nested enum-why is it used?

Question

I have seen constructs with an enum declared inside an enum. What is this used for ?

Answer 1

Enums in Java can't be extended, so if you wanna collate strongly-related enums in one place you can use these nested enum constructs. For example:

public enum DepartmentsAndFaculties
{
   UN (null, "UN", "University"),
   EF (UN,   "EF", "Engineering Faculty"),
   CS (EF,   "CS", "Computer Science & Engineering"),
   EE (EF,   "EE", "Electrical Engineering");

   private final DepartmentsAndFaculties parent;
   private final String code, title;

   DepartmentsAndFaculties(DepartmentsAndFaculties parent, String code, String title)
   {
       this.parent = parent;
       this.code   = code;
       this.title  = title;
   }

   public DepartmentsAndFaculties getParent()
   {
       return parent;
   }

   public String getCode()
   {
       return code;
   }

   public String getTitle()
   {
       return title;
   }
}

Here, inner enums consist of {parent enum, code, title} combinations. Example usage:

 DepartmentsAndFaculties cs = DepartmentsAndFaculties.CS;
 cs.getTitle();

You can see the power of nested enums when constructing hierarchical entities/enums.