diff --git a/src/java/davmail/util/StringUtil.java b/src/java/davmail/util/StringUtil.java
index 61c0301a..b294400f 100644
--- a/src/java/davmail/util/StringUtil.java
+++ b/src/java/davmail/util/StringUtil.java
@@ -21,7 +21,7 @@ package davmail.util;
 /**
  * Various string handling methods
  */
-public class StringUtil {
+public final class StringUtil {
     private StringUtil() {
     }