Java generic enum functionality using java 8 default methods and utility class

问题

Below is the failed attempt which I came up with, referring to Java Generic Enum class using Reflection.

Wanted to find a better way to do this. Couple of issues I find with this approach:

Each and every time I need to pass the class type. Example - EnumUtility.fromKey(Country.class, 1)
fromSet is duplicated in both City and Country

‌

public enum City implements BaseEnumInterface {

    TOKYO(0), NEWYORK(1);

    private final int key;

    public static Set<Integer> fromValue(Set<City> enums) {
        return enums.stream().map(City::getKey).collect(Collectors.toSet());
    }

    public int getKey() {
        return key;
    }

    private City(int key) {
        this.key = key;
    }
}


public enum Country implements BaseEnumInterface {

    USA(0), UK(1);

    private final int key;

    public static Set<Integer> fromSet(Set<Country> enums) {
        return enums.stream().map(Country::getKey).collect(Collectors.toSet());
    }

    public int getKey() {
        return key;
    }

    private Country(int key) {
        this.key = key;
    }

}


public class EnumUtility {

    public static <E extends Enum<E> & BaseEnumInterface> E fromKey(Class<E> enumClass, Integer key) {
        for (E type : enumClass.getEnumConstants()) {
            if (key == type.getKey()) {
                return type;
            }
        }
        throw new IllegalArgumentException("Invalid enum type supplied");
    }

    public static <E extends Enum<E> & BaseEnumInterface> Set<Integer> fromSet(Class<E> enumClass, Set<E> enums) {
        return enums.stream().map(BaseEnumInterface::getKey).collect(Collectors.toSet());
    }   
}


interface BaseEnumInterface {

    int getKey();

}


public class EnumTester {

    public static void main(String args[]) {
        System.out.println(EnumUtility.fromKey(Country.class, 1));
    }
}

回答1:

There is no way to avoid passing in the enum class to fromKey. How else would you know which enum constants to check for the requested key? Note: The second parameter in that method should be of type int, not Integer. Using == on Integer instances will not compare numeric values, it will compare object references!

EnumUtility.fromSet should work fine, so each enum class does not need a fromSet method at all. Note that EnumUtility.fromSet does not need a Class argument at all, and in fact your code is not using that parameter.

来源：https://stackoverflow.com/questions/43895415/java-generic-enum-functionality-using-java-8-default-methods-and-utility-class

标签

java

generics

enums

interface

java-8