Define an interface to generate FlowId

Change-Id: I9279082b7a0803670fed0e38e6f04f7002e64b9d
diff --git a/src/main/java/net/onrc/onos/api/flowmanager/FlowIdGenerator.java b/src/main/java/net/onrc/onos/api/flowmanager/FlowIdGenerator.java
new file mode 100644
index 0000000..e3ab9e3
--- /dev/null
+++ b/src/main/java/net/onrc/onos/api/flowmanager/FlowIdGenerator.java
@@ -0,0 +1,13 @@
+package net.onrc.onos.api.flowmanager;
+
+/**
+ * An generator of {@link FlowId}.
+ */
+public interface FlowIdGenerator {
+    /**
+     * Generates a globally unique {@link FlowId} instance.
+     *
+     * @return a globally unique {@link FlowId} instance.
+     */
+    FlowId getNextId();
+}