Collections problem

[fahad siddiqui]

I use a hashtable which contains a String as a key and an arraylist as a value.
This hashtable is iterated on the jsp and first the string is displayed, then another iteration takes place for the arraylist for the string key and hence, submenu type of stuffs gets displayed.

Now, i want to display a particular String and its arraylist iterated on the top of the page and the rest as is.

Tree map sorts them, so i was thinking, is it possible to implement a comparator to the tree map which would never compare anything and hence, tree map would retain data in the order it was added and hence, displayed properly.

Can someone advise on this. Please provide a proper code for comparator implemented class
or some other solution for this.


Regards,
Fahad

[Shailesh Chandra]

First of all your question doesn't fit into JDBC.

Now back to your problem, I would suggest you create a comparator and user TreeMap in your code.

Create instance of TreeMap by passing your comparator in constructor.

If you need help on comparator, visit the URL

http://www.onjava.com/pub/a/onjava/2003/03/12/java_comp.html?page=2


Shailesh

[fahad siddiqui]

I knew that a comparator needs to be used, but i needed an example code on the same which would make no checking and hence, would just store the data in the treemap as is entered.

All the examples on google gives a code for comparison and when i return false in every case no matter of what compare method returns, then it doesnt give correct results.

[Shailesh Chandra]

You need to write a comparator like this

public class MyComparator implements Comparator {
 
    /**
     * Default constructor
     *
     */
     String strRquiredOnTop = "TOP_STRING";
    public MyComparator(){        
    }
    
    
    /**
     * Override compare()
     */
    public int compare(Object object1, Object object2) {
    String value1 = (String) object1;
    String value2 = (String) object2;
    
     if(value1 != null && strRquiredOnTop.equals(value1))
     {
     return 1;
     }
else if(value2 != null && strRquiredOnTop.equals(value2))
     {
     return 1;
     }
     else if ((value1 instanceof Comparable) && (value1 instanceof String)) {
value1 = (value1 == null) ? "" : value1;
value2 = (value2 == null) ? "" : value2;
 
return ((Comparable) value1).compareTo(value2);
} 
else if ((value2 instanceof Comparable) && (value2 instanceof String)) {
value1 = (value1 == null) ? "" : value1;
value2 = (value2 == null) ? "" : value2;
return ((Comparable) value2).compareTo(value1);
            }
    else
    {
    retrun value1.compareTo(value2);
    }
    
    return 0;
   }
}
 

Shailesh
[ September 05, 2006: Message edited by: Shailesh Chandra ]

[Jeanne Boyarsky]

I'm going to move this to Java in General since - as pointed out - it is not a JDBC question.

[Garrett Rowe]

is it possible to implement a comparator to the tree map which would never compare anything and hence, tree map would retain data in the order it was added and hence, displayed properly.

Sounds like a LinkedHashMap could be of some use here.

[Naseem Khan]

Originally posted by fahad siddiqui:

Tree map sorts them, so i was thinking, is it possible to implement a comparator to the tree map which would never compare anything and hence, tree map would retain data in the order it was added and hence, displayed properly.

As your key is an String, so TreeMap will always sort the key. If you want to maintain insertion order for the keys, use LinkedHashMap. It maintains insertion order. Why you are looking for a comparator.

In those cases where you don't have source code with you and supppose your key class does not implement Comparable interface and you want to use the instances of the class as a key in TreeMap, there Comparator is very useful. You can write a seperate Comparator which will actually sort the key as per the logic written in compare() method.

Naseem

[Ilja Preuss]

Originally posted by fahad siddiqui:
[QB]
Tree map sorts them, so i was thinking, is it possible to implement a comparator to the tree map which would never compare anything and hence, tree map would retain data in the order it was added and hence, displayed properly.

No, that wouldn't work - TreeSet heavily depends on the comparator defining a true ordering for orgizing its whole structure.

Like already suggested by someone else, a LinkedHashMap is what you should use.