Java enum companion object (or equivalent)

Question
1

With scala enums I often like to do one of the following two things to avoid having to import the enum:

package object foo {
  type MyScalaEnum = path.to.MyScalaEnum
  val  MyScalaEnum = path.to.MyScalaEnum
}

and/or

def foo(f: MyScalaEnum.type => MyScalaEnum) = doSomething(f(MyScalaEnum)) // so can use eg .foo(_.MyValue); incidentally, does this technique have a name?

Unfortunately neither the val assignment nor the .type call seem to be work for java enums

Any suggestions? Thanks!

2

I once struggled with similar problem (aliasing Java class for its static methods) and I don’t think it’s possible. Java classes don’t have an actual object so there’s no runtime value for JavaClass.type

1 Like
3

Thanks, that’s what I suspected too.

I toyed with javap:

$ cd ~/scala
$ cat Day.scala
object Day extends Enumeration {  
  val SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY = Value
}

$ scalac Day.scala

$ javap Day.class 
Compiled from "Day.scala"
public final class Day {
  public static scala.Enumeration$Value SATURDAY();
  public static scala.Enumeration$Value FRIDAY();
  public static scala.Enumeration$Value THURSDAY();
  public static scala.Enumeration$Value WEDNESDAY();
  public static scala.Enumeration$Value TUESDAY();
  public static scala.Enumeration$Value MONDAY();
  public static scala.Enumeration$Value SUNDAY();
  public static scala.Enumeration$ValueSet$ ValueSet();
  public static scala.Enumeration$ValueOrdering$ ValueOrdering();
  public static scala.Enumeration$Value withName(java.lang.String);
  public static scala.Enumeration$Value apply(int);
  public static int maxId();
  public static scala.Enumeration$ValueSet values();
  public static java.lang.String toString();
}

$ javap Day$.class 
Compiled from "Day.scala"
public final class Day$ extends scala.Enumeration {
  public static Day$ MODULE$;
  public static {};
  public scala.Enumeration$Value SUNDAY();
  public scala.Enumeration$Value MONDAY();
  public scala.Enumeration$Value TUESDAY();
  public scala.Enumeration$Value WEDNESDAY();
  public scala.Enumeration$Value THURSDAY();
  public scala.Enumeration$Value FRIDAY();
  public scala.Enumeration$Value SATURDAY();
}

$ cd ~/java
$ cat Day.java
public enum Day {
    SUNDAY, MONDAY, TUESDAY, WEDNESDAY,
    THURSDAY, FRIDAY, SATURDAY 
}

$ javac Day.java
$ javap Day.class 
Compiled from "Day.java"
public final class Day extends java.lang.Enum<Day> {
  public static final Day SUNDAY;
  public static final Day MONDAY;
  public static final Day TUESDAY;
  public static final Day WEDNESDAY;
  public static final Day THURSDAY;
  public static final Day FRIDAY;
  public static final Day SATURDAY;
  public static Day[] values();
  public static Day valueOf(java.lang.String);
  static {};
}

There is indeed no Day$.class equivalent for java, though it makes me wonder how Day.MONDAY works in java (I’d expect to have to do new Day().MONDAY, but my java is rusty)

4

ah, it’s probably because of that (java can’t have top-level static classes).

Maybe a macro annotation approach that would annotate an object and take the java enum as input to reproduce all the values in the object… That seems awfully complicated though. I tried toying macro paradise the other day but ran into a lot of version problems with it.