Question

There is a following situation:
A class is given, with a generic field in it. Is it possible to instantiate the field at runtime with an instance of T?

class Root<T>{
   T var;

   Root(){
     //instantiate var with an instance of T here
   }
}

Answer 1

If you have a class which extends Root<T> and sets the type parameter in its extends clause, then it’s possible to dig it out using getGenericSuperclass(). For example Guice uses this approach to provide type parameter information to the injected classes.

Otherwise, the best bet is to pass a Class<T> instance as constructor parameter and use it. This will also result in simpler code, though a bit more boilerplate for each instantiation of the class.