IGroupable.java

// Compare IGroupable (with a type parameter) here with IGroupableX below (without a type argument).
// In what situation will the generics version buy you something that the non-generic version won't.

interface IGroupable<E extends IGroupable<E>> {
    E fraction(double fraction);
    
    public static class AlphaGroupable implements IGroupable<AlphaGroupable> {
        @Override
        public AlphaGroupable fraction(double fraction) { return null; }
    }
    
    public static class BetaGroupable implements IGroupable<BetaGroupable> {
        @Override
        public BetaGroupable fraction(double fraction) { return null; }
    }
    
    // Fraction returns an IGroupable that is not the containing class.
    public static class GammaGroupable implements IGroupable<BetaGroupable> {
        @Override
        public BetaGroupable fraction(double fraction) { return null; }
    }
}

interface IGroupableX {
    IGroupableX fraction(double fraction);
    
    public static class AlphaGroupable implements IGroupableX {
        @Override
        public AlphaGroupable fraction(double fraction) { return null; }
    }
    
    public static class BetaGroupable implements IGroupableX {
        @Override
        public BetaGroupable fraction(double fraction) { return null; }
    }
    
    // Fraction returns an IGroupable that is not the containing class.
    public static class GammaGroupable implements IGroupableX {
        @Override
        public BetaGroupable fraction(double fraction) { return null; }
    }
}