Use lower bound wildcard for API to be more flexible

Change-Id: Ia6bcec5afb2e872b55bae4bbd9139ad35ce01284
diff --git a/core/api/src/main/java/org/onosproject/net/newresource/ResourceService.java b/core/api/src/main/java/org/onosproject/net/newresource/ResourceService.java
index 1a2fa2c..c28720c 100644
--- a/core/api/src/main/java/org/onosproject/net/newresource/ResourceService.java
+++ b/core/api/src/main/java/org/onosproject/net/newresource/ResourceService.java
@@ -66,7 +66,7 @@
      * @param resources resources to be allocated
      * @return non-empty list of allocation information if succeeded, otherwise empty list
      */
-    List<ResourceAllocation> allocate(ResourceConsumer consumer, List<Resource> resources);
+    List<ResourceAllocation> allocate(ResourceConsumer consumer, List<? extends Resource> resources);
 
     /**
      * Transactionally allocates the specified resources to the specified user.