concurrent reduction

ConcReductTest.java

import java.util.stream.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;

public class ConcReductTest {
  static public void main(String[] args) {
    ConcurrentMap<Integer, AtomicInteger> result =
      IntStream.range(1, 10_000_001)
        .boxed()
        .unordered()  // invoke concurrent reduction
        .parallel()
        .collect(Collector.of(
          () -> new ConcurrentHashMap<>(),
          (m,v) -> {
            AtomicInteger a = m.putIfAbsent(v % 10, new AtomicInteger(v));
            if (a != null)
              a.getAndAdd(v);
          },
          (m,n) -> { // NOT USED
            System.out.println("!");
            return null;
          },
          Collector.Characteristics.CONCURRENT
        ));
    System.out.println("r="+result);
  }
}