diff --git a/src/test/java/org/apache/commons/lang3/StringsUtilCapitalizeTest.java b/src/test/java/org/apache/commons/lang3/StringsUtilCapitalizeTest.java
new file mode 100644
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+++ b/src/test/java/org/apache/commons/lang3/StringsUtilCapitalizeTest.java
@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.lang3;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import org.junit.jupiter.api.Test;
+
+
+public class StringsUtilCapitalizeTest {
+      @Test
+    void capitalize_NormalLowercaseWord_ReturnsCapitalized() {
+        assertEquals("Hello", StringUtils.capitalize("hello"));
+    }
+
+    @Test
+    void capitalize_AlreadyCapitalizedWord_ReturnsSameString() {
+        assertEquals("Hello", StringUtils.capitalize("Hello"));
+    }
+
+    @Test
+    void capitalize_AllUppercaseWord_ReturnsSameString() {
+        assertEquals("HELLO", StringUtils.capitalize("HELLO"));
+    }
+
+    @Test
+    void capitalize_EmptyString_ReturnsSameString() {
+        assertEquals("", StringUtils.capitalize(""));
+    }
+
+    @Test
+    void capitalize_NullInput_ReturnsNull() {
+        assertNull(StringUtils.capitalize(null));
+    }
+
+    @Test
+    void capitalize_SingleLowercaseChar_ReturnsCapitalizedChar() {
+        assertEquals("A", StringUtils.capitalize("a"));
+    }
+
+    @Test
+    void capitalize_NonLetterFirstChar_ReturnsSameString() {
+        assertEquals("1hello", StringUtils.capitalize("1hello"));
+    }
+
+    @Test
+    void capitalize_WhitespaceOnly_ReturnsSameString() {
+        assertEquals("   ", StringUtils.capitalize("   "));
+    }
+}