Given data:

I am presented with a collection:

Collection<?> collection

This collection may contain anything. It could be "normal" classes, or it could be enum values.

The class is known at this point in my code. I am given:

Class<?> clazz

Which is guaranteed to be the class of objects contained in the collection. There is no class-mixing.

Goal:

Should the clazz of the objects in the collection be any kind of enum, i wish to create an EnumSet of the objects contained. For my purposes, i could just take the given collection an run with it. However enum sets are just way more efficient afaik.

What i have achieved:

1. Determined if i am dealing with an Enum type of class by using:

if (Enum.class.isAssignableFrom(clazz)) {
    //noinspection unchecked
    Class<? extends Enum<?>> enumClass = ((Class<? extends Enum<?>>) clazz);
    System.out.println("enumClass: "   enumClass.getSimpleName()); // prints the correct class!
    // what now? :-(
}

What i am struggling with:

Anything beyond that... It feels like i have already tried every way of casting things at the wall and seeing what sticks. And when it comes to trying to create a generic EnumSet, i have never even gotten to the point where my IDE would let me compile.

Thank you for reading!

Answer found

From the answer of @JayC667 below (thank you very much for your effort) i have abstracted the following answer for my purposes:

if (Enum.class.isAssignableFrom(clazz)) {
    EnumSet<?> enumSet = collection.isEmpty() ? EnumSet.noneOf(((Class<? extends Enum>) clazz)) : EnumSet.copyOf((Collection<Enum>) collection);
}

CodePudding user response：

I am not sure if this answers your question...

Central piece is EnumSet.copyOf((Collection<TestEnum1>) collection). Code works OK for the test, but I'm not sure where you're going with it, so... good luck ;-)

package stackoverflow;

import java.util.ArrayList;
import java.util.Collection;
import java.util.EnumSet;
import java.util.HashSet;
import java.util.Set;

public class EnumSetMess {

    public enum TestEnum1 {
        A, B, C;
    }
    public enum TestEnum2 {
        A, B, C;
    }


    public static void main(final String[] args) {
        {
            final ArrayList<TestEnum1> list = new ArrayList<>();
            list.add(TestEnum1.A);
            list.add(TestEnum1.B);
            list.add(TestEnum1.C);
            final Set<?> res = x(list, TestEnum1.class);
            System.out.println("Set: "   res.getClass().getSimpleName());
            for (final Object r : res) {
                System.out.println("\t"   r);
            }
        }
        {
            final ArrayList<TestEnum2> list = new ArrayList<>();
            list.add(TestEnum2.A);
            list.add(TestEnum2.B);
            list.add(TestEnum2.C);
            final Set<?> res = x(list, TestEnum2.class);
            System.out.println("Set: "   res.getClass().getSimpleName());
            for (final Object r : res) {
                System.out.println("\t"   r);
            }
        }
        {
            final ArrayList<String> list = new ArrayList<>();
            list.add("sA");
            list.add("sB");
            list.add("sC");
            final Set<?> res = x(list, String.class);
            System.out.println("Set: "   res.getClass().getSimpleName());
            for (final Object r : res) {
                System.out.println("\t"   r);
            }
        }
    }


    static Set<?> x(final Collection<?> collection, final Class<?> clazz) {
        if (Enum.class.isAssignableFrom(clazz)) {
            //noinspection unchecked
            final Class<? extends Enum<?>> enumClass = ((Class<? extends Enum<?>>) clazz);
            System.out.println("enumClass: "   enumClass.getSimpleName()); // prints the correct class!
            // what now? :-(
            final EnumSet<?> ret = EnumSet.copyOf((Collection<TestEnum1>) collection);
            return ret;
        }
        return new HashSet<>(collection);
    }

}

The output is

enumClass: TestEnum1
Set: RegularEnumSet
    A
    B
    C
enumClass: TestEnum2
Set: RegularEnumSet
    A
    B
    C
Set: HashSet
    sC
    sA
    sB