core: make NumberUtils.formatDouble locale-independent (#150)

src/main/java/org/vaadin/addons/dramafinder/element/utils/NumberUtils.java

@@ -1,6 +1,8 @@
 package org.vaadin.addons.dramafinder.element.utils;
 
 import java.text.DecimalFormat;
+import java.text.DecimalFormatSymbols;
+import java.util.Locale;
 
 public class NumberUtils {
 
@@ -9,8 +11,11 @@ public static String formatDouble(double value) {
         if (value == Math.rint(value)) {
             return String.valueOf((long) value);
         } else {
-            // Use DecimalFormat to avoid scientific notation
-            DecimalFormat df = new DecimalFormat("0.######");
+            // Use DecimalFormat to avoid scientific notation. Locale.ROOT keeps the
+            // decimal separator a "." regardless of the JVM default locale, matching
+            // what the browser reports for native input value/step/min/max.
+            DecimalFormat df = new DecimalFormat("0.######",
+                    DecimalFormatSymbols.getInstance(Locale.ROOT));
             return df.format(value);
         }
     }

src/test/java/org/vaadin/addons/dramafinder/element/utils/NumberUtilsTest.java

@@ -0,0 +1,55 @@
+package org.vaadin.addons.dramafinder.element.utils;
+
+import java.util.Locale;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class NumberUtilsTest {
+
+    private final Locale defaultLocale = Locale.getDefault();
+    private final Locale defaultFormatLocale = Locale
+            .getDefault(Locale.Category.FORMAT);
+
+    @AfterEach
+    void restoreLocale() {
+        Locale.setDefault(defaultLocale);
+        Locale.setDefault(Locale.Category.FORMAT, defaultFormatLocale);
+    }
+
+    @Test
+    void formatDouble_wholeValue_noFractionalPart() {
+        assertEquals("5", NumberUtils.formatDouble(5d));
+        assertEquals("0", NumberUtils.formatDouble(0d));
+        assertEquals("-3", NumberUtils.formatDouble(-3d));
+    }
+
+    @Test
+    void formatDouble_fractionalValue_dotSeparator() {
+        assertEquals("0.5", NumberUtils.formatDouble(0.5));
+        assertEquals("5.5", NumberUtils.formatDouble(5.5));
+        assertEquals("-1.25", NumberUtils.formatDouble(-1.25));
+    }
+
+    @Test
+    void formatDouble_commaSeparatorLocale_stillUsesDot() {
+        Locale.setDefault(Locale.FRANCE);
+
+        assertEquals("0.5", NumberUtils.formatDouble(0.5));
+        assertEquals("5.5", NumberUtils.formatDouble(5.5));
+    }
+
+    @Test
+    void formatDouble_commaSeparatorFormatCategory_stillUsesDot() {
+        Locale.setDefault(Locale.Category.FORMAT, Locale.GERMANY);
+
+        assertEquals("0.5", NumberUtils.formatDouble(0.5));
+    }
+
+    @Test
+    void formatDouble_smallValue_noScientificNotation() {
+        assertEquals("0.000001", NumberUtils.formatDouble(0.000001));
+    }
+}