Bnbs.java

package com.mywonderfulbnb.bnb.definition

interface Bnbs {
  Optional<Bnb> load(BnbId);
  void save(Bnb);
}

RoomName.java

package com.mywonderfulbnb.bnb.definition

final class RoomName {
  private String value;

  public RoomName(String value) {
    if (value.isEmpty()) {
      throw new RoomNameCannotBeEmpty();
    }
  
    this.value = value;
  }
}

final class RoomNameCannotBeEmpty extends Exception {}

Service.java

package com.mywonderfulbnb.bnb.definition

final class Service {
  private Bnbs Bnbs;
  private ActivatedOwners activatedOwners;

  public Service(Bnbs bnbs, ActivatedOwners activatedOwners) {
    this.bnbs = bnbs;
    this.activatedOwners = activatedOwners;
  }

  public addBnb(UUID ownerUuid, UUID bnbUuid, String nameStr) {
    ownerId = new OwnerId(ownerUuid);
    bnbId = new BnbId(bnbUuid);
    name = new Name(nameStr);
    
    assertOwnerIsActivated(ownerId);
    
    bnb = new Bnb(bnbUuid, ownerId, name);
    
    bnbs.save(bnb);
  }
  
  public addRoom(UUID ownerUuid, UUID bnbUuid, String roomNameStr) {
    ownerId = new OwnerId(ownerUuid);
    bnbId = new BnbId(bnbUuid);
    roomName = new RoomName(roomNameStr);
    
    bnb = loadBnb(bnbId)
    
    bnb.addRoomByName(roomName);
    
    bnbs.save(bnb);
  }
  
  private assertOwnerIsActivated(OwnerId ownerId) {
    itExists = activatedOwners.existsOneWithId(ownerId);
    
    if (!itExists) {
      throw new ActivatedOwnerNotFound(ownerId);
    }
  }
  
  private Bnb loadBnb(BnbId bnbid) {
    maybeBnb = bnbs.load(bnbId);
    
    if (!maybeBnb.isPresent()) {
      throw new BnbNotFound(bnbId);
    }
    
    return maybeBnb.get();
  }
}