Apply suggestions from code review

Co-authored-by: Lukasz Cwik <lcwik@google.com>

sdks/java/harness/src/main/java/org/apache/beam/fn/harness/debug/DataSampler.java

@@ -51,10 +51,10 @@ public DataSampler(int maxSamples, int sampleEveryN) {
   }
 
   // Maximum number of elements in buffer.
-  private int maxSamples = 10;
+  private final int maxSamples = 10;
 
   // Sampling rate.
-  private int sampleEveryN = 1000;
+  private final int sampleEveryN = 1000;
 
   // The fully-qualified type is: Map[ProcessBundleDescriptorId, [PCollectionId, OutputSampler]].
   // The DataSampler object lives on the same level of the FnHarness. This means that many threads
@@ -142,7 +142,7 @@ public Map<String, List<byte[]>> samplesFor(Set<String> descriptors, Set<String>
                   return;
                 }
 
-                samples.putIfAbsent(pcollectionId, new ArrayList<>());
+                samples.putIfAbsent(pcollectionId, Collections.EMPTY_LIST);
                 samples.get(pcollectionId).addAll(outputSampler.samples());
               });
         });

sdks/java/harness/src/main/java/org/apache/beam/fn/harness/debug/OutputSampler.java

@@ -33,7 +33,7 @@
  */
 public class OutputSampler<T> {
   private final Coder<T> coder;
-  private final List<T> buffer = new ArrayList<>();
+  private final List<T> buffer = new ArrayList<>(maxElements);
   private static final Logger LOG = LoggerFactory.getLogger(OutputSampler.class);
 
   // Maximum number of elements in buffer.