Problem

We have multiple enum types that have some public static EnumType valueOfText(String text), for the purpose of mapping the contents of a data file cell to enum.

I'm trying to write a generic enum util that takes a comma-separated string and return multiple enum values. For example, we have the following enum:

public enum Frequency {
    SEMI_ANNUAL("S"), MONTHLY("M"), QUARTERLY("Q"), ANNUAL("A")

    public final String textValue;

    public Frequency(String textValue) {
        this.textValue = textValue;
    }

    public static Frequency valueOfText(String textValue) {
        for (Frequency frequency : values()) {
            if (frequency.textValue.equals(textValue))
                return frequency;
        }
        return null;
    }
}

and string "A,S" which we want to convert to [Frequency.ANNUAL, Frequency.SEMI_ANNUAL].

Attempted solution

I create some EnumUtils like so:

import java.util.stream.Collectors

public final class EnumUtils {
    public static final String LIST_SEPARATOR = ",";

    public static <E extends Enum<E>> List<E> CreateFromText(String text) {
        List<String> textList = text.split(this.LIST_SEPARATOR)

        return textList.stream()
                .map { txt ->
                    E.valueOfText(txt)
                }
                .collect(Collectors.toList())
    }
}

What happen after said solution

We go to use it, like this:

EnumUtils.CreateFromText<Frequency>(row[3])

and the IDE compain, immediately, about the <>.

How can we specify enum type in this?

CodePudding user response：

In Groovy you can do it if you pass the actual Class instead of just using a type parameter.

enum Frequency {
    SEMI_ANNUAL("S"), MONTHLY("M"), QUARTERLY("Q"), ANNUAL("A")

    final String textValue;

    Frequency(String textValue) {
        this.textValue = textValue;
    }

    static Frequency valueOfText(String textValue) {
        return values().find { it.textValue == textValue }
    }
}

final class EnumUtils {
    static <E extends Enum<E>> List<E> createFromText(Class<E> clazz, String text) {
        return text.split(",").collect { clazz.valueOfText(it) }
    }
}

EnumUtils.createFromText(Frequency, "S,M")

The same idea won't work in Java, since clazz won't have valueOfText at compile time.

Perhaps the Util class doesn't save you much typing, though:

"S,M".split(",").collect(Frequency.&valueOfText)