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javasortinghashmap
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add-semi-colons

Reputation: 18800

Sorting Descending order: Java Map

What I want to do is sort a map by value. I went over many questions that are available on the stackoverflow site and found out following solution that does what I want but missing a small thing.

Link1: Sorting Map

But the issue I am running into is that by default this is sorted by ascending order by value. I want to order it by descending order:

So what I did was I created a class that implements a comparator

class MyComparator implements Comparator {
    Map map;
    public MyComparator(Map map) {
        this.map = map;
    }
    public int compare(Object o1, Object o2) {
        return ((Integer) map.get(o2)).compareTo((Integer) map.get(o1));
    }
}

And then I pass my map to the treemap,

MyComparator comp = new MyComparator(myMap);
Map<String, Integer> newMap = new TreeMap(comp);
newMap.putAll(myMap);

This seems like bad approach because I feel this is inefficient. Is there a way to change the solution in the link to do ordering on descending order by default.

Upvotes: 39

Views: 83536

Answers (5)

Holger
Holger

Reputation: 298113

You should use new TreeMap<>(Collections.reverseOrder());.

Map<String, Integer> newMap = new TreeMap<>(Collections.reverseOrder());
newMap.putAll(myMap);

or to reverse an existing comparator like the value-comparator Collections.reverseOrder(comparator). It works like your approach swapping the two objects before invoking compare/compareTo.

Upvotes: 135

Farman
Farman

Reputation: 66

This will work :

      TreeMap<Integer, Integer> reverseInteger=new TreeMap<>(new Comparator<Integer>() {

        @Override
        public int compare(Integer o1, Integer o2) {
            return o2>o1?1:o2==o1?0:-1;
        }
    });

Upvotes: 4

OCarneiro
OCarneiro

Reputation: 327

You could simply invert the return value of your compare method by adding a minus sign at the beginning:

return -((Integer) map.get(o2)).compareTo((Integer) map.get(o1));

Upvotes: 2

Nilesh Jadav
Nilesh Jadav

Reputation: 910

    TreeMap<Long,String> treeMap = new TreeMap<Long,String>();

    NavigableMap <Long, String> nmap = treeMap.descendingMap();

    Set<Long, String> set = nmap.entrySet();

    Iterator<Long, String> iterator = set.iterator();

now u can iterate over iterator and extract the value using iterator.hasNext() and iterator.next() methods ......

Upvotes: 7

Nicole
Nicole

Reputation: 33197

To change the solution in the link to sort by descending order, just reverse the condition:

...
// Note: this comparator imposes orderings that are inconsistent with equals.    
public int compare(String a, String b) {
    if (base.get(a) >= base.get(b)) {
        return 1; // For ascending, return -1;
    } else {
        return -1; // For ascending, return 1;
    } // returning 0 would merge keys
}
...

Upvotes: 0

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