Merge pull request #439 from y-higuchi/add_annotation

Add @Override to methods which implements an Interface.
diff --git a/src/main/java/net/onrc/onos/graph/GraphDBConnection.java b/src/main/java/net/onrc/onos/graph/GraphDBConnection.java
index b24907a..bebcdd2 100644
--- a/src/main/java/net/onrc/onos/graph/GraphDBConnection.java
+++ b/src/main/java/net/onrc/onos/graph/GraphDBConnection.java
@@ -128,7 +128,7 @@
 	 */
 	@Override
 	public Boolean isValid() {
-		return (graph != null || graph.isOpen());
+		return (graph != null && graph.isOpen());
 	}
 
 	/**