Java 7 Generics

generics.java

interface Entity<ID> {
    ID getId();
    setId( ID id );
}
   

class Repository<ID, ENTITY extends Entity<ID>> {
  Map<ID, ENTITY> identityMap = new HashMap<>();
  void add( ENTITY entity ) {
    identityMap.put( entity.getId(), entity );
  }
}

// ID could be an Integer, UUID, String, or anything, ENTITY could be anything that 
// implements the Entity interface, and it's all strict typed at compile time, so you can't
// add a UUID, User to a repository specified with Integer, Post