Get type name for generic parameter of generic class [duplicate]

You can't do this in general because of type erasure - an instance of A<String> doesn't know the type of T. If you need it, one way is to use a type literal:

public class A<T>
{
    private final Class<T> clazz;

    public A<T>(Class<T> clazz)
    {
        this.clazz = clazz;
    }

    // Use clazz in here
}

Then:

A<String> x = new A<String>(String.class);

It's ugly, but that's what type erasure does :(

An alternative is to use something like Guice's TypeLiteral. This works because the type argument used to specify a superclass isn't erased. So you can do:

A<String> a = new A<String>() {};

a now refers to a subclass of A<String>, so by getting a.getClass().getSuperClass() you can eventually get back to String. It's pretty horrible though.

You can get the name of the generics from the subclass. See this example. We Define a parent class like this:

public class GetTypeParent<T> {

    protected String getGenericName()
    {
        return ((Class<T>) ((ParameterizedType) getClass()
                .getGenericSuperclass()).getActualTypeArguments()[0]).getTypeName();
    }
}

We then define its child class in this way:

public class GetTypeChild extends GetTypeParent<Integer> {
    public static void main(String[] args) {
        GetTypeChild getTypeChild = new GetTypeChild();
        System.out.println(getTypeChild.getGenericName());
    }
}

You can see that in the main method, or in any instance method, I am capable to get the name of the generics type, in this case the main will print: java.lang.Integer.

Short answer: Impossible.

Slightly longer answer: Once your code is compiled, the type parameters is discarded. Thus, Java cannot know what you set there. You could, however, pass the class in question to your object and operate on it:

public class Example<T> {
  private final Class<T> clazz;

  public Example(Class<T> clazz){
     this.clazz = clazz;
  }
...
}

This certainly is possible so long as the type argument is specified via a subclass of A:

public class B extends A<String> {}

Class<?> typeArg = TypeResolver.resolveRawArgument(A.class, B.class);
assert typeArg == String.class;

TypeResolver is available via TypeTools.