Generic Enum with generic type Enum

Question

I have the following interface and class definition.

interface A
{
}

class B
{
    public static enum C implements A
    {
        c1, c2, c3;
    }

    public static enum D implements A
    {
        d1, d2, d3;
    }

    public static enum E implements A
    {
        e1, e2, e3;
    }
}

Now I have a class where I declare a Map and assign the enum as key and set a value.

class Test
{
    private Map<C, String> myMap;

    public void assignVal()
    {
        myMap = new EnumMap<C, String>(C.class);
        myMap.put(C.c1, String.valueOf(1));
    }
}

Question: As you see myMap is tied to the enum C. I want to create a generic version of myMap, so I can assign any enum value in the class B.

I already went thru the stackoverflow post: How to implement enum with generics?

Why not just use `Map` and `new HashMap<>()`? – Andreas Jan 18 '17 at 21:46 — Andreas, Jan 18 '17 at 21:46
@Andreas Maybe `A` has other implementations. – shmosel Jan 18 '17 at 21:49 — shmosel, Jan 18 '17 at 21:49

teppic · Answer 1 · 2017-01-18T22:14:20.620

1

You can't do this with EnumMap.

EnumMap requires a key of a single type — the constructor is there to enforce this by tying the generic type to a concrete type.

Under the hood it builds a cache of possible key values that it uses to enforce run-time type safety.

You'll need to use another type of map if you want to allow keys from your hierarchy.

edited Jan 18 '17 at 22:14

answered Jan 18 '17 at 22:02

teppic

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score 0 · Answer 2 · edited Jan 18 '17 at 22:00

0

Try this:

private Map<Enum<? extends A>, String> myMap;

This puts a bound on the type to any Enum that implements A.

This is equivalent to <? extends Enum<?> & A>, but easier to type.

edited Jan 18 '17 at 22:00

Andy Turner

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answered Jan 18 '17 at 21:41

Bohemian

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Really? Not `? extends Enum> & A`? This doesn't look like it'd work because of the bounds on `Enum`. – Andy Turner Jan 18 '17 at 21:42
@andy they are the same: see http://stackoverflow.com/questions/14122783/is-there-a-difference-between-the-generic-bounds-enumt-foo-and-enum-ext – Bohemian Jan 18 '17 at 21:45
@andy sorry - original post had a syntax error. Fixed now. – Bohemian Jan 18 '17 at 21:46
2

But can you put into a map with upper-bounded key type? – Andy Turner Jan 18 '17 at 21:47
1

Cannot `put()` with `Map extends Enum extends A>, String>`. Just change to `Map, String>`. --- Of course, that's not enough, because `myMap = new EnumMap(C.class)` won't work. That will then have to be `myMap = new HashMap<>()`. – Andreas Jan 18 '17 at 21:53
@Andreas but the whole thing about `EnumMap`s is that the keys are from a specific, enumerated class. You don't want them to accept keys from multiple enums. – Andy Turner Jan 18 '17 at 21:57
@AndyTurner Who said it has to be an `EnumMap`? – shmosel Jan 18 '17 at 21:59
@shmosel OP: "`myMap = new EnumMap`". – Andy Turner Jan 18 '17 at 21:59
@AndyTurner But OPs question is about how to make a map accept multiple enums: *"I want to create a generic version of myMap, so I can assign any enum value in the class B"*. Note how class `B` has **3** enum classes, named `C`, `D`, and `E`. So question really says: *so I can assign any enum value from `C`, `D`, or `E`.* – Andreas Jan 18 '17 at 21:59
@AndyTurner Yes, I see that it *is*, but not that it needs to be. If that were the case, OP's request is obviously impossible. – shmosel Jan 18 '17 at 22:00
@Andreas Your suggestion may be unintuitive, since one might expect keys in the map to be of type `A`. I would prefer the simpler `Map`. Another option is to create a custom class to capture multiple bounds. – shmosel Jan 18 '17 at 22:06
@shmosel I agree on `Map`, that why I [suggested it](http://stackoverflow.com/questions/41729902/generic-enum-with-generic-type-enum/41729984?noredirect=1#comment70654534_41729902) 20 mins ago. `Map, String>` enhances that by *also* requiring the key to be an `enum`. – Andreas Jan 18 '17 at 22:08
@Andreas I'm well aware of your suggestion, as I responded to it. This solution enhances type safety (my initial concern) but reduces usability. – shmosel Jan 18 '17 at 22:11
@shmosel How does it reduce usability? OP wants a `Map` that can take `C`, `D`, and `E` as key. Since Bohemian already removed the `? extends` that prevented `put()`, the type declaration supports that. No loss of usability. – Andreas Jan 18 '17 at 22:13
1

@Andreas Like I said, it means keys are not of type `A`. For example, `A a = myMap.keySet().iterator().next();` is a compile error. – shmosel Jan 18 '17 at 22:15
Map, String> didnot work due to bound mismatch. Based on the discussion, should I conclude that this is not doable? – curious Jan 18 '17 at 22:25
@Bohemian *This is equivalent to ` extends Enum> & A>`, but easier to type.* No it's not; see my previous comment. – shmosel Jan 18 '17 at 22:29

leeyuiwah · Answer 3 · 2017-01-18T23:17:31.123

I would second what teppic said -- you cannot use EnumMap generically with all subclasses of A. It must be constructed with a specific concrete class. See the JavaDoc of EnumMap

https://docs.oracle.com/javase/7/docs/api/java/util/EnumMap.html

I see that you seems to have two options here

Use other types of Map implementations (e.g. HashMap)

Map<A, String> myMap1 = new HashMap<>();
myMap1.put(C.c1, C.c1.name());
System.out.println(myMap1);

The output would be:

{c1=c1}

Implement yet another subclasses of A so that you can use EnumMap

If, for some reasons, you really want to use EnumMap, and you do not want to implement all the values of C, D, E into one enum, you still have the following solution.

Implement a new subclasses of A (let's call it SuperCde in the following code example) that has all the possible values of C, D, and E, and it has a static method getSuperCde() that serves as a bridge:

public static enum SuperCde implements A{
    c1,c2,c3,
    d1,d2,d3,
    e1,e2,e3
    ;

    public static SuperCde getSuperCde(A a) {
        if (a instanceof C) {
            C cValue = (C) a;
            switch (cValue) {
            case c1: return SuperCde.c1;
            case c2: return SuperCde.c2;
            case c3: return SuperCde.c3;
            default: throw new IllegalArgumentException();              
            }
        } else if (a instanceof D) {
            D dValue = (D) a;
            switch (dValue) {
            case d1: return SuperCde.d1;
            case d2: return SuperCde.d2;
            case d3: return SuperCde.d3;
            default: throw new IllegalArgumentException();              
            }
        } else if (a instanceof E) {
            E eValue = (E) a;
            switch (eValue) {
            case e1: return SuperCde.e1;
            case e2: return SuperCde.e2;
            case e3: return SuperCde.e3;
            default: throw new IllegalArgumentException();              
            }
        } else {
            throw new IllegalArgumentException();
        }
    }
}

And then you can use the following code:

Map<SuperCde, String> myMap2 = new EnumMap<SuperCde, String>(SuperCde.class);       
myMap2.put(SuperCde.getSuperCde(C.c1), C.c1.name());
System.out.println(myMap2);

The output would be:

{c1=c1}

score 0 · Answer 4 · answered Jan 19 '17 at 01:48

As pointed out by others you cannot use EnumMap. Also, while using <E extends Enum<E> & A> works, the version for bounded wildcards <? extends Enum<?> & A> does not exist. Given that, you could wrap your map key with an object that does the static type checking using the <E extends Enum<E> & A> construct.

class EnumAWrapper{
    final Enum<?> enumObj;
    final A aObj;

    <E extends Enum<E> & A> EnumAWrapper(E enumA){
        enumObj = enumA;
        aObj = enumA;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((aObj == null) ? 0 : aObj.hashCode());
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        EnumAWrapper other = (EnumAWrapper) obj;
        if (aObj == null) {
            if (other.aObj != null)
                return false;
        } else if (!aObj.equals(other.aObj))
            return false;
        return true;
    }

}

Here is a demo:

Map<EnumAWrapper, String> map = new HashMap<>();

//does compile
map.put(new EnumAWrapper(C.c1), "");
map.put(new EnumAWrapper(D.d1), "");
map.put(new EnumAWrapper(E.e1), "");
A aObj = map.keySet().iterator().next().aObj;
Enum<?> enumObj = map.keySet().iterator().next().enumObj;

//does not compile (given that enum F does not implement A)
map.put(new EnumAWrapper(new A(){}), "");
map.put(new EnumAWrapper(F.f1), "");

Generic Enum with generic type Enum

4 Answers4