CODEWITHSUNDEEP
javagenerics
lbj-ub
lbj-ub

Reputation: 1423

Inserting values in a typed map

Why would the code below result in the following error when trying to add to the Map?

Wrong 1st argument type. Found: 'com.test.Test.SomeEnums', required 'T'

public class Test {
   public enum SomeEnums implements SomeType {
       A;

       public <T extends Enum<T> & SomeType> Map<T, Object> buildMap() {
            Map<T, Object> map = new HashMap<>();
            map.put(SomeEnums.A, new Object());
            return map;
       }
   }
}

public interface SomeType {
}

Any ideas?

Upvotes: 1

Views: 52

Answers (1)

ernest_k
ernest_k

Reputation: 45329

The problem is that map.put(SomeEnums.A, new Object()) is not always safe for Map<T, Object>. Although SomeEnums is a valid substitute for extends Enum<T> & SomeType, it is not always the concrete type parameter.

For example, consider this second enum:

enum OtherEnum implements SomeType {
    B;
}

If the same method is to be called:

Map<OtherEnum, Object> otherMap = Test.SomeEnums.A.buildMap();

This is a valid call given the signature of buildMap(). However, the problem is that the method is adding an incorrect map key:

map.put(SomeEnums.A, new Object()); 
//SomeEnums.A is not always of type <T>, so this is not allowed.

The code will compile with a type cast (map.put((T) SomeEnums.A, new Object())) - with a warning, but that's unsafe and likely not the point of the generic method.

Upvotes: 1

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