how to compeer Arraylist contains hashtables by key name

[ben josh]

Hello all
i have Arraylist that contains hashtables
how can i sort the ArrayList by the hashtables key?

[Campbell Ritchie]

Sorting depends on comparisons.

Find the sort method of the Collections class.
Find the Comparator interface and set it up to compare the keys from the Hashtable entries.

Alternatively it is possible to get a Set of the map entries. Check whether you can sort that set by the keys of the entry.

It seems slightly odd that you are putting Hashtables into an ArrayList in the first place.

[ben josh]

well it kind of data structure i need to have in my application so i bent to that .
can i get some kind of example how to set up comparator for this scenario ?
link or tutorial
thanks

[Campbell Ritchie]

Have you read the Comparator interface in the API? Work out a way to get it to return 0 if the two objects are "equal" negative number if less and positive number if more. Also implement the "equals" method of Comparator. Then you pass the Comparator object to the sort() method as a parameter.

[Campbell Ritchie]

. . . and there is mention of Comparator in the Java tutorial under Collections . . . algorithms.

[ben josh]

hello all
after reading some tutorials i still cant do this comparison right .. here is what i have :
my data strut :

public class Tuple extends Hashtable{
 
 @Override
 public synchronized Object get(Object key) {
  // TODO Auto-generated method stub
  return super.get(key);
 }
 
 
 @Override
 public synchronized Object put(Object key, Object value) {
  // TODO Auto-generated method stub
  return super.put(key, value);
 }
 
 public String getKeybyIndex(int i){
  Enumeration e = super.keys();
  String key = null;
  int j = 1 ;
  while(e.hasMoreElements()){
   key = (String)e.nextElement();
   if(i == j )
    break;
   ++j;
  }
  return key;
 }
 
}
 
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);
 
}
 

 
}
 

 
and my comperetor class and main class
 
class Comp implements Comparator {
 
public int compare (Object o1, Object o2) {
 

String s1 = ((Tuple)o1).getKeybyIndex(1).toString().toUpperCase() ; 
 
String s2 = ((Tuple)o2).getKeybyIndex(1).toString().toUpperCase() ; 
 
return s1.compareTo ( s2 ) ; 
 
}
 
}
 

public class Main {
 

 
 
public static void main(String[] args) {
 
 
 
Tuple t1 = new Tuple();
 
Tuple t2 = new Tuple();
 
Tuple t3 = new Tuple();
 

t1.put("1","1");
 
t1.put("2","2");
 
t1.put("3","3");
 

TupleList tls = new TupleList();
 
tls.addTuple(t3);
 
tls.addTuple(t2);
 
tls.addTuple(t1);
 

Collections.sort ( (List<Tuple> :wink:  tls, new Comp ( ) ) ; 
 

System.out.println(tls);
 
}
 

}
 

but after all that im getting

Exception in thread "main" java.lang.ClassCastException: com.test.ds.TupleList
 
at com.test.ds.Main.main(Main.java:38)

and i cant understand why , can someone please help me here ?

Thanks

[Garrett Rowe]

The current problem is here.

 TupleList tls = new TupleList();
 
tls.addTuple(t3);
 
tls.addTuple(t2);
 
tls.addTuple(t1);
 
 
Collections.sort ( (List<Tuple>)  tls, new Comp ( ) ) ; 
 
 
System.out.println(tls);   

a TupleList is not a List<Tuple> and can't be made one even with a cast. Internally a TupleList contains a reference to a List<Tuple>, but this is irrelevant. Either you can make your TupleList class implement List<Tuple> or you can sort the internal List<Tuple> by some other means.


On another note, since you are using generics in the rest of your project, you could eliminate the casts in your Comp.compare() method and make it typesafe by changing it to:

class Comp implements Comparator<Tuple> {
 
public int compare (Tuple t1, Tuple t2) {
 
 
   String s1 = t1.getKeybyIndex(1).toString().toUpperCase() ; 
 
   String s2 = t2.getKeybyIndex(1).toString().toUpperCase() ; 
 
   return s1.compareTo ( s2 ) ; 
 
}

[ March 16, 2007: Message edited by: Garrett Rowe ]

[Garrett Rowe]

Originally posted by Campbell Ritchie:
Have you read the Comparator interface in the API? Work out a way to get it to return 0 if the two objects are "equal" negative number if less and positive number if more. Also implement the "equals" method of Comparator...

Why would it be helpful to override the equals() method here? Normally Comparators that I've used have no state.

[Campbell Ritchie]

Override equals?
I thought you had to because I thought there was an equals method in Comparator, but maybe I was mistaken. Thank you.

[ben josh]

Hi
even so i did your changes also to the TupleList and the comp class i still have
exceptions

Exception in thread "main" java.lang.NullPointerException
 
at java.util.Arrays.sort(Arrays.java:1219)
 
at java.util.Collections.sort(Collections.java:159)
 
at com.test.ds.Main.main(Main.java:38)

i cant understand it ...

[Garrett Rowe]

I thought there was an equals method in Comparator...

There is but according to the documentation:

Note that it is always safe not to override Object.equals(Object). However, overriding this method may, in some cases, improve performance by allowing programs to determine that two distinct Comparators impose the same order.

I've just never written one where this would be useful.

[Garrett Rowe]

Hi
even so i did your changes also to the TupleList and the comp class i still have
exceptions

Which line is line 38? Exactly what changes did you make?

[ben josh]

here is what i did :
in the Tuplelist class now lookes like this :

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);
}

[B] public void sort(Comp comparator) {
        Collections.sort (list, comparator);
}[/B]
}

and my main class looks like this :

public static void main(String[] args) {
// TODO Auto-generated method stub
Tuple t1 = new Tuple();
Tuple t2 = new Tuple();
Tuple t3 = new Tuple();

t1.put("1","1");
t1.put("2","2");
t1.put("3","3");

TupleList tls = new TupleList();
tls.addTuple(t3);
tls.addTuple(t2);
tls.addTuple(t1);

[B] tls.sort(new Comp()); [/B]
System.out.println(tls);
}

and im geting this excption :

Exception in thread "main" java.lang.NullPointerException
at com.test.ds.Comp.compare(Main.java:12)
at com.test.ds.Comp.compare(Main.java:1)
at java.util.Arrays.mergeSort(Arrays.java:1284)
at java.util.Arrays.sort(Arrays.java:1223)
at java.util.Collections.sort(Collections.java:159)
at com.test.ds.TupleList.sort(TupleList.java:20)
at com.test.ds.Main.main(Main.java:38)

maybe because the Tuple when created doing allocation of 10 elements that are null ?
i reallydont know ..
help

[ben josh]

some one?
is there any sulotion for this problem ?