Add short-cut path taking when the paremter type is equal to own type

Change-Id: Ifbc3ea11c901b1496adcc8d0a372c86cd27969e2

@@ -109,9 +109,13 @@ final class EncodableDiscreteResources implements DiscreteResources {
 
     @Override
     public DiscreteResources add(DiscreteResources other) {
-        Set<DiscreteResource> union = Sets.union(values(), other.values());
+        if (other instanceof EncodableDiscreteResources) {
+            return of(parent, Sets.union(this.values(), other.values()));
+        } else if (other instanceof EmptyDiscreteResources) {
+            return this;
+        }
 
-        return of(parent, union);
+        return DiscreteResources.of(Sets.union(this.values(), other.values()));
     }
 
     @Override

@@ -81,9 +81,13 @@ final class GenericDiscreteResources implements DiscreteResources {
     // returns a new instance, not mutate the current instance
     @Override
     public DiscreteResources add(DiscreteResources other) {
-        Set<DiscreteResource> newValues = new LinkedHashSet<>(this.values);
-        newValues.addAll(other.values());
-        return new GenericDiscreteResources(newValues);
+        if (other instanceof GenericDiscreteResources) {
+            return new GenericDiscreteResources(Sets.union(this.values(), other.values()));
+        } else if (other instanceof EmptyDiscreteResources) {
+            return this;
+        }
+
+        return DiscreteResources.of(Sets.union(this.values(), other.values()));
     }
 
     @Override

@@ -93,6 +93,15 @@ final class UnifiedDiscreteResources implements DiscreteResources {
 
     @Override
     public DiscreteResources add(DiscreteResources other) {
+        if (other instanceof UnifiedDiscreteResources) {
+            UnifiedDiscreteResources cast = (UnifiedDiscreteResources) other;
+            return new UnifiedDiscreteResources(
+                    this.generics.add(cast.generics),
+                    this.encodables.add(cast.encodables));
+        } else if (other instanceof EmptyDiscreteResources) {
+            return this;
+        }
+
         return of(Sets.union(this.values(), other.values()));
     }