将一种枚举类型转换为另一种枚举类型的通用方法

Question

I have two enum as below

First

public enum firstEnum {
    ALL_PRODUCT,
    SPECIFIC_COLLECTION,
    SPECIFIC_PRODUCT
}

second

public enum secondEnum {
    PRODUCT,
    COLLECTION,
    PRODUCT
}

I am trying to create a generic method to convert one type to another as below

public record EnumCasting() {
    public <E1,E2> E2 CastOneEnumToAnother(E1 enumFirst, E2 enumSecond) {
         return // some logic to convert
    }
}

Not sure how can I achieve this, any help will be appreciated. firstEnum.ALL_Product should return secondEnum.Product, firstEnum.SPECIFIC_COLLECTION should return secondEnum.COLLECTION

Answer 1

Here's one way to do it.

interface MappableEnum<E> { 
    E counterpart();
}

enum E1 {
    A, B, C;
}

enum E2 implements MappableEnum<E1> {
    AA(E1.A), BB(E1.B), CC(E1.C);

    private final E1 counterpart;

    E2(E1 counterpart) { this.counterpart = counterpart; }

    E1 counterpart() { return counterpart; }
}

If you want to have multiple mappings from the same enum, you can use EnumMap<E1, E2> to capture the mapping. Build the EnumMap in the constructor of E1 using the same technique. This way, the mapping between enums is explicit.

Trying to do the mapping implicitly based on ordinals, as another answer suggested, is a world of hurt. If someone asks about the wrong pair of enums, you will either get a garbage answer or an exception when one has more constants than the other. And even if they use it correctly, if someone later inserts a new enum constant into one and doesn't insert it in the exactly right place in the other, your code is broken.

If you want to be doing this kind mapping, the two classes are coupled, and you have to capture that coupling somewhere, either in the type system ( implements ) or as explicitly initialized data ( EnumMap ).

Answer 2

I think TacheDeChoco is on the right track, but he misses the necessary generics declarations. Also, since generics are implemented through type erasure, you need to pass in the class of the second enum type.

Assuming the mapping strategy is as simple as "taking the corresponding enum values being at the exact same position" , the following could to the job:

public record EnumCaster() {
    public static <E1 extends Enum<E1>, E2 extends Enum<E2>> E2 CastOneEnumToAnother(E1 input, Class<E2> e2) {
        int pos = input.ordinal();
        return e2.getEnumConstants()[pos];
    }

    public static void main(String[] args) {
        System.out.println(firstEnum.ALL_PRODUCT);
        System.out.println(CastOneEnumToAnother(firstEnum.ALL_PRODUCT, secondEnum.class));
    }
}

Answer 3

Assuming the mapping strategy is as simple as "taking the corresponding enum values being at the exact same position ", the following could to the job:

public record EnumCasting() {
    public <E1 extends Enum<E1>, E2 extends Enum<E2>> E2 CastOneEnumToAnother(
            E1 input, Class<? extends E2> e2Class) {
        int pos = input.ordinal();
        return e2Class.getEnumConstants()[pos];
    }
}

Answer 4

Considering that you need return the value of the same position of second Enum, you can try this:

public class Main {

    public static void main(String[] args) throws Exception {

        SecondEnum selectedEnum = castOneEnumToAnother(FirstEnum.ALL_PRODUCT, SecondEnum.values()); // ok
        System.out.println("Selected enum: " + selectedEnum);

        selectedEnum = castOneEnumToAnother(FirstEnum.SPECIFIC_PRODUCT, SecondEnum.values()); //exception
        System.out.println("Selected enum: " + selectedEnum);
    }

    public static <E1 extends Enum<E1>, E2 extends Enum<E2>> E2 castOneEnumToAnother(E1 enumInput, E2[] enumValues) throws Exception {
        int pos = enumInput.ordinal();

        if (pos >= enumValues.length) {
            throw new Exception("Not found value in position " + pos + " for " + enumValues[0].getDeclaringClass().getSimpleName());
        }
        return enumValues[pos];
    }
}

enum FirstEnum {
    ALL_PRODUCT, SPECIFIC_COLLECTION, SPECIFIC_PRODUCT
}

enum SecondEnum {
    PRODUCT, COLLECTION //, PRODUCT
}