Cannot convert fom enum to interface it implements

Question

I'm in Java 7 and I have the following enum:

public enum BooleanEnum implements StringRepresentable{
    YES {
        @Override
        public String getStringRepresentation() {
            return "true";
        }
    },
    NO {
        @Override
        public String getStringRepresentation() {
            return "false";
        }
    };
    public abstract String getStringRepresentation();
}

Now I have the method:

List<StringRepresentable> getValues(){
    return Arrays.asList(BooleanEnum.values()); //Type mismatch: 
                  //cannot convert from List<BooleanEnum> to List<StringRepresentable>
}

What's wrong with that enum? It implements the interface, therefore the code should have compiled fine.

**[It's because List is not a subclass of List](http://stackoverflow.com/questions/2745265/is-listdog-a-subclass-of-listanimal-why-arent-javas-generics-implicitly-p)** — Neeraj Jain, Apr 13 '15 at 12:16

Jon Skeet · Accepted Answer · 2015-04-13T16:41:03.507

It implements the interface, therefore the code should have compiled fine.

No, because the type argument is being inferred as BooleanEnum - and a List<BooleanEnum> isn't a List<StringRepresentation>... you can add instances of other StringRepresentation implementations to the latter.

Four possible options:

Specify that you're returning a list of some subclass of StringRepresentation:

List<? extends StringRepresentation> get Values() {
    // Implementation as before
}

Specify the type argument:

return Arrays.<StringRepresentation>asList(BooleanEnum.values());

Use an intermediate variable for clarity:

StringRepresentation[] array = BooleanEnum.values();
return Arrays.asList(array);

Don't return a List<StringRepresentation> at all; return an Iterable<StringRepresentation> at which point you can use EnumSet instead:
```
Iterable<? extends StringRepresentable> getValues() {
    return EnumSet.allOf(BooleanEnum.class);
}
```

score 2 · Answer 2 · answered Apr 13 '15 at 12:10

If this compiled, someone could do this:

List<StringRepresentation> values = getValues();
values.set(0, new SomeOtherStringRepresentation());

(since modifying the list returned by Arrays.asList modifies the original array) and then you'd have stored a SomeOtherStringRepresentation in the original array of BooleanEnums! That's obviously not allowed.

Instead, you could make a copy, say as an ArrayList:

List<StringRepresentation> getValues(){
    return new ArrayList<StringRepresentation>(Arrays.asList(BooleanEnum.values()));
}

score 2 · Answer 3 · answered Apr 13 '15 at 12:10

First BooleanEnum.values() returns an BooleanEnum[], then with Arrays.asList you return a List<BooleanEnum>.

Since a List<BooleanEnum> is not a subset of List<StringRepresentation>, this code will not compile.

You could in theory get this code to compile by returning a List<? extends StringRepresentation>, however I would not recommend that as it is not really useful to the caller side.

score 2 · Answer 4 · answered Apr 13 '15 at 12:11

2

The type of your List is StringRepresentation, and BooleanEnum just implements StringRepresentable, an interface can't be converted into any types of class which implements the interface.

answered Apr 13 '15 at 12:11

SilentKnight

13,761
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score 2 · Answer 5 · answered Apr 13 '15 at 12:12

2

You should return something like:

 return Arrays.<StringRepresentable>asList(BooleanEnum.values());

Since List of BooleanEnum is not same as List of StringRepresentable.

answered Apr 13 '15 at 12:12

SMA

36,381
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73

Cannot convert fom enum to interface it implements

5 Answers5