How can I use reflection to create a generic parameterized class in Java?
I have
public class SomeClass<T> {
public SomeClass<T>() {
}
}
and I need an instance of it.
I've tried variations of
Class c = Class.forName("SomeClass");
but could not find a syntax that would allow me to get an appropriately typed instance, like, say
SomeType instance = (SomeType)Class.forName("SomeClass<SomeType>").createInstance();
So, how could I go about doing this?
Java uses erasure-based generics (i.e., the type parameters are erased at runtime—for example, List<Integer> and List<String> are treated as the same type at runtime). Since reflection is inherently a runtime feature, the type parameters are not used or involved at all.
In other words, you can only instantiate the raw type (SomeClass, not SomeClass<T>) when you're using reflection. You will then have to manually cast the type to the generic version (and generate an unchecked warning).
SomeClass<?> in your source code. The type parameter is ever only useful at compile-time, and since you don't have that info at compile-time, ? is the way to express that.SomeClass<Type1> is the same type as SomeClass<Type2>: both are of type SomeClass. That is why the kinds of things you can do with type parameters in Java is very restricted compared to what you can do in, say, C#. (Java == erasure generics, i.e., type parameters lost at runtime. C# == reified generics, i.e., type parameters preserved at runtime.)See/search for "Type erasure". Generics are for compile time and they are not availble at runtime so what you are trying is not possible. You need to use the raw type for reflection
Because of type erasure, at runtime SomeClass is all there is left of SomeClass<SomeType>.
