Use enum that implements interface as parameter as key in EnumMap

Question

I want to have a family of child classes that extend BaseClass, which has in turn a EnumMap defined. Depending on the IDE I plug this code in. It tells me that the Constructor

EnumMap<B, Integer>()

is not defined, respectively B is not within the specified bounds.

interface A {}
enum B implements A {hello, world}

abstract class BaseClass {
    protected EnumMap<? extends A, Integer> baseMap;
}

class ChildClass extends BaseClass {
    public ChildClass () {
        baseMap = new EnumMap<B,Integer>();
    }
}

E.g. the third last line gives me troubles. I don't see what is wrong.

score 3 · Accepted Answer · answered Apr 02 '18 at 12:31

3

EnumMap takes the enum in the constructor:

baseMap = new EnumMap<B,Integer>(B.class);

And as we're in a post Java 7 world

baseMap = new EnumMap<>(B.class);

I would suggest using a Map in the declaration - program to the interface:

protected Map<? extends A, Integer> baseMap;

answered Apr 02 '18 at 12:31

Boris the Spider

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Thanks, it worked partially :) Two questions: 1.) How would I put an object into the map now, `baseMap.put(B.hello, 10);` gives me a weird error message... 2.) Why do suggest map over EnumMap in this example, especially as I already know that I want to use enum type keys? – Felix Crazzolara Apr 02 '18 at 12:47
For 2) becuase they're not an `enum`, they are an `A` - this is an `interface` and there is no reason something else could not implement it. For 1) due to [PECS](https://stackoverflow.com/a/19739576/2071828) you can not call method that takes `? extends` as an argument - you would need to use `Map` if you want to `put()`. – Boris the Spider Apr 02 '18 at 12:49
@Felix.C given the quantity of questions about co/contra-variance on SO, you are far from the only person to think so... – Boris the Spider Apr 02 '18 at 13:03
It still struggle :( I use `protected Map baseMap;` with `new EnumMap(B.class);`. I mean B is an A and EnumMap is a map, why does this not work? – Felix Crazzolara Apr 02 '18 at 13:18
Because, as you know `A` is not an `Enum` and a `Map` is not the same as a `Map`... – Boris the Spider Apr 02 '18 at 13:19

Use enum that implements interface as parameter as key in EnumMap

1 Answers1