Make use of Java 8 to simplify computeLeaseCounts() (#847)

It's possible to remove quite a few lines of code from the
method using Java 8 features (a `groupingBy` Collector, etc).

Java 8 is the minimum supported version of the KCL since
#176 was
merged in July 2017.

See also:

https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html#package.description
http://cr.openjdk.java.net/~briangoetz/lambda/lambda-translation.html

amazon-kinesis-client/src/main/java/software/amazon/kinesis/leases/dynamodb/DynamoDBLeaseTaker.java

@@ -40,6 +40,9 @@
 import software.amazon.kinesis.metrics.MetricsScope;
 import software.amazon.kinesis.metrics.MetricsUtil;
 
+import static java.util.stream.Collectors.groupingBy;
+import static java.util.stream.Collectors.summingInt;
+
 /**
  * An implementation of {@link LeaseTaker} that uses DynamoDB via {@link LeaseRefresher}.
  */
@@ -539,30 +542,16 @@ private List<Lease> chooseLeasesToSteal(Map<String, Integer> leaseCounts, int ne
  * @return map of workerIdentifier to lease count
  */
  private Map<String, Integer> computeLeaseCounts(List<Lease> expiredLeases) {
- Map<String, Integer> leaseCounts = new HashMap<>();
  // The set will give much faster lookup than the original list, an
  // important optimization when the list is large
  Set<Lease> expiredLeasesSet = new HashSet<>(expiredLeases);
 
- // Compute the number of leases per worker by looking through allLeases and ignoring leases that have expired.
- for (Lease lease : allLeases.values()) {
- if (!expiredLeasesSet.contains(lease)) {
- String leaseOwner = lease.leaseOwner();
- Integer oldCount = leaseCounts.get(leaseOwner);
- if (oldCount == null) {
- leaseCounts.put(leaseOwner, 1);
- } else {
- leaseCounts.put(leaseOwner, oldCount + 1);
- }
- }
- }
+ Map<String, Integer> leaseCounts = allLeases.values().stream()
+ .filter(lease -> !expiredLeasesSet.contains(lease))
+ .collect(groupingBy(Lease::leaseOwner, summingInt(lease -> 1)));
 
- // If I have no leases, I wasn't represented in leaseCounts. Let's fix that.
- Integer myCount = leaseCounts.get(workerIdentifier);
- if (myCount == null) {
- myCount = 0;
- leaseCounts.put(workerIdentifier, myCount);
- }
+ // If I have no leases, I won't be represented in leaseCounts. Let's fix that.
+ leaseCounts.putIfAbsent(workerIdentifier, 0);
 
  return leaseCounts;
  }