From ff9e733993b73914c2f1b6677a70f485b8ae0d56 Mon Sep 17 00:00:00 2001
From: Gary Gregory <garydgregory@gmail.com>
Date: Mon, 18 May 2026 08:19:48 -0400
Subject: [PATCH] FastDatePrinter WeekYear rule calls Calendar.getWeekYear()
 without checking isWeekDateSupported()

The method would throw UnsupportedOperationException
---
 .../commons/lang3/time/FastDatePrinter.java   |  4 ++-
 .../lang3/time/FastDateFormatTest.java        | 30 +++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/src/main/java/org/apache/commons/lang3/time/FastDatePrinter.java b/src/main/java/org/apache/commons/lang3/time/FastDatePrinter.java
index 6063e8aafb2..42a309f02d1 100644
--- a/src/main/java/org/apache/commons/lang3/time/FastDatePrinter.java
+++ b/src/main/java/org/apache/commons/lang3/time/FastDatePrinter.java
@@ -862,7 +862,9 @@ private static final class WeekYear implements NumberRule {
 
         @Override
         public void appendTo(final Appendable buffer, final Calendar calendar) throws IOException {
-            rule.appendTo(buffer, calendar.getWeekYear());
+            // Some Calendar implementations (JapaneseImperialCalendar) do not support week-dates.
+            // Fall back to Calendar.YEAR in that case.
+            rule.appendTo(buffer, calendar.isWeekDateSupported() ? calendar.getWeekYear() : calendar.get(Calendar.YEAR));
         }
 
         @Override
diff --git a/src/test/java/org/apache/commons/lang3/time/FastDateFormatTest.java b/src/test/java/org/apache/commons/lang3/time/FastDateFormatTest.java
index b992164e862..6c084b634ff 100644
--- a/src/test/java/org/apache/commons/lang3/time/FastDateFormatTest.java
+++ b/src/test/java/org/apache/commons/lang3/time/FastDateFormatTest.java
@@ -110,6 +110,36 @@ private AtomicLongArray measureTime(final Format printer, final Format parser) t
         return totalElapsed;
     }
 
+    /**
+     * Pre-patch: UnsupportedOperationException when formatting a Japanese Imperial Calendar with 'Y' (week-year) pattern.
+     * <p>
+     * Post-patch: falls back to Calendar.YEAR and formats successfully.
+     * </p>
+     */
+    @Test
+    void testJapaneseImperialCalendarWeekYearDoesNotThrow() {
+        final Locale japaneseImperial = new Locale("ja", "JP", "JP");
+        final Calendar japaneseCal = Calendar.getInstance(japaneseImperial);
+        final FastDateFormat fdp = FastDateFormat.getInstance("YYYY-MM-dd");
+        assertNotNull(fdp.format(japaneseCal), "Formatting JapaneseImperialCalendar with YYYY pattern must not throw UnsupportedOperationException");
+    }
+
+    /**
+     * Pre-patch: UnsupportedOperationException when formatting a Japanese Imperial
+     * <p>
+     * Calendar with 'Y' (week-year) pattern. Post-patch: falls back to Calendar.YEAR and formats successfully.
+     * </p>
+     * <p>
+     * Also test that a regular Gregorian calendar works fine with YYYY.
+     * </p>
+     */
+    @Test
+    void testGregorianCalendarWeekYearWorks() {
+        final Calendar cal = Calendar.getInstance();
+        final FastDateFormat fdp = FastDateFormat.getInstance("YYYY-MM-dd");
+        assertNotNull(fdp.format(cal), "Formatting Gregorian Calendar with YYYY pattern should always work");
+    }
+
     @DefaultLocale(language = "en", country = "US")
     @Test
     void test_changeDefault_Locale_DateInstance() {