🐛 Fix RecurrenceRule#setByPart, fixes #127 (#128)

src/main/java/org/dmfs/rfc5545/recur/RecurrenceRule.java

@@ -1960,8 +1960,10 @@ public void setByDayPart(List<WeekdayNum> value)
         {
             mParts.remove(Part.BYDAY);
         }
-
-        mParts.put(Part.BYDAY, value);
+        else
+        {
+            mParts.put(Part.BYDAY, value);
+        }
     }
 
 

src/test/java/org/dmfs/rfc5545/recur/RecurrenceRuleTest.java

@@ -21,6 +21,8 @@
 import org.dmfs.rfc5545.Weekday;
 import org.junit.jupiter.api.Test;
 
+import java.util.Collections;
+
 import static org.dmfs.jems2.hamcrest.matchers.LambdaMatcher.having;
 import static org.dmfs.jems2.hamcrest.matchers.fragile.BrokenFragileMatcher.throwing;
 import static org.dmfs.jems2.hamcrest.matchers.single.SingleMatcher.hasValue;
@@ -157,4 +159,32 @@ void testAllDayUntilAndDateTimeStart() throws InvalidRecurrenceRuleException
             is(having(
                 r -> () -> r.iterator(DateTime.parse("20230301T000000")), is(throwing(IllegalArgumentException.class)))));
     }
+
+    /**
+     * see https://github.com/dmfs/lib-recur/issues/127
+     */
+    @Test
+    public void testSetByDayPart() throws InvalidRecurrenceRuleException {
+        RecurrenceRule rrule = new RecurrenceRule("FREQ=MONTHLY;BYMONTHDAY=31;COUNT=3");
+        rrule.setByDayPart(Collections.emptyList());
+
+        assertThat(rrule,
+            allOf(validRule(DateTime.parse("20230501T000000"),
+                    walking(),
+                    results(3)),
+                generates("20230501T000000",
+                    "20230531T000000",
+                    "20230731T000000",
+                    "20230831T000000")));
+
+        rrule.setByPart(null);
+        assertThat(rrule,
+            allOf(validRule(DateTime.parse("20230501T000000"),
+                    walking(),
+                    results(3)),
+                generates("20230501T000000",
+                    "20230531T000000",
+                    "20230731T000000",
+                    "20230831T000000")));
+    }
 }