Question

Java Hashmap Sorting

Asked by: lcor

I have a sorting issue with a Hashmap.  My constraint is that I MUST use the Hashmap and work with existing code.  I do a database query and place the results in a Hashmap.  When I iterate thru the Hashmap, it loses the original alphabetical sorting done by the database.  So, my problem is that I must sort the results coming out of the Hashmap which is then placed into another class.

Here's the code:

Hashmap hm = (database results)
Iterator it =  hm.keySet().iterator();

while (it.hasNext()) {
    String key = (String) it.next();
    STring value = (String) hm.get(key);
                                                                    <<<------------------I must do alphabetical sorting before
                                                                    <<<-----------------placing results (value) into this other class
    some_other_class.addVAlue(new Integer(value), "title");
}
 
There isn't a great number of values (most a couple hundred) so if another loop is okay.

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Asked On
2007-10-03 at 10:05:56ID22869869
Tags

hashmap

,

java

,

sort

Topics

Java Standard Tag Library (JSTL) & Server Tags

,

Java Programming Language

Participating Experts
3
Points
500
Comments
8

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Answers

 

by: CEHJPosted on 2007-10-03 at 10:08:17ID: 20008247

HashMap has no order. Use LinkedHashMap to preserve order

 

by: CEHJPosted on 2007-10-03 at 10:13:50ID: 20008294

(So if you preserve the order, you won't need to sort it again)

 

by: Bart_CrPosted on 2007-10-03 at 11:49:43ID: 20009030

If you need to go with the HashMap for getting the results, wrap them in a TreeMap to get them sorted again:

        TreeMap tm = new TreeMap(getDatabaseResults());
        Iterator it = tm.keySet().iterator();

        while (it.hasNext()) {
            String key = (String) it.next();
            String value = (String) tm.get(key);
            some_other_class.addVAlue(new Integer(value), "title");
        }

 

by: CEHJPosted on 2007-10-03 at 11:53:44ID: 20009059

Well of course you don't need to take the performance hit of sorting at all, other than at the db. Just put them in to the LinkedHashMap as they come out of the ResultSet

 

by: Bart_CrPosted on 2007-10-03 at 11:57:02ID: 20009087

I'd go with CEHJ's solution if you can alter the outcome from the database to a LinkedHashMap. If not you'll need to go with the sorting.

 

by: objectsPosted on 2007-10-03 at 17:04:26ID: 20010987

use the follwoiwing:

List list = new ArrayList(hm.values());
Arrays.sort(list);
while (it.hasNext()) {
    String value = (String) it.next();
    some_other_class.addVAlue(new Integer(value), "title");
}

 

by: lcorPosted on 2007-10-04 at 09:48:41ID: 20015626

Everyone had great input but I ended up using Bart_Cr's solution in the end.

 

by: CEHJPosted on 2007-10-04 at 12:21:59ID: 20016954

What made you want to sort them twice?

20120131-EE-VQP-002

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