Sorting a LinkedHashMap keyset by values object property

Question

I have this LinkedHashMap containing Integer indexes and object Paire for values :

Map<Integer, Paire> population1 = new LinkedHashMap<>();

My Paire class is rather simple and looks like this :

 public class Paire {

     float valeur;
     int index;

 public Paire(LinkedList<Sommet> liste, float valeur, int index) {

    this.liste = liste;
    this.valeur = valeur;
    this.index = index;
}

Now I want to store a keyset of my map, sorted by the float value from my class (valeur), in a LinkedList :

List<Integer> selection1 = new LinkedList(population1.keySet());

I know I could use Collection.sort to sort values and then track back their respective keys if those values were simple Strings or numbers, but I'm a little lost here. I feel there are simple and fast ways to do this without intermediate lists and variables. In addition, my code's execution needs to be as fast as possible (genetic algorithm for TSP).

@LeonardBrünings SortedMap doesn't work with sorting values. — Louis Wasserman
– Louis Wasserman, Commented Jun 29, 2017 at 20:41

Louis Wasserman · Accepted Answer · 2017-06-29 20:40:36Z

1

Collections.sort(selection1, new Comparator<Integer>() {
  @Override public int compare(Integer key1, Integer key2) {
    return Float.compare(
         population1.get(key1).valeur, population1.get(key2).valeur);
  }
});

If you care about speed, though, LinkedList is never your friend. Use ArrayList.

answered Jun 29, 2017 at 20:40

Louis Wasserman

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1 Comment

Flo Fst Over a year ago

Works incredibly well! Thanks. I do a lot of getFirst() and removeFirst(), this is why I use LinkedList. Thanks for the advice anyway!

Slava Vedenin · Accepted Answer · 2017-06-29 21:10:43Z

0

I recommend using stream (in my opinion, it is should be much faster then Collections.sort):

List<Integer> list = population1.entrySet().stream().sorted((e1, e2) -> Float.

compare(e1.getValue().valeur,e2.getValue().valeur)).map(Map.Entry::getKey)

  .collect(Collectors.toList());

I do a lot of getFirst() and removeFirst(), this is why I use LinkedList.

Better revert order of ArrayList and you can use:

    list.get(list.size() - 1)

instead of

 getFirst()

and

 list.remove(list.size() - 1)

instead of

 removeFirst()

Because getting and deleting last element of ArrayList is very very fast.

P.S. LinkedList is very very slow in almost any case

answered Jun 29, 2017 at 21:10

Slava Vedenin

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1 Comment

Flo Fst Over a year ago

This is a nice advice, I really thought LinkedList was only slower for insertion and looking for items in the middle of the list. The tip for getting and deleting faster with ArrayList is also very useful.

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