How to declare a Typed Class in Java?

Question

I need to declare an instance of Map.class, but the Map is typed... So I need something like this:

Class <Map<String, String>> clazz = Map.class;

This line causes a compile error. What is the clean way of expressing this?

Could you provide some insight on what are you trying to achieve? — alexk, Oct 29 '15 at 10:28
@alexk, I have a situation where I am invoking a method that receives a `Class` as a parameter. In my case, it would be the `Class`. If I write `Map.class`, it works fine, but I dislike the warning... I'm looking for a way to get rid of the warning (not using `@SupressWarnings`). — Martin, Oct 29 '15 at 14:00

Paul Boddington · Accepted Answer · 2015-10-29T10:49:44.240

5

You can do it with casts.

Class<Map<String, String>> clazz = (Class<Map<String, String>>) (Object) Map.class;

This generates a warning but compiles.

The problem with it is that due to type erasure, the resulting object at runtime is no different from Map.class (it doesn't know about the type).

If you need an object that truly represents Map<String, String> you can look into using java.lang.reflect.Type or Guava's TypeToken.

edited Oct 29 '15 at 10:49

answered Oct 29 '15 at 10:44

Paul Boddington

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Excellent answer, sir. I've learned something new. – javatutorial Oct 29 '15 at 10:47
Not a problem. Glad I could help. – Paul Boddington Oct 29 '15 at 10:50
1

This works so far, but I don't like to do casting... I would like some solution that doesn't involve casting, or better said, that doesn't generate warnings (@SupressWarnings is not a solution, just a workaround)... – Martin Oct 29 '15 at 13:57
1

I agree this is not a satisfactory solution. There is no way of doing this without a warning sadly. – Paul Boddington Oct 29 '15 at 15:14
Based on this last comment, I will accept the answer... There is indeed no "100% clean" way of doing it... – Martin Apr 05 '16 at 15:42

score 2 · Answer 2 · answered Oct 29 '15 at 10:31

2

import java.util.*;

public class Test{
  public static void main(String [] args){
   Class <Map> clazz = Map.class;
   System.out.println(clazz);
  }
}

will print

interface java.util.Map

answered Oct 29 '15 at 10:31

Tahar Bakir

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score 0 · Answer 3 · edited Oct 29 '15 at 10:31

0

Map<String, String> clazz = new HashMap<String, String>()

Remember that Map is an interface, not instantiable directly but only via a class implementing it, like HashMap or TreeMap

edited Oct 29 '15 at 10:31

KristofMols

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answered Oct 29 '15 at 10:29

morels

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`class` is a reserved word in Java, you can't use it as a variable name – JonK Oct 29 '15 at 10:31
3

The OP needs a Class, not a Map. – Paul Boddington Oct 29 '15 at 10:31
@JonK right, I did not care the name of the variable. thx. – morels Oct 29 '15 at 10:32
1

@morels: I think here the OP has another problem. You can get a `Class` object from an interface. But here he has a typed Map, not a raw Map, so the compiler won't let him get a `Class` object. Sorry, but I'm downvoting until you edit your answer. – javatutorial Oct 29 '15 at 10:39
I don't think that's what the OP is looking for. OP wants a Class type and not an instance of a Map. – Avi Oct 29 '15 at 10:48

How to declare a Typed Class in Java?

3 Answers3