Java: Pick a random value from an enum?

Question

If I have an enum like this:

public enum Letter {
    A,
    B,
    C,
    //...
}

What is the best way to pick one randomly? It doesn't need to be production quality bulletproof, but a fairly even distribution would be nice.

I could do something like this

private Letter randomLetter() {
    int pick = new Random().nextInt(Letter.values().length);
    return Letter.values()[pick];
}

But is there a better way? I feel like this is something that's been solved before.

Answer 1

The only thing I would suggest is caching the result of values() because each call copies an array. Also, don't create a Random every time. Keep one. Other than that what you're doing is fine. So:

public enum Letter {
  A,
  B,
  C,
  //...

  private static final List<Letter> VALUES =
    Collections.unmodifiableList(Arrays.asList(values()));
  private static final int SIZE = VALUES.size();
  private static final Random RANDOM = new Random();

  public static Letter randomLetter()  {
    return VALUES.get(RANDOM.nextInt(SIZE));
  }
}

Answer 2

Combining the suggestions of cletus and helios,

import java.util.Random;

public class EnumTest {

    private enum Season { WINTER, SPRING, SUMMER, FALL }

    private static final RandomEnum<Season> r =
        new RandomEnum<Season>(Season.class);

    public static void main(String[] args) {
        System.out.println(r.random());
    }

    private static class RandomEnum<E extends Enum> {

        private static final Random RND = new Random();
        private final E[] values;

        public RandomEnum(Class<E> token) {
            values = token.getEnumConstants();
        }

        public E random() {
            return values[RND.nextInt(values.length)];
        }
    }
}

Edit: Oops, I forgot the bounded type parameter, <E extends Enum>.

Answer 3

If you do this for testing you could use Quickcheck (this is a Java port I've been working on).

import static net.java.quickcheck.generator.PrimitiveGeneratorSamples.*;

TimeUnit anyEnumValue = anyEnumValue(TimeUnit.class); //one value

It supports all primitive types, type composition, collections, different distribution functions, bounds etc. It has support for runners executing multiple values:

import static net.java.quickcheck.generator.PrimitiveGeneratorsIterables.*;

for(TimeUnit timeUnit : someEnumValues(TimeUnit.class)){
    //..test multiple values
}

The advantage of Quickcheck is that you can define tests based on a specification where plain TDD works with scenarios.