

But this won't ensure consistency across methods, so if you have:

public <X> Class<X> getClazz();
public <X> void handle(X message);

You can't say that the <X> is the same for both methods.

answered May 1, 2012 at 10:55

beerbajay

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2 Comments

Louis Wasserman Over a year ago

To be clear, if you provide a method public <X> Class<X> getClazz(), the caller of the method would get to specify X.

beerbajay Over a year ago

Oops, yeah I should have noted that. The caller could also therefore specify a different X per call.

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