CODEWITHSUNDEEP
javahashmapcomparator
pmdaly
pmdaly

Reputation: 1222

How sort a LinkedHashMap by the size of each key's value?

I have a LinkedHashMap< Integer, HashSet< Integer > > and I want to sort the keys based on decreasing order of the size of their values. I currently have this fragment from another thread but I'm not sure how to change it to work with my case.

myMap.entrySet().stream()
         .sorted(Map.Entry.comparingByValue(Comparator.reverseOrder()))
         .collect(Collectors.toMap(
                  Map.Entry::getKey,
                  Map.Entry::getValue,
                  (x,y) -> {throw new AssertionError();},
                  LinkedHashMap::new
         ));

Here's what I've tried

List<Map.Entry<String, Integer>> entries =
  new ArrayList<Map.Entry<String, Integer>>(map.entrySet());
Collections.sort(entries, new Comparator<Map.Entry<String, Integer>>() {
  public int compare(Map.Entry<String, Integer> a, Map.Entry<String,Integer> b){
    return a.getValue().size().compareTo(b.getValue().size());
   }
});
Map<String, Integer> sortedMap = new LinkedHashMap<String, Integer>();
for (Map.Entry<String, Integer> entry : entries) {
  sortedMap.put(entry.getKey(), entry.getValue());
}

I just added the .size() in there. I found it from this thread. The size I'm referring to is the number of items in the HashSet. It is usually just getValue().size() but it's not working here.

Sorting LinkedHashMap

Upvotes: 1

Views: 794

Answers (1)

Mick Mnemonic
Mick Mnemonic

Reputation: 7956

What you've tried was close. Here's a fixed version:

List<Map.Entry<Integer, Set<Integer>>> entries = new ArrayList<>(
            map.entrySet());
Collections.sort(entries,
        new Comparator<Map.Entry<Integer, Set<Integer>>>() {
            public int compare(Map.Entry<Integer, Set<Integer>> a,
                    Map.Entry<Integer, Set<Integer>> b) {
                return Integer.compare(
                          a.getValue().size(),
                          b.getValue().size());
            }
        });

Map<Integer, Set<Integer>> sortedMap = new LinkedHashMap<>();
for (Map.Entry<Integer, Set<Integer>> entry : entries) {
    sortedMap.put(entry.getKey(), entry.getValue());
}

Setting up with

 LinkedHashMap<Integer, Set<Integer>> map = new LinkedHashMap<>();
 map.put(1, new HashSet<>(Arrays.asList(1,2,3)));
 map.put(2, new HashSet<>(Arrays.asList(1,2)));
 map.put(3, new HashSet<>(Arrays.asList(1)));

and running 

System.out.println(sortedMap);

after sorting outputs

{3=[1], 2=[1, 2], 1=[1, 2, 3]}

Upvotes: 1

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