Passing a class with type parameter as type parameter for generic method in Java

Question

This is actually possible in Java, using some "tricks". Don't succumb to pressure from the C# fanatics! (j/k)

The "trick" is to create a class that extends a generic type, and access the value of the type parameter of the parent class through the Type returned by .getGenericSuperclass() or .getGenericInterfaces().

This is quite cumbersome. To simplify our lives, Google has already written most of the boring part of the code for us, and made it available through Guava.

Check the TypeToken class, which does exactly what you want. For example:

TypeToken<List<String>> stringListTok = new TypeToken<List<String>>() {};

Then you pass around a TypeToken<T> instead of a Class<T> and that's all. It provides you with methods to do reflection on the type represented by T.

What this is doing internally is simply calling .getClass().getGenericSuperclass() (or ...Interfaces()), then some ugly casts from Type to ParameterizedType and retrieving all the information from there (.getActualTypeArguments(), etc).

Finally, if you want to do something similar with Dependency Injection (ie, suppose you need to inject a Class<T> on the constructor of a class, or you want to get an instance of some parameterized interface, in which the instance should depend on the type parameter), Google Guice (a DI container from Google) has a very similar mechanism to solve the problem, called TypeLiteral. The use and the code behind the scenes are almost identical to TypeToken from Guava. Check it here: TypeLiteral