Use more concrete type for return value

Same resources are not allowed for a parent and Set is more suitable than
Collection

Change-Id: Ib45179819d81376678bf8949864b12b5bd721085
diff --git a/core/store/dist/src/main/java/org/onosproject/store/newresource/impl/ConsistentResourceStore.java b/core/store/dist/src/main/java/org/onosproject/store/newresource/impl/ConsistentResourceStore.java
index f13481c..defc955 100644
--- a/core/store/dist/src/main/java/org/onosproject/store/newresource/impl/ConsistentResourceStore.java
+++ b/core/store/dist/src/main/java/org/onosproject/store/newresource/impl/ConsistentResourceStore.java
@@ -17,6 +17,7 @@
 
 import com.google.common.annotations.Beta;
 import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Maps;
 import org.apache.felix.scr.annotations.Activate;
 import org.apache.felix.scr.annotations.Component;
@@ -389,16 +390,16 @@
     }
 
     @Override
-    public Collection<Resource> getChildResources(Resource parent) {
+    public Set<Resource> getChildResources(Resource parent) {
         checkNotNull(parent);
         if (!(parent instanceof DiscreteResource)) {
             // only Discrete resource can have child resource
-            return ImmutableList.of();
+            return ImmutableSet.of();
         }
 
         Versioned<Set<Resource>> children = childMap.get((DiscreteResource) parent);
         if (children == null) {
-            return ImmutableList.of();
+            return ImmutableSet.of();
         }
 
         return children.value();