added Creating Groups to frontend and saving them in Database. Also prepared some Methods for Snapshot db

src/main/java/mops/gruppen2/service/EventService.java

@@ -0,0 +1,51 @@
+package mops.gruppen2.service;
+
+import com.fasterxml.jackson.core.JsonProcessingException;
+import mops.gruppen2.domain.EventDTO;
+import mops.gruppen2.domain.event.Event;
+import mops.gruppen2.repository.EventRepository;
+import org.springframework.stereotype.Service;
+
+@Service
+public class EventService {
+    private final SerializationService serializationService;
+    private final EventRepository eventStore;
+
+    public EventService(SerializationService serializationService, EventRepository eventStore) {
+        this.serializationService = serializationService;
+        this.eventStore = eventStore;
+    }
+
+
+    public void saveEvent(Event event){
+        EventDTO eventDTO = getDTO(event);
+        eventStore.save(eventDTO);
+
+    }
+
+    public EventDTO getDTO(Event event){
+        EventDTO eventDTO = new EventDTO();
+        eventDTO.setGroup_id(event.getGroup_id());
+        eventDTO.setUser_id(event.getUser_id());
+        try {
+            eventDTO.setEvent_payload(serializationService.serializeEvent(event));
+        } catch (JsonProcessingException e) {
+            e.printStackTrace();
+        }
+        return  eventDTO;
+    }
+
+    public Long checkGroup() {
+        Long tmpId = 1L;
+        Iterable<EventDTO> eventDTOS = eventStore.findAll();
+        for (EventDTO event : eventDTOS) {
+            if (event.getGroup_id() == null) {
+                return tmpId;
+            }
+            if (tmpId <= event.getGroup_id()) {
+                tmpId++;
+            }
+        }
+        return tmpId;
+    }
+}