java map getting incompatible types with child class of an abstract class

Question

I'm trying to create a map like this

Map<String, Class<abstractClass>> map = HashMap<String, Class<abstractClass>>();

For its input, I tried

map.put("exampleString", childClass.class);

But this doesn't work, the IDE says wrong 2nd argument type.

Found: java.lang.Class<childClass>, required java.lang.Class<abstractClass>

Even though there's an inheritance relationship between them. Does anyone know how to solve this problem?

Thanks in advance!

Unrelated: please follow Java naming conventions. Class names go UpperCase always. — GhostCat, Jun 07 '18 at 04:26

score 1 · Accepted Answer · answered Jun 07 '18 at 05:51

You need a generic wildcard:

Class<? extends abstractClass>

This is because generic parameters by default are invariant. You could only put abstractClass.class in the map before the change.

By adding <? extends abstractClass>, you are making the generic parameter contravariant. In other situations, you can change extends to super to achieve covariance. There is an acronym for remembering when to use extends and when to use super, called PECS.

score 0 · Answer 2 · answered Jun 07 '18 at 03:32

0

It looks to me that the childClass.class does not extend / implement the abstract class.

Please show the childClass signature.

answered Jun 07 '18 at 03:32

Ilan Keshet

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public class childClass extends abstractClass – Cheryl Jun 07 '18 at 03:38

score 0 · Answer 3 · answered Jun 07 '18 at 03:40

0

Are you sure you don't just want:

Map<String, abstractClass> map = HashMap<String, abstractClass>();
map.put("exampleString", childClass);

?

What you have is you are taking the actual meta signature of the class and putting it into a map, instead of the actual reference to a class

answered Jun 07 '18 at 03:40

Ilan Keshet

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I want to get the class type I would use with different input of strings, instead of objects. So I think I have to use Class<> and .class..? – Cheryl Jun 07 '18 at 03:42
With this implementation, I get expression expected from the childClass part. – Cheryl Jun 07 '18 at 03:44
Try this: Class extends abstractClass> – Ilan Keshet Jun 07 '18 at 03:45

score 0 · Answer 4 · answered Jun 07 '18 at 03:40

0

Actually I solved this problem by changing

Class<abstractClass>

into

Class<? extends abstractClass>

answered Jun 07 '18 at 03:40

Cheryl

39
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java map getting incompatible types with child class of an abstract class

4 Answers4