fix: require transfers stop ids when transfer_type is empty

main/src/main/java/org/mobilitydata/gtfsvalidator/validator/TransferStopIdsConditionalValidator.java

@@ -35,6 +35,9 @@
  *   <li>{@link MissingRequiredFieldNotice} - {@code from_stop_id} is missing or {@code to_stop_id}
  *       is missing for all transfer types except for in-seat transfer types
  * </ul>
+ *
+ * <p>An empty {@code transfer_type} means a recommended transfer point, which is the same as {@code
+ * transfer_type=0}, so both stop ids are required for it as well.
  */
 @GtfsValidator
 public class TransferStopIdsConditionalValidator extends FileValidator {
@@ -49,9 +52,7 @@ public TransferStopIdsConditionalValidator(GtfsTransferTableContainer transfersC
   @Override
   public void validate(NoticeContainer noticeContainer) {
     for (GtfsTransfer transfer : transfersContainer.getEntities()) {
-      if (transfer.hasTransferType()) {
-        validateTransferEntity(transfer, noticeContainer);
-      }
+      validateTransferEntity(transfer, noticeContainer);
     }
   }
 

main/src/test/java/org/mobilitydata/gtfsvalidator/validator/TransferStopIdsConditionalValidatorTest.java

@@ -94,4 +94,42 @@ public void testTransferFromStopIdNoInSeatTransfer() {
       noticeContainer.getValidationNotices().clear();
     }
   }
+
+  /**
+   * This test is used to verify that the validator generates a notice when the stop ids are missing
+   * and the {@code transfer_type} is empty, which is a recommended transfer point.
+   */
+  @Test
+  public void testTransferMissingStopIdsEmptyTransferType() {
+    GtfsTransferTableContainer gtfsTransferTableContainer =
+        GtfsTransferTableContainer.forEntities(
+            ImmutableList.of(new GtfsTransfer.Builder().build()), noticeContainer);
+
+    new TransferStopIdsConditionalValidator(gtfsTransferTableContainer).validate(noticeContainer);
+
+    assertThat(noticeContainer.getValidationNotices())
+        .containsExactlyElementsIn(
+            Arrays.asList(
+                new MissingRequiredFieldNotice(
+                    GtfsTransfer.FILENAME, 0, GtfsTransfer.FROM_STOP_ID_FIELD_NAME),
+                new MissingRequiredFieldNotice(
+                    GtfsTransfer.FILENAME, 0, GtfsTransfer.TO_STOP_ID_FIELD_NAME)));
+  }
+
+  /**
+   * This test is used to verify that the validator does not generate a notice when the stop ids are
+   * present and the {@code transfer_type} is empty.
+   */
+  @Test
+  public void testTransferStopIdsPresentEmptyTransferTypeNoNotice() {
+    GtfsTransferTableContainer gtfsTransferTableContainer =
+        GtfsTransferTableContainer.forEntities(
+            ImmutableList.of(
+                new GtfsTransfer.Builder().setFromStopId("stop1").setToStopId("stop2").build()),
+            noticeContainer);
+
+    new TransferStopIdsConditionalValidator(gtfsTransferTableContainer).validate(noticeContainer);
+
+    assertThat(noticeContainer.getValidationNotices()).isEmpty();
+  }
 }