help with determing if a node is duplicate and if so prevent from adding to list

[jputneyCS]

//my code works perfectly, i just need to stop it from adding duplicate nodes. any help, ill be extremely thankful.



public class OrderedList<T extends Comparable<? super T>> extends List<T>
{
public void add(T o)
{
if(head == null)
{
head = new ListNode<T>(o, head);
}
else
{
if(head.data.compareTo(o)>0)
{
head = new ListNode<T>(o, head);
}
else
{
ListNode<T> current = new ListNode<T>(head.data, head);
ListNode<T> previous = new ListNode<T>(null, head);
boolean add = true;

while(current!= null && current.data.compareTo(o)<0)
{
previous = current;
current = current.next;
}
// i think this is where i need to check for duplicity

previous.next = new ListNode<T>(o,current);

}
}
}
}

[pbrockway2]

I think you're right: once you've moved the current pointer so that previous and all others (towards the head) are strictly greater than current then check previous.next. If it is equal to current then the method can end (throw an exception or whatever).

(I'm assuming that by duplicate you mean "compares equal" - this is the semantics that Set uses. And your OrderedList is more or less a SortedSet.)

Also, unless I'm missing something your line

previous.next = new ListNode<T>(o,current);

adds current in the correct position in the list. But what happens to the rest of the list - the ones that used to follow previous? Won't they be "forgotten"?

[jputneyCS]

im using the code to find the position that i should add the element thats being passed into the list. when i run it all of my elements print out correctly. i just have to exclude elements thats already in the list. sorry if i dont understand what you mean by losing previous data.....

while(current!= null && current.data.compareTo(o)<0)
{
previous = current;
current = current.next;
}

[pbrockway2]

Sorry I've woken up! It's the ListNode that has a reference to the next node, right?

In that case it all looks good. Just check, at the place you marked, the value in the current node and o. If they are equal just return or throw an exception.

[jputneyCS]

ok, i added the conditon. i think what im trying to do is if the node after the positon i want to add is not equal to the node im adding then add.....but its throwing a nullPointerException on the condition

while(current!= null && current.data.compareTo(o)<0)
{
previous = current;
current = current.next;
}
// i think this is where i need to check for duplicity
if(!current.equals(o))
previous.next = new ListNode<T>(o,current);

[jputneyCS]

ok, i figured it out thanks very much. i ended up using the condition

while(current!= null && current.data.compareTo(o)<0)
{
previous = current;
current = current.next;
}
// i think this is where i need to check for duplicity
if(current == null || !current.data.equals(o))
previous.next = new ListNode<T>(o,current);

[pbrockway2]

Great - I'm glad you've got it figured out. current==null is the case where you have reached the end of the list.