Fix NPE in AbstractPathService.

Was triggering NPE when used in plain PathManager

- implement PathService to inherit interface javadoc

Change-Id: I345ec84ed3e61a383574fd58679fb00291b4bba0
diff --git a/core/net/src/main/java/org/onosproject/net/topology/impl/PathManager.java b/core/net/src/main/java/org/onosproject/net/topology/impl/PathManager.java
index 9a04fff..5c41ff4 100644
--- a/core/net/src/main/java/org/onosproject/net/topology/impl/PathManager.java
+++ b/core/net/src/main/java/org/onosproject/net/topology/impl/PathManager.java
@@ -18,13 +18,17 @@
 import org.apache.felix.scr.annotations.Activate;
 import org.apache.felix.scr.annotations.Component;
 import org.apache.felix.scr.annotations.Deactivate;
+import org.apache.felix.scr.annotations.Reference;
+import org.apache.felix.scr.annotations.ReferenceCardinality;
 import org.apache.felix.scr.annotations.Service;
 import org.onosproject.net.DisjointPath;
 import org.onosproject.net.ElementId;
 import org.onosproject.net.Link;
 import org.onosproject.net.Path;
+import org.onosproject.net.host.HostService;
 import org.onosproject.net.topology.LinkWeight;
 import org.onosproject.net.topology.PathService;
+import org.onosproject.net.topology.TopologyService;
 import org.onosproject.net.topology.AbstractPathService;
 import org.slf4j.Logger;
 
@@ -45,11 +49,19 @@
 @Service
 public class PathManager extends AbstractPathService implements PathService {
 
-
     private final Logger log = getLogger(getClass());
 
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected TopologyService topologyService;
+
+    @Reference(cardinality = ReferenceCardinality.MANDATORY_UNARY)
+    protected HostService hostService;
+
     @Activate
     public void activate() {
+        // initialize AbstractPathService
+        super.topologyService = this.topologyService;
+        super.hostService = this.hostService;
         log.info("Started");
     }