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

Question

I have a small problem in java while using genericity. I have a class A :

public class A<T>

In a method of A, I need to get the type name of T. Is there a way to find the string s using T ?

(If I create A<String> temp = new A<String>();, I want to be able to get java.lang.String at one point - I have to use genericity because one of my methods will have to return a List<T>).

This seems quite easy but I do not see how to do it.

Answer 1

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.

Answer 2

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.