diff --git a/src/main/java/net/onrc/onos/ofcontroller/bgproute/RestClient.java b/src/main/java/net/onrc/onos/ofcontroller/bgproute/RestClient.java
index a9f2abe..9606d24 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/bgproute/RestClient.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/bgproute/RestClient.java
@@ -35,7 +35,7 @@
 				log.warn("The content received from {} is not json", str);
 			}		
 
-			BufferedReader br = new BufferedReader(new InputStreamReader((conn.getInputStream()))); 
+			BufferedReader br = new BufferedReader(new InputStreamReader((conn.getInputStream())));
 			String line;
 			while ((line = br.readLine()) != null) {
 				response.append(line);
diff --git a/src/main/java/net/onrc/onos/ofcontroller/core/internal/TopoLinkServiceImpl.java b/src/main/java/net/onrc/onos/ofcontroller/core/internal/TopoLinkServiceImpl.java
index 7c897d4..5e01578 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/core/internal/TopoLinkServiceImpl.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/core/internal/TopoLinkServiceImpl.java
@@ -75,7 +75,7 @@
 
 	}
 
-	private class ExtractLink implements PipeFunction<PathPipe<Vertex>, Link> {
+	private static class ExtractLink implements PipeFunction<PathPipe<Vertex>, Link> {
 		@Override
 		public Link compute(PathPipe<Vertex> pipe) {
 			long s_dpid = 0;
diff --git a/src/main/java/net/onrc/onos/ofcontroller/flowprogrammer/FlowProgrammer.java b/src/main/java/net/onrc/onos/ofcontroller/flowprogrammer/FlowProgrammer.java
index e94d948..4915cc7 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/flowprogrammer/FlowProgrammer.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/flowprogrammer/FlowProgrammer.java
@@ -44,7 +44,7 @@
 				       IOFSwitchListener {
     // flag to enable FlowSynchronizer
     private static final boolean enableFlowSync = true;
-    protected static Logger log = LoggerFactory.getLogger(FlowProgrammer.class);
+    protected static final Logger log = LoggerFactory.getLogger(FlowProgrammer.class);
     protected volatile IFloodlightProviderService floodlightProvider;
     protected volatile IControllerRegistryService registryService;
     protected volatile IRestApiService restApi;
diff --git a/src/main/java/net/onrc/onos/ofcontroller/flowprogrammer/FlowSynchronizer.java b/src/main/java/net/onrc/onos/ofcontroller/flowprogrammer/FlowSynchronizer.java
index 740e782..23bf7ae 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/flowprogrammer/FlowSynchronizer.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/flowprogrammer/FlowSynchronizer.java
@@ -102,6 +102,10 @@
 	    Set<FlowEntryWrapper> graphEntries = getFlowEntriesFromGraph();
 	    long step1 = System.nanoTime();
 	    Set<FlowEntryWrapper> switchEntries = getFlowEntriesFromSwitch();
+	    if (switchEntries == null) {
+	    	log.debug("getFlowEntriesFromSwitch() failed");
+	    	return null;
+	    }
 	    long step2 = System.nanoTime();
 	    SyncResult result = compare(graphEntries, switchEntries);
 	    long step3 = System.nanoTime();
@@ -216,12 +220,15 @@
 	    } catch (IOException e) {
 		// TODO Auto-generated catch block
 		e.printStackTrace();
+		return null;
 	    } catch (InterruptedException e) {
 		// TODO Auto-generated catch block
 		e.printStackTrace();
+		return null;
 	    } catch (ExecutionException e) {
 		// TODO Auto-generated catch block
 		e.printStackTrace();
+		return null;
 	    }
 
 	    Set<FlowEntryWrapper> results = new HashSet<FlowEntryWrapper>();
@@ -338,7 +345,7 @@
 	 */
 	@Override
 	public boolean equals(Object obj){
-	    if(obj.getClass() == this.getClass()) {
+	    if(obj != null && obj.getClass() == this.getClass()) {
 		FlowEntryWrapper entry = (FlowEntryWrapper) obj;
 		// TODO: we need to actually compare the match + actions
 		return this.flowEntryId.equals(entry.flowEntryId);
diff --git a/src/main/java/net/onrc/onos/ofcontroller/topology/TopologyManager.java b/src/main/java/net/onrc/onos/ofcontroller/topology/TopologyManager.java
index 4feaa8e..d447580 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/topology/TopologyManager.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/topology/TopologyManager.java
@@ -211,7 +211,7 @@
      */
     @Override
     public void dropTopology(Topology topology) {
-	topology = null;
+    // nothing to do
     }
 
     /**
diff --git a/src/main/java/net/onrc/onos/ofcontroller/util/Dpid.java b/src/main/java/net/onrc/onos/ofcontroller/util/Dpid.java
index bd91daa..81223d2 100644
--- a/src/main/java/net/onrc/onos/ofcontroller/util/Dpid.java
+++ b/src/main/java/net/onrc/onos/ofcontroller/util/Dpid.java
@@ -13,7 +13,7 @@
 @JsonDeserialize(using=DpidDeserializer.class)
 @JsonSerialize(using=DpidSerializer.class)
 public class Dpid {
-    static public long UNKNOWN = 0;
+    static public final long UNKNOWN = 0;
 
     private long value;