StringBuffer -> StringBuilder

My use case does not require thread-safety.

Change-Id: Ibb4d639c2980ede301577ab1e39958f5856444c7
diff --git a/src/main/java/net/onrc/onos/core/datastore/ObjectExistsException.java b/src/main/java/net/onrc/onos/core/datastore/ObjectExistsException.java
index 08be74f..13f7240 100644
--- a/src/main/java/net/onrc/onos/core/datastore/ObjectExistsException.java
+++ b/src/main/java/net/onrc/onos/core/datastore/ObjectExistsException.java
@@ -14,12 +14,12 @@
 
     public ObjectExistsException(final IKVTableID tableID, final byte[] key,
                                  final Throwable cause) {
-        super(ByteArrayUtil.toHexStringBuffer(key, ":")
+        super(ByteArrayUtil.toHexStringBuilder(key, ":")
                 + " already exist on table:" + tableID, cause);
     }
 
     public ObjectExistsException(final IKVTableID tableID, final byte[] key) {
-        super(ByteArrayUtil.toHexStringBuffer(key, ":")
+        super(ByteArrayUtil.toHexStringBuilder(key, ":")
                 + " already exist on table:" + tableID);
     }
 }