EDIT

import java.util.HashSet;

import java.util.List;

import java.util.Map;

import java.util.UUID;

import org.springframework.beans.factory.annotation.Autowired;

    private Map<UUID, Registration> registrationsByUUID = new HashMap<>();

    private Map<String, Registration> registrationsByRegID = new HashMap<>();

    private Map<String, RegistrationDTO> registrationDTOsByRegID = new HashMap<>();

    private Collection<CdmBase> cdmEntities = new HashSet<>();

        registrationsByUUID.put(reg.getUuid(), reg);

        registrationsByRegID.put(reg.getSpecificIdentifier(), reg);

        registrationDTOsByRegID.put(reg.getSpecificIdentifier(), dto);

    }

    private void mergeBack(){

        return registrationDTOsByRegID.values();

    }

    /**

     * @param registrationID

     * @return