Compute the flow ID at build time rather than when fromApp() is called.

Depending on the order that the application calls the builder methods,
there may not be any selector, priority, etc. added at the time fromApp() is
called. This means there's no entropy to the hash function so all flows end
up with the same flow ID.

Change-Id: I9badedc343fdf6f3ee7f73547955cd84302d9226

@@ -291,6 +291,7 @@ public class DefaultFlowRule implements FlowRule {
     public static final class Builder implements FlowRule.Builder {
 
         private FlowId flowId;
+        private ApplicationId appId;
         private Integer priority;
         private DeviceId deviceId;
         private Integer tableId = 0;
@@ -307,7 +308,7 @@ public class DefaultFlowRule implements FlowRule {
 
         @Override
         public FlowRule.Builder fromApp(ApplicationId appId) {
-            this.flowId = computeFlowId(appId);
+            this.appId = appId;
             return this;
         }
 
@@ -357,16 +358,22 @@ public class DefaultFlowRule implements FlowRule {
 
         @Override
         public FlowRule build() {
-            checkNotNull(flowId != null, "Either an application" +
+            checkArgument(flowId != null || appId != null, "Either an application" +
                     " id or a cookie must be supplied");
-            checkNotNull(selector != null, "Traffic selector cannot be null");
-            checkNotNull(timeout != null || permanent != null, "Must either have " +
+            checkNotNull(selector, "Traffic selector cannot be null");
+            checkArgument(timeout != null || permanent != null, "Must either have " +
                     "a timeout or be permanent");
-            checkNotNull(deviceId != null, "Must refer to a device");
-            checkNotNull(priority != null, "Priority cannot be null");
+            checkNotNull(deviceId, "Must refer to a device");
+            checkNotNull(priority, "Priority cannot be null");
             checkArgument(priority >= MIN_PRIORITY, "Priority cannot be less than " +
                     MIN_PRIORITY);
 
+            // Computing a flow ID based on appId takes precedence over setting
+            // the flow ID directly
+            if (appId != null) {
+                flowId = computeFlowId(appId);
+            }
+
             return new DefaultFlowRule(deviceId, selector, treatment, priority,
                                        flowId, permanent, timeout, tableId);
         }