diff --git a/src/main/java/com/not2excel/api/util/Colorizer.java b/src/main/java/com/not2excel/api/util/Colorizer.java
index be955bd..b94f833 100644
--- a/src/main/java/com/not2excel/api/util/Colorizer.java
+++ b/src/main/java/com/not2excel/api/util/Colorizer.java
@@ -15,7 +15,7 @@
  * All rights Reserved
  * Please read included LICENSE file
  */
-public class Colorizer
+public final class Colorizer
 {
     private static final int MAX_SIZE = 1000;
     private static final Map<String, String>    colorizedStrings = new ConcurrentHashMap<String, String>();
diff --git a/src/main/java/com/not2excel/api/util/StringUtil.java b/src/main/java/com/not2excel/api/util/StringUtil.java
index 2c8cff5..329e1b7 100644
--- a/src/main/java/com/not2excel/api/util/StringUtil.java
+++ b/src/main/java/com/not2excel/api/util/StringUtil.java
@@ -8,7 +8,7 @@
  * All rights Reserved
  * Please read included LICENSE file
  */
-public class StringUtil
+public final class StringUtil
 {
     private static final Random random = new Random();