StreamMapInvertingCompilerBug.java

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class StreamMapInvertingCompilerBug {
  public static void main(String... args) {
    Map<Integer, Long> numberCount =
        Stream.of(1, 4, 2, 5, 3, 5, 1, 3, 5, 3, 2, 1, 4, 5)
            .collect(Collectors.groupingBy(
                Function.identity(),
                Collectors.counting()
            ));

    // This compiles fine
    SortedMap<Long, List<Integer>> invertedMapAscending =
        numberCount.entrySet().stream()
            .collect(Collectors.groupingBy(
                Map.Entry::getValue,
                TreeMap::new,
                Collectors.mapping(
                    Map.Entry::getKey,
                    Collectors.toList()
                )));
    System.out.println("invertedMapAscending = " +
        invertedMapAscending);

    // this does not compile, but IntelliJ says it is OK
    /*
    SortedMap<Long, List<Integer>> invertedMapDescending1 =
        numberCount.entrySet().stream()
            .collect(Collectors.groupingBy(
                Map.Entry::getValue,
                () -> new TreeMap<>(Comparator.reverseOrder()),
                Collectors.mapping(
                    Map.Entry::getKey,
                    Collectors.toList()
                )));
    System.out.println("invertedMapDescending1 = " +
        invertedMapDescending1);
     //*/

    // this compiles, but needs the explicit generic type for
    // TreeMap, but IntelliJ marks that as unnecessary
    SortedMap<Long, List<Integer>> invertedMapDescending2 =
        numberCount.entrySet().stream()
            .collect(Collectors.groupingBy(
                Map.Entry::getValue,
                () -> new TreeMap<Long, List<Integer>>(Comparator.reverseOrder()),
                Collectors.mapping(
                    Map.Entry::getKey,
                    Collectors.toList()
                )));
    System.out.println("invertedMapDescending2 = " +
        invertedMapDescending2);
  }
}