Addresses comments in PR.

src/main/java/org/opentripplanner/apis/transmodel/model/timetable/DatedServiceJourneyQuery.java

@@ -13,7 +13,6 @@
 import org.opentripplanner.apis.transmodel.mapping.TransitIdMapper;
 import org.opentripplanner.apis.transmodel.model.EnumTypes;
 import org.opentripplanner.apis.transmodel.support.GqlUtil;
-import org.opentripplanner.framework.collection.CollectionUtils;
 import org.opentripplanner.transit.api.request.TripOnServiceDateRequest;
 import org.opentripplanner.transit.api.request.TripOnServiceDateRequestBuilder;
 import org.opentripplanner.transit.model.framework.FeedScopedId;
@@ -94,6 +93,9 @@ public static GraphQLFieldDefinition createQuery(
           .type(new GraphQLList(new GraphQLNonNull(Scalars.GraphQLString)))
       )
       .dataFetcher(environment -> {
+        // The null safety checks are not needed here - they are taken care of by the request
+        // object, but reuse let's use the mapping method and leave this improvement until all APIs
+        // are pushing this check into the domain request.
         var authorities = mapIDsToDomainNullSafe(environment.getArgument("authorities"));
         var lines = mapIDsToDomainNullSafe(environment.getArgument("lines"));
         var serviceJourneys = mapIDsToDomainNullSafe(environment.getArgument("serviceJourneys"));
@@ -102,10 +104,6 @@ public static GraphQLFieldDefinition createQuery(
         var operatingDays = environment.<List<LocalDate>>getArgument("operatingDays");
         var alterations = environment.<List<TripAlteration>>getArgument("alterations");
 
-        if (CollectionUtils.isEmpty(operatingDays)) {
-          throw new IllegalArgumentException("At least one operatingDay must be provided.");
-        }
-
         TripOnServiceDateRequestBuilder tripOnServiceDateRequestBuilder = TripOnServiceDateRequest
           .of()
           .withOperatingDays(operatingDays)
@@ -114,15 +112,11 @@ public static GraphQLFieldDefinition createQuery(
           .withServiceJourneys(serviceJourneys)
           .withReplacementFor(replacementFor);
 
-        if (!CollectionUtils.isEmpty(privateCodes)) {
-          tripOnServiceDateRequestBuilder =
-            tripOnServiceDateRequestBuilder.withPrivateCodes(privateCodes);
-        }
+        tripOnServiceDateRequestBuilder =
+          tripOnServiceDateRequestBuilder.withPrivateCodes(privateCodes);
 
-        if (!CollectionUtils.isEmpty(alterations)) {
-          tripOnServiceDateRequestBuilder =
-            tripOnServiceDateRequestBuilder.withAlterations(alterations);
-        }
+        tripOnServiceDateRequestBuilder =
+          tripOnServiceDateRequestBuilder.withAlterations(alterations);
 
         return GqlUtil
           .getTransitService(environment)

src/main/java/org/opentripplanner/framework/collection/ListUtils.java

@@ -69,4 +69,13 @@ public static <T> List<T> ofNullable(T input) {
       return List.of(input);
     }
   }
+
+  /**
+   * This method converts the given collection to an instance of a List. If the input is
+   * {@code null} an empty collection is returned. If not the {@link List#copyOf(Collection)} is
+   * called.
+   */
+  public static <T> List<T> nullSafeImmutableList(Collection<T> c) {
+    return (c == null) ? List.of() : List.copyOf(c);
+  }
 }

src/main/java/org/opentripplanner/transit/api/request/TripOnServiceDateRequest.java

@@ -2,6 +2,7 @@
 
 import java.time.LocalDate;
 import java.util.List;
+import org.opentripplanner.framework.collection.ListUtils;
 import org.opentripplanner.transit.model.framework.FeedScopedId;
 import org.opentripplanner.transit.model.timetable.TripAlteration;
 
@@ -33,13 +34,13 @@ protected TripOnServiceDateRequest(
     if (operatingDays == null || operatingDays.isEmpty()) {
       throw new IllegalArgumentException("operatingDays must have at least one date");
     }
-    this.operatingDays = List.copyOf(operatingDays);
-    this.authorities = List.copyOf(authorities);
-    this.lines = List.copyOf(lines);
-    this.serviceJourneys = List.copyOf(serviceJourneys);
-    this.replacementFor = List.copyOf(replacementFor);
-    this.privateCodes = List.copyOf(privateCodes);
-    this.alterations = List.copyOf(alterations);
+    this.operatingDays = ListUtils.nullSafeImmutableList(operatingDays);
+    this.authorities = ListUtils.nullSafeImmutableList(authorities);
+    this.lines = ListUtils.nullSafeImmutableList(lines);
+    this.serviceJourneys = ListUtils.nullSafeImmutableList(serviceJourneys);
+    this.replacementFor = ListUtils.nullSafeImmutableList(replacementFor);
+    this.privateCodes = ListUtils.nullSafeImmutableList(privateCodes);
+    this.alterations = ListUtils.nullSafeImmutableList(alterations);
   }
 
   public static TripOnServiceDateRequestBuilder of() {

src/main/java/org/opentripplanner/transit/api/request/TripOnServiceDateRequestBuilder.java

@@ -7,12 +7,12 @@
 
 public class TripOnServiceDateRequestBuilder {
 
-  private List<FeedScopedId> authorities = List.of();
-  private List<FeedScopedId> lines = List.of();
-  private List<FeedScopedId> serviceJourneys = List.of();
-  private List<FeedScopedId> replacementFor = List.of();
-  private List<String> privateCodes = List.of();
-  private List<TripAlteration> alterations = List.of();
+  private List<FeedScopedId> authorities;
+  private List<FeedScopedId> lines;
+  private List<FeedScopedId> serviceJourneys;
+  private List<FeedScopedId> replacementFor;
+  private List<String> privateCodes;
+  private List<TripAlteration> alterations;
   private List<LocalDate> operatingDays;
 
   protected TripOnServiceDateRequestBuilder() {}