MAPREDUCE-7354. Use empty array constant present in TaskCompletionEvent to avoid creating redundant objects (#3123)

Reviewed-by: Hui Fei <ferhui@apache.org>
Reviewed-by: Akira Ajisaka <aajisaka@apache.org>

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-app/src/main/java/org/apache/hadoop/mapreduce/v2/app/job/impl/JobImpl.java

@@ -146,7 +146,7 @@ public class JobImpl implements org.apache.hadoop.mapreduce.v2.app.job.Job,
     EMPTY_TASK_ATTEMPT_COMPLETION_EVENTS = new TaskAttemptCompletionEvent[0];
 
   private static final TaskCompletionEvent[]
-    EMPTY_TASK_COMPLETION_EVENTS = new TaskCompletionEvent[0];
+    EMPTY_TASK_COMPLETION_EVENTS = TaskCompletionEvent.EMPTY_ARRAY;
 
   private static final Logger LOG = LoggerFactory.getLogger(JobImpl.class);
 

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/test/java/org/apache/hadoop/mapreduce/TestJob.java

@@ -50,7 +50,7 @@ public class TestJob {
     when(client.getTaskReports(jobid, TaskType.REDUCE)).thenReturn(
         new TaskReport[0]);
     when(client.getTaskCompletionEvents(jobid, 0, 10)).thenReturn(
-        new TaskCompletionEvent[0]);
+        TaskCompletionEvent.EMPTY_ARRAY);
     Job job = Job.getInstance(cluster, status, new JobConf());
     Assert.assertNotNull(job.toString());
   }

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/test/java/org/apache/hadoop/mapreduce/TestJobMonitorAndPrint.java

@@ -45,7 +45,6 @@ import org.apache.log4j.Layout;
 import org.apache.log4j.Level;
 import org.apache.log4j.Logger;
 import org.apache.log4j.WriterAppender;
-import org.mockito.invocation.InvocationOnMock;
 import org.mockito.stubbing.Answer;
 
 /**
@@ -81,15 +80,9 @@ public class TestJobMonitorAndPrint {
         1f, 1f, State.SUCCEEDED, JobPriority.HIGH, "tmp-user", "tmp-jobname",
         "tmp-queue", "tmp-jobfile", "tmp-url", true);
 
-    doAnswer(
-        new Answer<TaskCompletionEvent[]>() {
-          @Override
-          public TaskCompletionEvent[] answer(InvocationOnMock invocation)
-              throws Throwable {
-            return new TaskCompletionEvent[0];
-          }
-        }
-        ).when(job).getTaskCompletionEvents(anyInt(), anyInt());
+    doAnswer((Answer<TaskCompletionEvent[]>) invocation ->
+        TaskCompletionEvent.EMPTY_ARRAY).when(job)
+        .getTaskCompletionEvents(anyInt(), anyInt());
 
     doReturn(new TaskReport[5]).when(job).getTaskReports(isA(TaskType.class));
     when(clientProtocol.getJobStatus(any(JobID.class))).thenReturn(jobStatus_1, jobStatus_2);

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-hs/src/main/java/org/apache/hadoop/mapreduce/v2/hs/UnparsedJob.java

@@ -183,7 +183,7 @@ public class UnparsedJob implements org.apache.hadoop.mapreduce.v2.app.job.Job {
   @Override
   public TaskCompletionEvent[] getMapAttemptCompletionEvents(
       int startIndex, int maxEvents) {
-    return new TaskCompletionEvent[0];
+    return TaskCompletionEvent.EMPTY_ARRAY;
   }
 
   @Override