I've a hashmap and i would like to sort it..
I've looked through the forum... but i still can't gather how i can go about solving it..
i changed my hashmap to treemap as i wanted to sort based on the key.
key is an integer value
and value of hashmap is a double.
please help/suggest..
You can't sort a HashMap (or a Hash - anything) because the nature of the beast is that it's based on a "hashing algorithm" which means that a given set of keys will order themselves a certain way in a table of a give size, but that order is effectively random. The nature of the table fixes the order.
TreeMap will autmatically maintain your integers in order. If you want the "natural" order of integer keys then you don't need to supply a comparitor because Integer implements Comparable.
(You can't sort a TreeMap either, but you can controll the way a particular TreeMap orders it's entries).
>>>TreeMap will autmatically maintain your integers in order.
if that's the case, do i simply do:
public void sorthashmap()
{
TreeMap myTreeMap;
myTreeMap = new TreeMap(hashMap);
}
would i need to return something? or convert it back to hashmap?
yes.
im currently having the key values
e.g a string: 3:1.0 5:1.0 29:1.0 30:1.0 7:1.0
ignoring the ":", notice that my key is the left hand object of ":" and my value is the right hand object of ":"
however my hashmap cant order the keys regardless of the values... i wanted them in ascending order smalll to big
3:1.0 5:1.0 7:1.0 29:1.0 30:1.0
Please kindly help okie?
I don't understand why...
I use:
TreeMap myTreeMap;
myTreeMap = new TreeMap(hashMap);
String result="";
Collection lines = myTreeMap.values();
Iterator i = lines.iterator();
Set words = myTreeMap.keySet();
for(Iterator it=words.iterator(); it.hasNext();)
{
result += (String)it.next() + " : " + (String)i.next() + " ";
}
System.out.println(result);
and nothing is displayed :(
Map hashmap;
List l = new ArrayList(hashmap.keySet());
Collections.sort(l);
Object key, value;
for(Iterator ito = l.iterator(); ito.hasNext();){
key = ito.next();
value = hashmap.get(key);
}
> >>>TreeMap will autmatically maintain your integers
> in order.
>
> if that's the case, do i simply do:
> > public void sorthashmap()
> {
>TreeMap myTreeMap;
>myTreeMap = new TreeMap(hashMap);
> }
>
> would i need to return something? or convert it back
> to hashmap?
No you can't sort a Map use the TreeMap instead of the HashMap and it will maintain the keys in their natural order. The above code would copy the entries from a HashMap into a TreeMap and you could then iterate over the TreeMap but really, why bother? Just us a TreeMap instead of a HashMap in the first place.
Hi,
I tried to use yours also:
List l = new ArrayList(hashMap.keySet());
Collections.sort(l);
Object okey, ovalue;
for(Iterator ito = l.iterator(); ito.hasNext();){
okey = ito.next();
ovalue = hashMap.get(okey);
result += (String)okey + " : " + (String)ovalue + " ";
}
System.out.println(result);
but why is it that result is always null?
i don't get it... *in desperate help*
Hi Malcolm.
TreeMap myTreeMap;
myTreeMap = new TreeMap(hashMap);
String result="";
Collection lines = myTreeMap.values();
Iterator i = lines.iterator();
Set words = myTreeMap.keySet();
for(Iterator it=words.iterator(); it.hasNext();)
{
result += (String)it.next() + " : " + (String)i.next() + " ";
}
System.out.println(result);
i have tried using a TreeMap...but i don't know why i cant print anything out...
i have to convert the hashmap to a treemap since it has been used in the first place by another person... and have developed from there...
please help. thanks!
> > but why is it that result is always null?
> >
> > i don't get it... *in desperate help*
>
> If the result is null, it means your map is empty in
> the first place.
Or maybe it is a custom Map implementation ? Maybe it's buggy and keySet() returns an empty Set ?
Have you had a result > 0 when you called hashmap.size() ? If yes, then try outputting hashmap.keySet().size().
You could try checking myTreeMap.size()
Using two separated iterators may be a little iffy. Better something like:
Iterator it = myTreeMap.entrySet().iterator();
while(it.hasNext()) {
Map.Entry entry = (Map.Entry)it.next();
result += (String) entry.getKey() + ":" + (String)entry.getValue() + " ";
}
I confess, though, I can't see why your code isn't working.
> Or maybe it is a custom Map implementation ? Maybe
> it's buggy and keySet() returns an empty Set ?
> Have you had a result > 0 when you called
> hashmap.size() ? If yes, then try outputting
> hashmap.keySet().size().
it's a custom map...
have tried to display keySet...the size is the same as hashmap size.
but strangely no result come out..
sigh... i think it's buggy. dont know why why why
> > > but why is it that result is always null?
> > >
> > > i don't get it... *in desperate help*
> >
> > If the result is null, it means your map is empty
> in
> > the first place.
>
> Or maybe it is a custom Map implementation ? Maybe
> it's buggy and keySet() returns an empty Set ?
> Have you had a result > 0 when you called
> hashmap.size() ? If yes, then try outputting
> hashmap.keySet().size().
> it's a custom map...
>
> have tried to display keySet...the size is the same
> as hashmap size.
>
> but strangely no result come out..
>
> sigh... i think it's buggy. dont know why why why
Hmm. Could you post the code of the keySet() method in that class ?
Also, you could try:
Map hashmap;
if( hashmap.keySet().size() > 0 ){
if( ! hashmap.keySet().iterator().hashNext() ){
System.err.println("The iterator is crap !");
}
else if( ! hashmap.containsKey(hashmap.keySet().iterator().next()) ){
System.err.println("The iterator is crap !");
}
}
Hi mind if you all look at this for a while..
public String sortHashSet()
{
TreeMap myTreeMap;
myTreeMap = new TreeMap(hashMap);
String result="";
try{
List l = new ArrayList(hashMap.keySet());
Iterator it = myTreeMap.entrySet().iterator();
while(it.hasNext()) {
Map.Entry entry = (Map.Entry)it.next();
result += (String) entry.getKey() + ":" + (String)entry.getValue() + " ";
}
System.out.println("hello" + myTreeMap.size());
}
catch(Exception e)
{ System.out.println(e.getMessage()); }
return result;
}
this is my entire code that is giving me problem...
STRANGELY it dosnt even go to the hello statement? what's wrong?
> Not much point in looking at functions of the
> HashMap, it's the TreeMap that's giving the problem
> now.
It's not a HashMap, the OP just said it was a custom Map implementation.
Are you sure the problem's with the TreeMap now ? I don't think so. OP, could you please clarify ?
> > Not much point in looking at functions of the
> > HashMap, it's the TreeMap that's giving the
> problem
> > now.
>
> It's not a HashMap, the OP just said it was a custom
> Map implementation.
> Are you sure the problem's with the TreeMap now ? I
> don't think so. OP, could you please clarify ?
sorry sorry it's my fault. the HashMap was not custom...
rather now im using a class that actually creates HashMap
and yup the string result always display null...cause of exceptions..
now i print stack trace...there's a whole lot of exceptions out...
sigh my programming isnt good.
please pardon me.
now i got to see what's the problem of the exceptions.
yes u are absolutely right!
now got some improvements!
manage to sort the keys BUT unfortunately it's accoding to alphabetical order:
14:2.0 24:1.0 29:1.0 3:1.0 30:1.0 35:3.0 4:4.0 5:1.0 7:1.0
note: keys are on the left of ":" and value on the right...
hmm...im using this now:
List l = new ArrayList(hashMap.keySet());
Collections.sort(l);
Object okey, ovalue;
for(Iterator ito = l.iterator(); ito.hasNext();){
okey = ito.next();
ovalue = hashMap.get(okey);
result += (String) okey + ":" + ((Weight)ovalue).getValue() + " ";
}
any suggestions?
> manage to sort the keys BUT unfortunately it's
> accoding to alphabetical order:
>
> any suggestions?
Yes: make your map contains instances of java.lang.Integer as keys, not simply Strings consisting of digits.
Or use a custom comparator, like:
Comparator c = new Comparator(){
public int compare(Object o1, Object o2){
int i1 = Integer.parseInt(o1.toString()), i2 = Integer.parseInt(o2.toString());
return i1 - i2;
}
};
List l = new ArrayList(hashmap.keySet());
Collections.sort(l, c);
...
