Spring Boot. how to Pass Optional<> to an Entity Class

The answers lack some job to do. Before you call get(), you should do some checking with isPresent(). Like so:

Optional<RoomEntity> optionalEntity =  roomRepository.findById(roomId);
if (optionalEntity.isPresent()) {
    RoomEntity roomEntity = optionalEntity.get();
    ...
}

Read this great article about optionals: https://dzone.com/articles/using-optional-correctly-is-not-optional

According to your error you are getting Optional<RoomEntity> from repository's findAll method and you are casting it to RoomEntity.

Instead of RoomEntity roomEntity = roomRepository.findById(roomId); do this

Optional<RoomEntity> optinalEntity = roomRepository.findById(roomId); RoomEntity roomEntity = optionalEntity.get();

First solution

You can implement JpaRepository instead of CrudRepository which provide a getOne method that returns an RoomEntity as you expect. (JpaRepository for JPA or MongoRepository for MongoDB) :

public interface RoomRepository extends JpaRepository<RoomEntity, Long> {
    List<RoomEntity> findAllById(Long id);
}

and

RoomEntity roomEntity = roomRepository.getOne(roomId);

Note that an EntityNotFoundException will be thrown if there's no RoomEntity for this roomId.

Second solution

The findById method of CrudRepository returns an optional so you must deal with it properly to get an RoomEntity if there is one. For example :

RoomEntity roomEntity = optionalEntity.roomRepository.findById(roomId).get();

In this case .get() will throw a NoSuchElementException if there's no RoomEntity for this roomId.

This article may help to understand optionals : http://www.baeldung.com/java-optional

Try this, it works for me

RoomEntity roomEntity = roomRepository.findById(roomId).orElse(null);