Java: How to get values from an arbitrary Enum class

Question

I have an enum containing reference to a class of enum like so:

enum Animal { 
  DOG(DayOfWeek.class),
  CAT(Month.class);

  public Class<? extends Enum> clazz;

  Animal(Class<? extends Enum> c) {
    this.clazz = c;
  }

  public String getValuesConcat() {
    String allValues = "";

    // The following line doesn't compile
    for (Object enumValue : EnumSet.allOf(clazz)) {
      allValues += " " + enumValue.toString();
    }

    return allValues;
  }
}

I couldn't figure out a way to store Enum class so that I can get the list of values from it later. Any clues?

You code compiles for me, and Animal.DOG.getValuesConcat() returns MONDAY TUESDAY WEDNESDAY THURSDAY FRIDAY SATURDAY SUNDAY. What error do you get? — tgdavies
– tgdavies, Commented Feb 23, 2021 at 4:40
Are you going to need the clazz member later? Or is it just to get the values? — Axel
– Axel, Commented Feb 23, 2021 at 4:56
FYI: This Question is derived from: Variable of type of enum class. — Basil Bourque
– Basil Bourque, Commented Feb 23, 2021 at 4:57

Basil Bourque · Accepted Answer · 2021-02-23 05:10:17Z

2

Rather than using an EnumSet, I think it's cleaner to use Class#getEnumConstants. This returns an array of your enum objects.

            for (Enum enumValue : clazz.getEnumConstants()) {
                allValues += " " + enumValue.name();
            }

Be aware that this code will NPE if clazz is not an Enum.

edited Feb 23, 2021 at 5:10

Basil Bourque

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answered Feb 23, 2021 at 4:44

tgdavies

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Comments

Basil Bourque · Accepted Answer · 2021-02-23 04:56:22Z

1

You can extend the correct Answer by tgdavies to get an unmodifiable Set of your enum objects.

Pass the array of enum objects returned by Class#getEnumConstants to the Set.of method.

Here is a code example equivalent to your Set.of( Animal.CAT.clazz.getEnumConstants() ) ;.

Set< Month > months = Set.of( Month.class.getEnumConstants() ) ;  // Pass array of enum objects to `Set.of` to produce an unmodifiable set.
        
System.out.println( "months: " + months ) ;

See this code run live at IdeOne.com.

months: [JANUARY, MAY, NOVEMBER, DECEMBER, MARCH, OCTOBER, JUNE, APRIL, AUGUST, JULY, SEPTEMBER, FEBRUARY]

answered Feb 23, 2021 at 4:56

Basil Bourque

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Comments

Axel · Accepted Answer · 2021-02-23 05:05:20Z

1

I totally overlooked that you need another enum's values.

Since the enum constructor is private anyway, why make your life hard by passing a Class object? Just pass the values instead. This should work as long as your enum contains at least one value:

enum Animal {
    DOG(DayOfWeek.values()),
    CAT(Month.values());

    public final Class<? extends Enum<?>> clazz;
    private final Enum<?>[] values;

    <T extends Enum<T>> Animal(T[] values) {
        this.clazz = (Class<? extends Enum<?>>) values[0].getClass();
        this.values = values;
    }

    public String getValuesConcat() {
        return Arrays.stream(values).map(Enum::name).collect(Collectors.joining(" "));
    }
}

answered Feb 23, 2021 at 5:05

Axel

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