how to compeer Arraylist contains hashtables by key name

You need to provide a [url=http://java.sun.com/j2se/1.4.2/docs/api/java/util/Comparator.html]Comparator[/url] for comparing hash tables, based on your criteria.

However, your order criteria are unclear: by the hashtables key

(note that a hash table contains a set of keys, not just one key.)

So you have to clearly define what would make a given hash table being before or after another one.

TimTheEnchantora at 2007-7-10 3:44:31 >

Hashtable is overkill in that case, IMHO. Why not using your own class, which would have key and value members (and could implement Comparable, btw.)

Anyway, if you still want/have to keep using hashtable, then your Comparator implementation should compare the only key of the Hashtables, something like: public int compare(Object o1, Object o2) {

return ((String)((Hashtable)o1).keySet().iterator().next()).compareTo(((Hashtable)o2).keySet().iterator().next());

}

(Fastidious, isn't it ?)

Then, use this comparator to [url=http://java.sun.com/j2se/1.4.2/docs/api/java/util/Collections.html#sort(java.util.List, java.util.Comparator)]sort[/url] your list.

TimTheEnchantora at 2007-7-10 3:44:31 >

The exception is thrown because the tls variable is not a List<Tuple> and you are trying to cast it.

You might simply add a sort method in your TupleList class:public class TupleList {

ArrayList<Tuple> list = new ArrayList();

public void addTuple(Tuple t) {

list.add(t);

}

public Tuple getTuple(int i) {

return list.get(i);

}

public void sort(Comp comparator) {

Collections.sort (list, comparator);

}

}

Note that, as soon as you don't define a specific behaviour in your TupleList class, why don't you simply use a List<Tuple> directly in your app ? (in other words, what is the reason for having a TupleList class ?)

Finally, your Tuple class looks weird to me. You said that "hash table i will have only one key and value", but nothing prevent it from hapening in your code. I cannot see why it has to extend Hashtable.

TimTheEnchantora at 2007-7-10 3:44:31 >

This NullPointerException comes from the fact that you are putting values in the Tuple t1, but you don't put any value int the two others:t1.put("1","1");

t1.put("2","2");

t1.put("3","3");

Therefore, the getKeyByIndex(1) used by your comparator returns null for t2 and t3.

Regards,

Tim

Anyway, my advise remains: you should review your design. Extending Hashtable is not the way to go here. And I cannot see any reason for not refactoring you classe(s). Your code could become clearer and more robust.

TimTheEnchantora at 2007-7-10 3:44:32 >