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?

Answer 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.

Answer 2

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

Answer 3

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 (eg 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}

Answer 4

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> <? 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), "");