Tag: enum

Enums for scala

Scala has very limited implementation of Enumeration. Enumerated objects can't extends other classes. Partial replacement for it is to use sealed classes. You can do pattern matching on them. When you ommit some possible value you will get compiler warning for not exhaustive pattern matching. One missing feature is that you can't get sorted values of all objects extending them. You can simple got it using my (40-lines) EnumOf class from scala-enum. Examples below.

Declaration

sealed abstract class Color(red: Double, green: Double, blue: Double)

object Color extends EnumOf[Color] {
  case object Red   extends Color(1, 0, 0)
  case object Green extends Color(0, 1, 0)
  case object Blue  extends Color(0, 0, 1)
  case object White extends Color(0, 0, 0)
  case object Black extends Color(1, 1, 1)
}

Usage

Color.values shouldEqual List(Red, Green, Blue, White, Black)

Color.valueOfOpt("Blue").value shouldEqual Blue
Color.valueOfOpt("NotExisiting").isEmpty shouldBe true
You can also enumerate on objects nested in instances

Declaration

case class DistanceFrom(srcCity: String, srcCoordinates: Coordinate) extends EnumOf[DistanceBetween] {

  case object ToBerlin extends DistanceFromSrcCityTo("Berlin", Coordinate(52.5075419, 13.4251364))
  case object ToNewYork extends DistanceFromSrcCityTo("New York", Coordinate(40.7033127, -73.979681))

  abstract class DistanceFromSrcCityTo(val destCity: String, val destCoordinates: Coordinate) extends DistanceBetween {
    override def srcCoordinates: Coordinate = DistanceFrom.this.srcCoordinates
  } 
}

sealed abstract class DistanceBetween {
  def srcCoordinates: Coordinate
  
  def destCity: String
  def destCoordinates: Coordinate

  def inKm: Int = Coordinate.distanceInKm(srcCoordinates, destCoordinates).toInt
}

Usage

val DistanceFromWarsaw = DistanceFrom("Warsaw", Coordinate(52.232938, 21.0611941))

DistanceFromWarsaw.ToBerlin.inKm shouldEqual 519
DistanceFromWarsaw.ToNewYork.inKm shouldEqual 6856

DistanceFromWarsaw.values.map(_.inKm) shouldEqual List(519, 6856)

Generic Enum converter for iBatis

My goal was to create a simple, extensible Enum converter that would work with iBatis. This seems like a trivial problem, but took me a while to find a proper solution. There were some additional preconditions:
  • all given Enums are jaxb generated objects - but any standard Java Enum should work
  • conversion was 1-to-1, no special conditions and processing
The example Enum for this problem looks like this one (copy&paste from jaxb generated source): 
@XmlType(name ="ServiceType") 
@XmlEnum
public enum ServiceType {

    @XmlEnumValue("stationary")
    STATIONARY("stationary"),
    @XmlEnumValue("mobile")
    MOBILE("mobile");
    private final String value;

    ServiceType(String v) {
        value = v;
    }

    public String value() {
        return value;
    }

    public static ServiceType fromValue(String v) {
        for (ServiceType c: ServiceType.values()) {
            if (c.value.equals(v)) {
                return c;
            }
        }
        throw new IllegalArgumentException(v);
    }

}
"No big deal", you say. I beg to differ. What I wanted to achieve was a simple construction which would look like this when used for another Enum (CommonEnumTypeHandler is the name of my generic converter): 
public class ServiceTypeHandler extends CommonEnumTypeHandler { }
Unfortunately due to the fact, that Java does not have reified generics, which is described in multiple places, I had to stick with passing through a Class type of my enum. So it looks like this: 
public class ServiceTypeHandler extends CommonEnumTypeHandler {

    public ServiceTypeHandler() {
        super(ServiceType.class);
    }
}
My final class has to look like this one below: 
import java.sql.SQLException;

import com.ibatis.sqlmap.client.extensions.ParameterSetter;
import com.ibatis.sqlmap.client.extensions.ResultGetter;
import com.ibatis.sqlmap.client.extensions.TypeHandlerCallback;

public abstract class CommonEnumTypeHandler implements TypeHandlerCallback {

    Class enumClass;

    public CommonEnumTypeHandler(Class clazz) {
        this.enumClass = clazz;
    }

    public void setParameter(ParameterSetter ps, Object o) throws SQLException {
        if (o.getClass().isAssignableFrom(enumClass)) {
            ps.setString(((T) o).name().toUpperCase());
        } else
            throw new SQLException("Excpected " + enumClass + " object than: " + o);
    }

    public Object getResult(ResultGetter rs) throws SQLException {
        Object o = valueOf(rs.getString());
        if (o == null)
            throw new SQLException("Unknown parameter type: " + rs.getString());
        return o;
    }

    public Object valueOf(String s) {
        return Enum.valueOf(enumClass, s.toUpperCase());
    }
}