[ONOS-3904] Derived data type and formatting fixes

Change-Id: I1d68899e0056fa0db6322e83f7e9d3ff9b3b1ee0
diff --git a/utils/yangutils/src/main/java/org/onosproject/yangutils/parser/impl/TreeWalkListener.java b/utils/yangutils/src/main/java/org/onosproject/yangutils/parser/impl/TreeWalkListener.java
index 08b1da7..20e086f 100644
--- a/utils/yangutils/src/main/java/org/onosproject/yangutils/parser/impl/TreeWalkListener.java
+++ b/utils/yangutils/src/main/java/org/onosproject/yangutils/parser/impl/TreeWalkListener.java
@@ -16,6 +16,8 @@
 
 package org.onosproject.yangutils.parser.impl;
 
+import java.util.Stack;
+
 import org.antlr.v4.runtime.ParserRuleContext;
 import org.antlr.v4.runtime.tree.ErrorNode;
 import org.antlr.v4.runtime.tree.TerminalNode;
@@ -57,8 +59,6 @@
 import org.onosproject.yangutils.parser.impl.listeners.ValueListener;
 import org.onosproject.yangutils.parser.impl.listeners.VersionListener;
 
-import java.util.Stack;
-
 /**
  * ANTLR generates a parse-tree listener interface that responds to events
  * triggered by the built-in tree walker. The methods in listener are just