public class IntersectionTypes {
  public static <S extends String & SafeString> void mainJava8() {
    Worker.safeWork(
      Validator.validate("Hello world!")
    );

    S s = Validator.validate("Intersection types must be aliased");
    Worker.safeWork(s);
  }
  public static /* look Ma, no generics for type aliasing! */ void mainJava10plus() {
    Worker.safeWork(
      Validator.validate("Hello world!")
    );

    var s = Validator.validate("Intersection types can just use `var`");
    Worker.safeWork(s);
  }
}

interface StringValue {}
interface SafeString extends StringValue {}
interface UnsafeString extends StringValue {}

class Worker {
  static <S extends String & SafeString> void safeWork(S s) {}
}

class Validator {
  static <T extends String & SafeString> T validate(String s) {
    // Check here, maybe throw an exception if invalid, and then if valid ...
    return CastMagic.cast(s);
  }
}

// https://www.gamlor.info/wordpress/2010/11/magic-cast-method-in-java/
class CastMagic {
    private CastMagic(){}

    public static <TSource,TTarget> TTarget cast(TSource toCast){
        return (TTarget) toCast;
    }
}