Add javadoc

- Changed constructor to protected, since
  TopologyObject itself is not intended to be instantiated.

Change-Id: I1b407f98504009c04d281ac11380509ec09f3607
diff --git a/src/main/java/net/onrc/onos/core/topology/TopologyObject.java b/src/main/java/net/onrc/onos/core/topology/TopologyObject.java
index fd2d0ce..2dfd613 100644
--- a/src/main/java/net/onrc/onos/core/topology/TopologyObject.java
+++ b/src/main/java/net/onrc/onos/core/topology/TopologyObject.java
@@ -1,11 +1,22 @@
 package net.onrc.onos.core.topology;
 
 
+/**
+ * Base class for Topology Objects.
+ */
 public class TopologyObject {
 
+    /**
+     * Topology instance this object belongs to.
+     */
     protected final Topology topology;
 
-    public TopologyObject(Topology topology) {
+    /**
+     * Constructor.
+     *
+     * @param topology Topology instance this object belongs to
+     */
+    protected TopologyObject(Topology topology) {
         this.topology = topology;
     }