I went through all the manuals out there and all SO questions but still unable to figure this out... I have a List (integer represents age): I need to: group the list by age, sort by group sizes (descending), sort by age (descending) So using the example above the result would have to be like this: What I tried: I

Grouping and Double sorting List

I went through all the manuals out there and all SO questions but still unable to figure this out…

I have a List (integer represents age):

List<Person> people = Arrays.asList
    (
            new Person("bob", 10),
            new Person("sue", 4),
            new Person("tom", 37),
            new Person("jim", 10),
            new Person("boo", 4),
            new Person("ekk", 53),
            new Person("joe", 10)
    );

I need to:

group the list by age,
sort by group sizes (descending),
sort by age (descending)

So using the example above the result would have to be like this:

{10=[bob, jim, joe],4=[sue, boo], 53=[ekk], 37=[tom]}

What I tried:

I tried with and without streams. I failed on both.

Note: I would lean toward no stream solution, because from my testing of the below code it seems like streams are much slower (I used System.nanotime()). These 3 operations will be done thousands of times each time, so it may make a slight difference.

Using streams here is what I did:

List<List<Person>> grpd = new ArrayList<>
    (
            people.stream()
                    .collect
                            (
                                    groupingBy(Person::getAge, toList())
                            )
                    .values()
    );
grpd = grpd.stream().sorted((a, b) -> Integer.compare(b.size(), a.size())).collect(toList());

No streams approach:

    Map<Integer, List<Person>> grouped = new HashMap<>();
    for (Person person : people)
    {
        if (grouped.containsKey(person._age))
        {
            grouped.get(person._age).add(person);
        } else
        {
            List<Person> p = new ArrayList<>();
            p.add(person);
            grouped.put(person._age, p);
        }
    }


    List<Map.Entry<Integer, List<Person>>> entries = new ArrayList<>(grouped.entrySet());
    Collections.sort(entries, new Comparator<Map.Entry<Integer, List<Person>>>()
    {
        @Override
        public int compare(Map.Entry<Integer, List<Person>> o1, Map.Entry<Integer, List<Person>> o2)
        {
            return Integer.compare(o2.getValue().size(), o1.getValue().size());
        }
    });
    Map<Integer, List<Person>> sortedBySize = new LinkedHashMap<>();
    for (Map.Entry<Integer, List<Person>> entry : entries)
    {
        sortedBySize.put(entry.getKey(), entry.getValue());
    }

Problem: I have no idea how to add the final sort on either case.

public class Person
{
    public String _name;
    public int _age;
    public int getAge() { return _age; }

    public Person(String name, int age)
    {
        _name = name;
        _age = age;
    }

    @Override
    public String toString()
    {
        return _name;
    }
}

Answer

As you’ve also asked about a non-stream solution, here it is:

Map<Integer, List<Person>> grouped = new HashMap<>();
people.forEach(person -> grouped.computeIfAbsent(
        person.getAge(), 
        k -> new ArrayList<>())
    .add(person));

This groups by age. Now let’s sort the entries, first by group size descending, then by age descending:

List<Map.Entry<Integer, List<Person>>> toSort = new ArrayList<>(grouped.entrySet());
toSort.sort(
    Comparator.comparingInt((Map.Entry<Integer, List<Person>> e) -> e.getValue().size())
              .reversed()
              .thenComparingInt(Map.Entry.comparingByKey().reversed()));

Now, toSort is a sorted list of entries. You need to put those entries into a new map:

Map<Integer, List<Person>> sorted = new LinkedHashMap<>();
toSort.forEach(e -> sorted.put(e.getKey(), e.getValue()));

And sorted holds the result you want.

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Answer