YARN-8969. AbstractYarnScheduler#getNodeTracker should return generic type to avoid type casting. Contributed by Wanqiang Ji.

(cherry picked from commit c7fcca0d7ec9e31d43ef3040ecd576ec808f1f8b)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/AbstractYarnScheduler.java

@@ -237,7 +237,7 @@ public abstract class AbstractYarnScheduler
   }
 
   @VisibleForTesting
-  public ClusterNodeTracker getNodeTracker() {
+  public ClusterNodeTracker<N> getNodeTracker() {
     return nodeTracker;
   }
 

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/FSPreemptionThread.java

@@ -229,7 +229,7 @@ class FSPreemptionThread extends Thread {
 
   private void trackPreemptionsAgainstNode(List<RMContainer> containers,
                                            FSAppAttempt app) {
-    FSSchedulerNode node = (FSSchedulerNode) scheduler.getNodeTracker()
+    FSSchedulerNode node = scheduler.getNodeTracker()
         .getNode(containers.get(0).getNodeId());
     node.addContainersForPreemption(containers, app);
   }