Comparator for sorting by frequency without Creating a Comparator implementation class

Question

Just curious to know on whether we can sort a List according to the frequency of repeated numbers using Java 8 without writing a Custom Comparator class.

I need to sort the given integers based on its frequency and then by the natural numerical order.

I'm getting error at Comparator.naturalOrder();

Here is the code which I tried:

Integer[] given = new Integer[]{0,0,1,22,11,22,22,11,44,555,55,66,77,88,99};
List<Integer> intList = Arrays.asList(given);


Map<Integer, Long> frequencyMap = intList.stream().collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
List<Integer> newList = intList.stream().sorted(Comparator.comparing(frequencyMap::get).thenComparing(Comparator.naturalOrder())).collect(Collectors.toList());
System.out.println(newList.toString());

The expected output is

[1, 44, 55, 66, 77, 88, 99, 555, 0, 0, 11, 11, 22, 22, 22]

PS: Used arrays in first line in order to avoid list.add() in multiple lines and for clear understanding.

Answer 1

Unfortunately, Java's type inference can't recognize the type of the compared object when chaining Comparator.comparing(frequencyMap::get) with thenComparing(Comparator.naturalOrder()) . Since the method signature of Map.get is get(Object) , the compiler infers Comparator<Object> as result type of Comparator.comparing(frequencyMap::get) .

You can fix this by inserting an explicit type. But note that you are not using the result of collect(Collectors.toList()) but just printing the original, unaffected List . On the other hand, you don't need the List when the array is given:

Integer[] given = {0,0,1,22,11,22,22,11,44,555,55,66,77,88,99};

Map<Integer, Long> frequencyMap = Arrays.stream(given)
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()));
Arrays.sort(given,
    Comparator.<Integer>comparingLong(frequencyMap::get)
       .thenComparing(Comparator.naturalOrder()));

System.out.println(Arrays.toString(given));

For printing without changing the array you can also use the following alternative

Arrays.stream(given)
    .collect(Collectors.groupingBy(Function.identity(), Collectors.counting()))
    .entrySet().stream()
    .sorted(Map.Entry.<Integer, Long>comparingByValue()
        .thenComparing(Map.Entry.comparingByKey()))
    .flatMap(e -> LongStream.range(0, e.getValue()).mapToObj(l -> e.getKey()))
    .forEach(System.out::println);

This sorts the groups instead of the individual values and prints identical values as often as they've been counted.

Answer 2

you need to add a type witness, minor compiler weakness:

 intList.stream()
        .sorted(Comparator.comparing((Integer x) -> frequencyMap.get(x))
                          .thenComparing(Comparator.naturalOrder()))
        .forEachOrdered(System.out::println);