Tighten visibility of constant

Change-Id: I05a54e4389d08499b21109868adec18ddbe2bb8d
diff --git a/core/net/src/main/java/org/onosproject/net/intent/impl/IntentManager.java b/core/net/src/main/java/org/onosproject/net/intent/impl/IntentManager.java
index d214c7f..5ec13bf 100644
--- a/core/net/src/main/java/org/onosproject/net/intent/impl/IntentManager.java
+++ b/core/net/src/main/java/org/onosproject/net/intent/impl/IntentManager.java
@@ -82,8 +82,8 @@
 
     private static final Logger log = getLogger(IntentManager.class);
 
-    public static final String INTENT_NULL = "Intent cannot be null";
-    public static final String INTENT_ID_NULL = "Intent key cannot be null";
+    private static final String INTENT_NULL = "Intent cannot be null";
+    private static final String INTENT_ID_NULL = "Intent key cannot be null";
 
     private static final EnumSet<IntentState> RECOMPILE
             = EnumSet.of(INSTALL_REQ, FAILED, WITHDRAW_REQ);