Update debug codes

Change-Id: I685ca0310c2132e566c473601e2b93e30072bb7e
diff --git a/src/main/java/net/onrc/onos/ofcontroller/util/Pair.java b/src/main/java/net/onrc/onos/ofcontroller/util/Pair.java
index 2245758..bdc24d3 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/util/Pair.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/util/Pair.java
@@ -17,4 +17,9 @@
 	this.first = first;
 	this.second = second;
     }
+
+    @Override
+    public String toString() {
+	return String.format("<%s, %s>", first, second);
+    }
 }