写方法，该方法按类返回/返回任何枚举的枚举常量

Question

I am getting the generics mixed up again...

With the help of this question , I've written methods to store and retrieve Enum values, not tied to a specific enum class:

public static void storeEnum(Enum<?> enumValue) {
    // Store it somehow
}

public static <E extends Enum<E>> E getEnum(E defaultValue) {
    String valueString = // get it somehow
    try {
        return (E) Enum.valueOf(defaultValue.getDeclaringClass(), valueString);
    } catch (Exception e) {
        return defaultValue;
    }
}

But now I do not want to have any default value any more, I want to have null in case of no luck!

First attempt:

public static <E extends Enum<E>> E getEnum(Class<? extends Enum> enumClass) {
    String valueString = // get it somehow
    try {
        return (E) Enum.valueOf(enumClass, valueString);
    } catch (Exception e) {
        return null;
    }
}

This compiles, but when I want to get a stored MyCoolEnum , the line MyCoolEnum mci = getEnum(MyCoolEnum.class); breaks the (Maven) compile run: incompatible types: inference variable E has incompatible upper bounds java.lang.Enum<E>,MyCoolEnum .

Of course, this comes from the ? . But how can I infer the type from the class?

• (Class<E extends Enum> enumClass) is not valid.
• (Class<Enum<E>> enumClass) breaks the valueOf ( The inferred type Enum<E> is not a valid substitute for the bounded parameter <T extends Enum<T>> ).

Answer 1

I can not reproduce your first error, but as an alternative, you should be able to use Class<E> ie:

public static <E extends Enum<E>> E getEnum(Class<E> enumClass) {
    String valueString = // get it somehow
    try {
        return Enum.valueOf(enumClass, valueString);
    } catch (Exception e) {
        return null;
    }
}

(Note that you can also drop the cast)