In Java 8 you can have default implementations in interface but I believe that will not solve your problem. Instead of changing the method signatures you can create a separate static method and call it with the class name within getUsers implementation. e.g.
createCreate new method: public static Collection<IUserEntity> getUsersStatic() { return staticUserEntityMap.values(); }
public static Collection<IUserEntity> getUsersStatic() { return staticUserEntityMap.values(); } callCall this method from getUsers: public Collection<IUserEntity> getUsers() { return UserEntity.getUsersStatic(); }:
public Collection<IUserEntity> getUsers() { return UserEntity.getUsersStatic(); }