在使用泛型时如何允许更精确的返回类型？

Question

I'm trying to model something like a "dynamic Enum " in Java. There is a generic inteface Domain which represents a set of allowable values, and an interface Point that represents a particular value. Like this:

public interface Domain<D extends Domain<D>> {

    Set<Point<D>> points();

}

public interface Point<D> { // D works a bit like a phantom type

    public D type();

}

My intention is to statically forbid accidental mixing of Point s from different Domain types.

I have the following Domain implementation:

public final class Symbols implements Domain<Symbols> {

    final Set<Point<Symbols>> symbols = new HashSet<>(); 

    public Symbols(final Set<String> values) {
        super();
        for (String value : values) {
            this.symbols.add(new SymbolPoint(value));
        }
    }

    @Override
    public Set<Point<Symbols>> points() {
        return symbols;
    }

    public class SymbolPoint implements Point<Symbols> {

        private final String symbol;

        ...
    }
}

It seems to work OK, but now I've hit a roadblock. I want the points() method of Symbols to return the type Set<SymbolPoint> . Which of course doesn't work because Set<SymbolPoint> is not a subtype of Set<Point<Symbols>> . How to make it work?

Answer 1

Your interface Domain should accept a wildcard for its points method.

interface Domain<D extends Domain<D>> {

    Set<? extends Point<D>> points();

}

You can now change Set<Point<Symbols>> to Set<SymbolPoint> in Symbols#points .