Looping through HashMap

[Frank Sikuluzu]

I created a HashMap putting values and keys, the keys are like "01", "02", "03" kind two digits string. The when I loop through the HashMap, I use

Set set= hm.keySet();
Iterator iter = set.iterator();
while(iter.hasNext()){
String currentKey = iter.next();
// print out this currentKey
}

The print out result shows it is NOT ordered by "01", "02", "03" or alphabetical letter order, which was what I expected.

Why ? and how do I get a loop that output keys in alphabetical letter or digital ascendant order ?

[Ernest Friedman-Hill]

Hi Frank,

HashMap does indeed store its keys in a seemingly random order. It's not really random, of course -- the order is related to the hashCode() values of the keys -- but it's not any kind of useful sorted order.

If you want a map in which they keys are kept in sorted order, use java.util.TreeMap instead. You pay for that sorting by giving up a little performance, but in many circumstances this really doesn't matter.

[Rick O'Shay]

Use a TreeMap since that guarantees natural ordering. If the natural order is not what you want you can create a treemap with your own comparator:

TreeMap( Comparator c )