@Target(value={METHOD,FIELD})
 @Retention(value=RUNTIME)
public @interface MapKeyClass

Specifies the type of the map key for associations of type java.util.Map. The map key can be a basic type, an embeddable class, or an entity. If the map is specified using Java generics, the MapKeyClass annotation and associated type need not be specified; otherwise they must be specified.

The MapKeyClass annotation is used in conjunction with ElementCollection or one of the collection-valued relationship annotations (OneToMany or ManyToMany). The MapKey annotation is not used when MapKeyClass is specified and vice versa.

 Example 1:
 @Entity
 public class Item {
 @Id int id;
 ...
 @ElementCollection(targetClass=String.class)
 @MapKeyClass(String.class)
 Map images; // map from image name to image filename
 ...
 }
 Example 2:
 // MapKeyClass and target type of relationship can be defaulted
 @Entity
 public class Item {
 @Id int id;
 ...
 @ElementCollection
 Map<String, String> images; 
 ...
 }
 Example 3:
 @Entity
 public class Company {
 @Id int id;
 ...
 @OneToMany(targetEntity=com.example.VicePresident.class)
 @MapKeyClass(com.example.Division.class)
 Map organization;
 }
 Example 4:
 // MapKeyClass and target type of relationship are defaulted
 @Entity
 public class Company {
 @Id int id;
 ...
 @OneToMany
 Map<Division, VicePresident> organization;
 }