Using EnumSet of a enum implementing an interface

I have an interface

public interface TerminalSymbol {
    // methods ...
}

an enum

// common usage enum that I need
public enum Common implements TerminalSymbol {
    EPSILON;

    @Override
    // methods ...
}

and I'd like to do something like:

enum Term implements TerminalSymbol {
    A, B, C, ...

    @Override
    // methods ...
}

EnumSet<? extends TerminalSymbol> terminalSymbols = EnumSet.allOf(Term.class);
terminalSymbol.add(Common.EPSILON); // this line gives me error

and that error is (in my case):

The method add(capture#1-of ? extends TerminalSymbol) in the type AbstractCollection<capture#1-of ? extends TerminalSymbol> is not applicable for the arguments (Common)

now I know that if I use Set<SomeInterface> I could prevent this type of error (and I could continue with my developing of my class representing a formal grammar) but I'd like to use EnumSet because it's likely to be more efficient than HashSet. How can I solve this problem?

Solution

EnumSet draws its efficiency from the key constraint that it contains strictly members of a single enum. It achieves this by efficiently storing its state with reliance on ordinal number of each enum member, basically behaving like a BitSet. So unfortunately you won't be able to reap its benefits if you want multiple enums.

The closest you can get to this goal is by devising your own numbering scheme which is distinct across all your enums, and using a BitSet to efficiently store them.