Add better return types

src/main/java/nl/tudelft/submit/service/StudentGroupService.java

@@ -277,15 +277,14 @@ public class StudentGroupService {
 	 *
 	 * @param personId the id of the person
 	 * @param moduleId the id of the module
-	 * @return          false if there are no other groups the people are in
 	 */
-	private Boolean checkIfPersonIsAlreadyInAGroup(Long personId, Long moduleId, String message)
+	private void checkIfPersonIsAlreadyInAGroup(Long personId, Long moduleId, String message)
 			throws GroupSwitchingException {
 		Optional<StudentGroupDetailsDTO> currentGroup = getGroupForPersonInModule(personId, moduleId);
 
 		// person is not in any other groups in the module
 		if (currentGroup.isEmpty()) {
-			return false;
+			return;
 		}
 
 		String personName = personCache.get(personId).orElseThrow().getDisplayName();
@@ -299,14 +298,13 @@ public class StudentGroupService {
 	 *
 	 * @param usernames list of usernames to be checked
 	 * @param moduleId  the id of the module
-	 * @return           whether if there is a person that is already in a group in the same module
+	 *
 	 */
-	private Boolean arePeopleAlreadyInAGroup(Long moduleId, List<String> usernames, String message) {
-		return usernames
-				.stream()
-				.map(name -> personCache.get(name).getId())
-				.map(id -> checkIfPersonIsAlreadyInAGroup(id, moduleId, message))
-				.reduce(false, (x, y) -> x || y);
+	private void arePeopleAlreadyInAGroup(Long moduleId, List<String> usernames, String message) {
+		for (String name : usernames) {
+			Long id = personCache.get(name).getId();
+			checkIfPersonIsAlreadyInAGroup(id, moduleId, message);
+		}
 	}
 
 	/**