I have an api function that wants a Class - how do I give it the class of a List?

Question

I'm using List.class because List<String>.class is wrong. Is there a syntax that gets the class of the parametrized class?

Code:

import java.util.List;
import org.apache.camel.Message;
...
    Message m = exchange.getIn();
    List<String> serviceIds = null;
    serviceIds = m.getHeader("serviceId", List.class);

Of course I would rather have:

    serviceIds = m.getHeader("serviceId", List<String>.class);

to ensure that getHeader would return null if a list of something not strings was in the header.

I'm not sure this is clear enough. Can you precise the exact problem ? You want "T" from an instance of List ? — Denys Séguret, Sep 21 '12 at 14:45
Julian, can you provide perhaps a code snippet, or more detailed information about the API method, and how you want to use it? — Craig Otis, Sep 21 '12 at 14:46

score 7 · Accepted Answer · answered Sep 21 '12 at 14:48

7

You cannot give it a class of List<String>, because there is no such class: the type of <String> is removed by the compiler during the process known as type erasure, leaving you with pure List.

answered Sep 21 '12 at 14:48

Sergey Kalinichenko

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Looks like I've got the best I can do. Not too impressed with Java here. Thanks for the input. – Julian Sep 21 '12 at 16:02

score 3 · Answer 2 · edited May 23 '17 at 11:56

I'm not entirely sure I understand your question, but if you have a List parameterized with some class, eg:

List<Cat> myCats = new ArrayList<Cat>();

And you have some API method that you want to call:

public static void doSomething(Class<T> clazz);

And you're looking to provide that method with Cat.class, there is unfortunately no way to determine (at runtime) the parameterized class that is being contained in the List. This information is erased at runtime.

I believe your question may be a duplicate of this one, but don't hesitate to correct me if I'm mistaken:

How to get the generic type at runtime?

I have an api function that wants a Class - how do I give it the class of a List?

2 Answers2