[ONOS-3222] Implement toString method for each Packet class

Change-Id: I17d72338d4202117d08b3dca9463be35a87a0c1e
diff --git a/utils/misc/src/main/java/org/onlab/packet/UDP.java b/utils/misc/src/main/java/org/onlab/packet/UDP.java
index a30c9a9..25221fa 100644
--- a/utils/misc/src/main/java/org/onlab/packet/UDP.java
+++ b/utils/misc/src/main/java/org/onlab/packet/UDP.java
@@ -22,6 +22,7 @@
 import java.util.HashMap;
 import java.util.Map;
 
+import static com.google.common.base.MoreObjects.toStringHelper;
 import static org.onlab.packet.PacketUtils.*;
 
 /**
@@ -303,4 +304,14 @@
             return udp;
         };
     }
+
+    @Override
+    public String toString() {
+        return toStringHelper(getClass())
+                .add("sourcePort", Integer.toString(sourcePort))
+                .add("destinationPort", Integer.toString(destinationPort))
+                .add("length", Short.toString(length))
+                .add("checksum", Short.toString(checksum))
+                .toString();
+    }
 }