Sorting a hashtable

[Craig]

What's the best way to sort a java.util.Hashtable which contains Comparable
objects?

 

[Frank]

What's the best way to sort a java.util.Hashtable which contains Comparable
objects?

To sort by keys you use a TreeMap (Use HashMap over Hashtable, btw..),
to sort the values you need to place them in a container that is/can be
sorted (List / TreeSet / array / whatever).

 

[Doni Ocena]

Try this:

i) get a collection of values by calling values() on your hashtable
ii) call toArray() on your collection from step (i)
iii) use Arrays.sort() on your array from step(ii)

Regards,
Doni

 

[Thomas Weidenfeller]

What's the best way to sort a java.util.Hashtable which contains Comparable
objects?

"best" in what sense? Least amount of code to write?

But first, a hashtable can't be sorted. It is a data structure which
maps keys to values, and because of the used algorithm (hashing),
always keeps the elements in the order of the hashes of the keys (with
gaps in between).

You must copy the elements to some other data structure. You can e.g.
extract either the keys or values so you can sort them separately.
Since you didn't specify if you want the keys or values sorted, I will
assume you want the keys sorted. In this case, however, the "best" (in
the sense of least code to write) would be to copy the whole hashtable
to a TreeMap:

TreeMap tm = new TreeMap(yourHashtable);
SortedSet sortedEntries = (SortedSet)tm.entrySet();

/Thomas

 

[John C. Bollinger]

What's the best way to sort a java.util.Hashtable which contains Comparable
objects?

You cannot sort a Hashtable (what would it even mean?), but you can
extract its values and sort them. For example:

public List sortedMapValues(Map map) { // a Hashtable is a Map
List rval = new ArrayList(map.values());

Collections.sort(rval); // assumes mutually Comparable values
return rval;
}

You can do other, similar things with the key set and entry set.

If you want a Map that always maintains its elements in order by key,
then you want a SortedMap, such as a TreeMap. You could probably
manipulate a LinkedHashMap in such a way as to cause its entry iteration
order to match some sorting scheme, but that would be way more trouble
than it was worth.