GUI -- Made OOTB formatters and comparators final classes.

Change-Id: I0ea59bf92d97b6d104b7d3770fb5f2e93308bbdc
diff --git a/core/api/src/main/java/org/onosproject/ui/table/cell/DefaultCellComparator.java b/core/api/src/main/java/org/onosproject/ui/table/cell/DefaultCellComparator.java
index fa49755..238116a 100644
--- a/core/api/src/main/java/org/onosproject/ui/table/cell/DefaultCellComparator.java
+++ b/core/api/src/main/java/org/onosproject/ui/table/cell/DefaultCellComparator.java
@@ -25,7 +25,10 @@
  * compares the resulting strings. Note that null values are acceptable and
  * are considered "smaller" than any non-null value.
  */
-public class DefaultCellComparator extends AbstractCellComparator {
+public final class DefaultCellComparator extends AbstractCellComparator {
+
+    // non-instantiable
+    private DefaultCellComparator() { }
 
     @Override
     protected int nonNullCompare(Object o1, Object o2) {