public class Test {

    interface Option <T> {

        T get();
    }

    static class None implements Option {

        public Object get() { throw new RuntimeException("wat"); }
    }

    static class Some <T> implements Option <T> {

        private T thing;

        public Some(T thing) {
            this.thing = thing;
        }

        public T get() { return thing; }

    }

    private static <T> T getOrElse(None opt, T alternative) {
        return alternative;
    }

    private static <T> T getOrElse(Some<T> opt, T alternative) {
        return opt.get();
    }

    public static void main(String[] args) {

        Option <String> a = new Some<>("a");
        Option <String> b = new None();

        // compiles and works fine
        System.out.println(getOrElse((Some <String>)a, "kek"));
        System.out.println(getOrElse((None) b, "kek"));

        // compile-time error
        System.out.println(getOrElse(a, "kek"));
        System.out.println(getOrElse(b, "kek"));
    }
}