How do I get a class instance of generic type T?

Question

I have a generics class, Foo. In a method of Foo, I want to get the class instance of type T, but I just can\'t call T.

Answer 1

I found a generic and simple way to do that. In my class I created a method that returns the generic type according to it's position in the class definition. Let's assume a class definition like this:

public class MyClass {

}

Now let's create some attributes to persist the types:

public class MyClass {

    private Class aType;

    private Class bType;

    private Class cType;

// Getters and setters (not necessary if you are going to use them internally)

    } 

Then you can create a generic method that returns the type based on the index of the generic definition:

   /**
     * Returns a {@link Type} object to identify generic types
     * @return type
     */
    private Type getGenericClassType(int index) {
        // To make it use generics without supplying the class type
        Type type = getClass().getGenericSuperclass();

        while (!(type instanceof ParameterizedType)) {
            if (type instanceof ParameterizedType) {
                type = ((Class) ((ParameterizedType) type).getRawType()).getGenericSuperclass();
            } else {
                type = ((Class) type).getGenericSuperclass();
            }
        }

        return ((ParameterizedType) type).getActualTypeArguments()[index];
    }

Finally, in the constructor just call the method and send the index for each type. The complete code should look like:

public class MyClass {

    private Class aType;

    private Class bType;

    private Class cType;


    public MyClass() {
      this.aType = (Class) getGenericClassType(0);
      this.bType = (Class) getGenericClassType(1);
      this.cType = (Class) getGenericClassType(2);
    }

   /**
     * Returns a {@link Type} object to identify generic types
     * @return type
     */
    private Type getGenericClassType(int index) {

        Type type = getClass().getGenericSuperclass();

        while (!(type instanceof ParameterizedType)) {
            if (type instanceof ParameterizedType) {
                type = ((Class) ((ParameterizedType) type).getRawType()).getGenericSuperclass();
            } else {
                type = ((Class) type).getGenericSuperclass();
            }
        }

        return ((ParameterizedType) type).getActualTypeArguments()[index];
    }
}