[ONOS-2499]update accordingly for ONOS-2408's review recommendation

Change-Id: I24e7c4acbe38e4286b994c5bf50d22def945fb29
diff --git a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/AtomicColumnType.java b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/AtomicColumnType.java
index e083754..881755a 100644
--- a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/AtomicColumnType.java
+++ b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/AtomicColumnType.java
@@ -34,7 +34,7 @@
      * @param baseType BaseType entity
      */
     public AtomicColumnType(BaseType baseType) {
-        checkNotNull(baseType, "BaseType is not null");
+        checkNotNull(baseType, "BaseType cannot be null");
         this.baseType = baseType;
         this.min = 1;
         this.max = 1;
@@ -47,7 +47,7 @@
      * @param max max constraint
      */
     public AtomicColumnType(BaseType baseType, int min, int max) {
-        checkNotNull(baseType, "BaseType is not null");
+        checkNotNull(baseType, "BaseType cannot be null");
         this.baseType = baseType;
         this.min = min;
         this.max = max;
diff --git a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/KeyValuedColumnType.java b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/KeyValuedColumnType.java
index 2419cdd..8407457 100644
--- a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/KeyValuedColumnType.java
+++ b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/KeyValuedColumnType.java
@@ -39,8 +39,8 @@
      */
     public KeyValuedColumnType(BaseType keyType, BaseType valueType, int min,
                                int max) {
-        checkNotNull(keyType, "keyType is not null");
-        checkNotNull(valueType, "valueType is not null");
+        checkNotNull(keyType, "keyType cannot be null");
+        checkNotNull(valueType, "valueType cannot be null");
         this.keyType = keyType;
         this.valueType = valueType;
         this.min = min;
diff --git a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/UuidBaseType.java b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/UuidBaseType.java
index 9824966..46e0d9f 100644
--- a/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/UuidBaseType.java
+++ b/ovsdb/rfc/src/main/java/org/onosproject/ovsdb/rfc/schema/type/UuidBaseType.java
@@ -16,6 +16,7 @@
 package org.onosproject.ovsdb.rfc.schema.type;
 
 import static com.google.common.base.MoreObjects.toStringHelper;
+import static com.google.common.base.Preconditions.checkNotNull;
 
 import java.util.Objects;
 
@@ -62,6 +63,7 @@
      * @param refType refType constraint
      */
     public UuidBaseType(String refTable, String refType) {
+        checkNotNull(refType, "refType cannot be null");
         this.refTable = refTable;
         this.refType = refType;
     }