Issue while sorting a map

Question

I have a map

Map<Integer, String> map = new TreeMap<Integer, String>();
map.put(21,"A");
map.put(9,"B");
map.put(23,"C");
map.put(25,"D");

I sorted it like this

List<Integer> list = new LinkedList(map.keySet());
Collections.sort(list, new Comparator<Integer>() {
@Override
public int compare(Integer o1, Integer o2) {
    return o1%10 - o2%10;
  }
});
Map sortedMap = new LinkedHashMap();
for(Integer key: list){
  sortedMap.put(key, map.get(key));
}
System.out.println(sortedMap);

Gives output {21=A, 23=C, 25=D, 9=B}

Even tried like this

Collections.sort(list);
Set set = map.entrySet();
Map result = new LinkedHashMap();
for (Iterator it = set.iterator(); it.hasNext();) {
    Map.Entry entry = (Map.Entry)it.next();
    result.put(entry.getKey(), entry.getValue());
}
System.out.println(result.toString());

Still same result

Why 9 is coming at last it must the first entry .

because 9 is biggest in ones place of all numbers !!! you are comparing last digit of all numbers... — Not a bug
– Not a bug, Commented Aug 25, 2014 at 12:33
Because you asked for it ! By explicitely stating % 10, you only keep the last digit of your numbers for the sort. :) — Orabîg
– Orabîg, Commented Aug 25, 2014 at 12:33
really this is stupidity how can I ignore that btw thanks for the help — Sarbong
– Sarbong, Commented Aug 25, 2014 at 12:38
The original map is already sorted. You only create a new ordered map that keeps the insertion order. What are you trying to achieve? — maba
– maba, Commented Aug 25, 2014 at 12:46

WeSt · Accepted Answer · 2014-08-25 12:28:41Z

1

Your compare function is comparing only the last digit of each number:

@Override
public int compare(Integer o1, Integer o2) {
    return o1 % 10 - o2 % 10; // a number modulo 10 yields it last digit, e.g. 25 % 10 = 5
  }
});

You are comparing the last digits of (21, 9, 23, 24) which are (1, 9, 3, 4). In this way, 9 is the biggest number and hence the last one in the list.

answered Aug 25, 2014 at 12:28

WeSt

2,7085 gold badges27 silver badges43 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Vinay Veluri · Accepted Answer · 2014-08-25 12:30:09Z

1

Try this !

 List<Integer> list = new LinkedList(map.keySet());
        Collections.sort(list, new Comparator<Integer>() {
            @Override
            public int compare(Integer o1, Integer o2) {
                return o1 - o2;
            }
        });

Is there any specific reason to use - % ? Which is changing the order of result.

This is the output with above

{9=B, 21=A, 23=C, 25=D}

answered Aug 25, 2014 at 12:30

Vinay Veluri

6,8755 gold badges36 silver badges59 bronze badges

Collectives™ on Stack Overflow

Issue while sorting a map

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related