ONOS-1479 -- GUI - augmenting topology view for extensibility:
- Added id field to property panel, as well as overloaded constructors.
- Added modify*Details() methods to UiTopoOverlay.
- Cleaned up use of string constants.
- Reworked RequestDetails in Topo view msg handler (and base).
- Fixed bug in topo UI where selected host title click caused exception on server.

Change-Id: Ib2a3cf60fae8ad8cda77a3b6933ee758262e6f3c
diff --git a/web/gui/src/main/java/org/onosproject/ui/impl/TopoOverlayCache.java b/web/gui/src/main/java/org/onosproject/ui/impl/TopoOverlayCache.java
index 3d6d900..f7690e8 100644
--- a/web/gui/src/main/java/org/onosproject/ui/impl/TopoOverlayCache.java
+++ b/web/gui/src/main/java/org/onosproject/ui/impl/TopoOverlayCache.java
@@ -74,12 +74,20 @@
         return isNullOrEmpty(id) ? NONE : overlays.get(id);
     }
 
+    /**
+     * Returns the current overlay instance.
+     * Note that this method always returns a reference; when there is no
+     * overlay selected the "NULL" overlay instance is returned.
+     *
+     * @return the current overlay
+     */
     public UiTopoOverlay currentOverlay() {
         return current;
     }
 
     /**
-     * Returns the number of overlays in the cache.
+     * Returns the number of overlays in the cache. Remember that this
+     * includes the "NULL" overlay, representing "no overlay selected".
      *
      * @return number of overlays
      */
@@ -88,16 +96,13 @@
     }
 
 
-
+    // overlay instance representing "no overlay selected"
     private static class NullOverlay extends UiTopoOverlay {
         public NullOverlay() {
             super(null);
         }
 
-        @Override
-        public void init() {
-        }
-
+        // override activate and deactivate, so no log messages are written
         @Override
         public void activate() {
         }
@@ -105,9 +110,5 @@
         @Override
         public void deactivate() {
         }
-
-        @Override
-        public void destroy() {
-        }
     }
 }