Custom sort for HashMap on Priority Queue [duplicate]

Question

I want to get the sorted key value pair on value, and if the values are equal, I wanna sort on keys asc.

I tried using PriorityQueue like following but I am getting null pointer for some reason. Please help me understand the issue and probably answer my query.

This is what I came up with, but unfortunately the output of the code is

PQ : [Hello, Yes]

Where as I was expecting

PQ : [No, Hello]

    public static void main(String[] args) {

        Map<String, Integer> map = new HashMap<>() {{
            put("Hello", 50);
            put("Yes", 50);
            put("No", 100);
        }};
        PriorityQueue<String> pq = new PriorityQueue<>((a, b) -> {
            if (map.get(a).equals(map.get(b))) return a.compareTo(b);
            else return Integer.compare(map.get(b), map.get(a));
        });
        verifyPQ(map, pq);
        System.out.println(pq);
    }

    private static void verifyPQ(Map<String, Integer> map, PriorityQueue<String> pq) {
        for (String key : map.keySet()) {
            pq.add(key);
            if (pq.size() > 2) {
                pq.poll();
            }
        }
}

@pjs thanks for the recommendation, I have updated it according to your review — Shubham Kumbhalkar
– Shubham Kumbhalkar, Commented Feb 24, 2024 at 17:47
Your comparator doesn’t make much sense, nor does your assertion that you are both getting an NPE and unexpected output. — user207421
– user207421, Commented Feb 24, 2024 at 18:26
1) that code is not throwing NullPointerException! 2) Does not compile/run as posted! 3) Using compare(map.get(b), map.get(a)) means that 100 comes before 50 so the "No" entry will be removed first; try compare(map.get(a), map.get(b)) — user85421
– user85421, Commented Feb 24, 2024 at 20:33
[ 4) be aware that a PriorityQueue only guarantees that the first element (head) ist sorted ] — user85421
– user85421, Commented Feb 24, 2024 at 20:38
you still didn't explain the "I am getting null pointer" ?? but code as posted is not running anyways !! — user85421
– user85421, Commented Feb 25, 2024 at 22:18

Eritrean · Accepted Answer · 2024-02-24 18:26:05Z

0

Why don't you use comparator instead of unnecessarily complicating things with priority queues?

If I have understood correctly, you want to sort by value in descending order and by key in ascending order.

Map<String, Integer> map = new HashMap<>();
map.put("Hello", 50);
map.put("Yes", 50);
map.put("No", 100);


map.entrySet()
   .stream()
   .sorted(Comparator.comparing(Entry<String,Integer>::getValue).reversed()
                     .thenComparing(Entry::getKey))
   .forEach(System.out::println);

If you only need the first two, just add a limit

map.entrySet()
   .stream()
   .sorted(Comparator.comparing(Entry<String,Integer>::getValue).reversed()
                     .thenComparing(Entry::getKey))
   .limit(2)
   .forEach(System.out::println);

answered Feb 24, 2024 at 18:26

Eritrean

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1 Comment

Shubham Kumbhalkar Over a year ago

Thanks @Eritrean, I wanted to use PriorityQueue because, that is what I thought would help me to solve this with custom sorting order. But this helps a lot. Just out of curiosity, can we achieve this with PriorityQueue? if so, how?

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