Static method in a generic class?

Question

In Java, I'd like to have something as:

class Clazz<T> {
  static void doIt(T object) {
    // shake that booty
  }
}

But I get

Cannot make a static reference to the non-static type T

I don't understand generics beyond the basic uses and thus can't make much sense of that. It doesn't help that I wasn't able to find much info on the internet about the subject.

Could someone clarify if such use is possible, by a similar manner? Also, why was my original attempt unsuccessful?

Answer 1

Java doesn't know what T is until you instantiate a type.

Maybe you can execute static methods by calling Clazz.doit(something) but it sounds like you can't.

The other way to handle things is to put the type parameter in the method itself:

static <U> void doIt(U object)

which doesn't get you the right restriction on U, but it's better than nothing....

Answer 2

When you specify a generic type for your class, JVM know about it only having an instance of your class, not definition. Each definition has only parametrized type.

Generics work like templates in C++, so you should first instantiate your class, then use the function with the type being specified.

Answer 3

Others have answered your question already, but in addition I can thoroughly recomment the O'Reilly "Java Generics" book. It's a subtle and complex subject at times, and if often seems to have pointless restrictions, but the book does a pretty good job of explaining why java generics are the way they are.

Answer 4

You can't use a class's generic type parameters in static methods or static fields. The class's type parameters are only in scope for instance methods and instance fields. For static fields and static methods, they are shared among all instances of the class, even instances of different type parameters, so obviously they cannot depend on a particular type parameter.

It doesn't seem like your problem should require using the class's type parameter. If you describe what you are trying to do in more detail, maybe we can help you find a better way to do it.