JPA 2.1 Attribute Converter - The better way to persist enums (http://www.thoughts-on-java.org/2013/10/jpa-21-type-converter-better-way-to.html)

findTrip.java

public List findTripsByVehicle(Vehicle vehicle) {
 Query query = this.em.createNamedQuery("Trip.findByVehicle");
 query.setParameter("vehicle", vehicle);
 return query.getResultList();
}

NamedQuery.java

@NamedQueries(@NamedQuery(name = "Trip.findByVehicle", query = "SELECT trip FROM Trip trip WHERE vehicle=:vehicle"))

TripEntity.java

@Entity
public class Trip {

 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
 private Integer id;

 private Vehicle vehicle;

 ...
}

Vehicle.java

public enum Vehicle {
 CAR("C"), BUS("B"), TRAIN("T"), PLANE("P");

 private String shortName;

 Vehicle(String shortName) {
  this.shortName = shortName;
 }

 public String getShortName() {
  return this.shortName;
 }

 public static Vehicle fromShortName(String shortName) {
  for (Vehicle v : Vehicle.values()) {
   if (v.getShortName().equals(shortName)) {
    return v;
   }
  }

  throw new IllegalArgumentException("No Vehicle with shortName ["
    + shortName + "] found.");
 }
}

VehicleConverter.java

@Converter(autoApply = true)
public class VehicleConverter implements AttributeConverter {

 @Override
 public String convertToDatabaseColumn(Vehicle vehicle) {
  return vehicle.getShortName();
 }

 @Override
 public Vehicle convertToEntityAttribute(String shortName) {
  return Vehicle.fromShortName(shortName);
 }
}