Issue with interface as Map key & value

Question

In a library project, I have :

public interface InterfaceA {
}

public interface InterfaceB {
}

public void myMethod(Map<? extends InterfaceA, List<? extends InterfaceB>> map) {
//do something
}

Then I have another project (having this library as a dependency) that contains two object implementing these interfaces :

public class ObjectA implements InterfaceA {
}

public class ObjectB implements InterfaceB {
}

When I try to call the library method myMethod like this :

HashMap<ObjectA, List<ObjectB>> hashMap = new HashMap<>();
//populate hashmap
myMethod(hashMap);

I get a compilation warning saying there is an argument mismatch.

What am I missing here ? Does it have something to do with the map ?

EDIT : The exact error (it's not a warning actually) is :

incompatible types: HashMap<ObjectA,List<ObjectB>> cannot be     converted to Map<? extends InterfaceA,List<? extends InterfaceB>>

Yes, the error is an argument mismatch. It's just that I did not copy/paste the code, and made a mistake. — nios, Aug 30 '16 at 12:58
Is there any chance you have another type declared somewhere called `InterfaceA`, `InterfaceB`, `ObjectA`, `ObjectB`, `List` or `Map` that is not the type you are expecting? — khelwood, Aug 30 '16 at 13:05
@khelwood No. I've checked imports on both files and they both import `java.util.List` and `java.util.Map`. — nios, Aug 30 '16 at 13:07
possible answer here: http://stackoverflow.com/questions/3575681/cant-cast-to-to-unspecific-nested-type-with-generics/3575895#3575895 — Jesper, Aug 30 '16 at 13:09

Jorn Vernee · Accepted Answer · 2016-08-30T13:16:52.963

3

Generics are invariant.

If your method declares:

Map<? extends InterfaceA, List<? extends InterfaceB>>

Then the second type parameter has to be exactly List<? extends InterfaceB>.

You can fix it by using:

Map<? extends InterfaceA, ? extends List<? extends InterfaceB>>

Instead.

edited Aug 30 '16 at 13:16

answered Aug 30 '16 at 13:07

Jorn Vernee

31,735
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Thanks! That did the trick, even if I don't clearly understand the syntax here. I clearly have to read a bit more about generics. – nios Aug 30 '16 at 13:11

score 1 · Answer 2 · answered Aug 30 '16 at 13:09

1

You either modify your Hashmap creation for this:

Map<? extends InterfaceA, List<? extends InterfaceB>> hashMap = new HashMap<>();

or modify your method definition for this:

public <A extends InterfaceA, B extends InterfaceB> void myMethod(Map<A, List<B>> map) {
    //do something
  }

answered Aug 30 '16 at 13:09

jalv1039

1,663
3
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1

What is the reason? – Erwin Bolwidt Aug 30 '16 at 13:10

score 0 · Answer 3 · answered Aug 30 '16 at 13:17

0

Declare your map as

HashMap<ObjectA, List<? extends InterfaceB>> hashMap = new HashMap<ObjectA, List<? extends InterfaceB>>();

answered Aug 30 '16 at 13:17

talex

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Issue with interface as Map key & value

3 Answers3