8369050: DecimalFormat Rounding Errors for Fractional Ties Near Zero

Reviewed-by: naoto

src/java.base/share/classes/java/text/DigitList.java

@@ -356,6 +356,11 @@ final class DigitList implements Cloneable {
         }
         decimalAt = count + exp;
 
+        // Eliminate trailing zeros.
+        while (count > 1 && digits[count - 1] == '0') {
+            --count;
+        }
+
         if (fixedPoint) {
             // The negative of the exponent represents the number of leading
             // zeros between the decimal and the first non-zero digit, for
@@ -382,11 +387,6 @@ final class DigitList implements Cloneable {
             // else fall through
         }
 
-        // Eliminate trailing zeros.
-        while (count > 1 && digits[count - 1] == '0') {
-            --count;
-        }
-
         // Eliminate digits beyond maximum digits to be displayed.
         // Round up if appropriate.
         round(fixedPoint ? (maximumDigits + decimalAt) : maximumDigits,

test/jdk/java/text/Format/DecimalFormat/RoundingTiesNearZeroTest.java

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8369050
+ * @summary Check rounding of DecimalFormat on tie cases when the maximum
+ *      fraction digits allowed is one less than the position of the first
+ *      significant digit in the double.
+ * @run junit RoundingTiesNearZeroTest
+ */
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+import java.math.RoundingMode;
+import java.text.NumberFormat;
+import java.util.Locale;
+import java.util.stream.Stream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class RoundingTiesNearZeroTest {
+
+    // Safe to re-use since we are not testing any fast-path cases
+    // so state is irrelevant
+    private static final NumberFormat format = NumberFormat.getInstance(Locale.US);
+
+    @ParameterizedTest
+    @MethodSource("ties")
+    void roundingTieTest(RoundingMode rm, int maxDigits, double db, String expected) {
+        format.setRoundingMode(rm);
+        format.setMaximumFractionDigits(maxDigits);
+        assertEquals(expected, format.format(db), "Rounding failed under " + rm);
+    }
+
+    static Stream<Arguments> ties() {
+        return Stream.of(
+                // 1) String is exact as binary
+                // 0.5 -> 0.5
+                Arguments.of(RoundingMode.HALF_EVEN, 0, 0.5, "0"),
+                Arguments.of(RoundingMode.HALF_UP, 0, 0.5, "1"),
+                Arguments.of(RoundingMode.HALF_DOWN, 0, 0.5, "0"),
+                // 2) String is rounded up from binary
+                // 0.0000005 -> 4.999999999999999773740559129431293428069693618454039096832275390625E-7
+                Arguments.of(RoundingMode.HALF_EVEN, 6, 0.0000005, "0"),
+                Arguments.of(RoundingMode.HALF_UP, 6, 0.0000005, "0"),
+                Arguments.of(RoundingMode.HALF_DOWN, 6, 0.0000005, "0"),
+                // 3) String is not rounded up from binary
+                // Non-exponential notation
+                // 0.05 -> 0.05000000000000000277555756156289135105907917022705078125
+                Arguments.of(RoundingMode.HALF_EVEN, 1, 0.05, "0.1"),
+                Arguments.of(RoundingMode.HALF_UP, 1, 0.05, "0.1"),
+                Arguments.of(RoundingMode.HALF_DOWN, 1, 0.05, "0.1"),
+                // Exponential notation
+                // 0.00005 -> 0.0000500000000000000023960868011929647991564706899225711822509765625
+                Arguments.of(RoundingMode.HALF_EVEN, 4, 0.00005, "0.0001"),
+                Arguments.of(RoundingMode.HALF_UP, 4, 0.00005, "0.0001"),
+                Arguments.of(RoundingMode.HALF_DOWN, 4, 0.00005, "0.0001")
+        );
+    }
+}

test/jdk/java/text/Format/NumberFormat/NumberRegression.java

@@ -30,7 +30,7 @@
  * 4125885 4134034 4134300 4140009 4141750 4145457 4147295 4147706 4162198
  * 4162852 4167494 4170798 4176114 4179818 4212072 4212073 4216742 4217661
  * 4243011 4243108 4330377 4233840 4241880 4833877 6177299 8008577 8227313
- * 8174269
+ * 8174269 8369050
  * @summary Regression tests for NumberFormat and associated classes
  * @library /java/text/testlib
  * @build HexDumpReader TestUtils
@@ -1776,7 +1776,7 @@ public class NumberRegression {
                 "2%", "1%", "2%", "2%", "1%",
                 "0%", "0%", "1%", "1%", "1%",
                 "0", "2", "0.2", "0.6", "0.04",
-                "0.04", "0.000", "0.002",
+                "0.04", "0.001", "0.002",
         };
         for (int i = 0; i < input.length; i++) {
             DecimalFormat format = new DecimalFormat(pattern[i]);