removal of elements in linkedlist of java

[dilip satyaraghava]

when i try to remove an element from linked list, an exception is arised.. java.util.ConcurrentModificationException...can any one suggest an example for removing element.......

[John Jai]

Welcome to the Ranch

Are you trying to remove the elements from the List while iterating with the for-each loop? Could you please show the code you have tried?

[Riaan Nel]

You can't remove elements from a list while you are looping through that list.

You could (although I'm not sure if it is the best solution) add all the items that you want to remove to a new list, then call removeAll() on your original list and pass in the new list (outside of a loop).

[Rob Spoor]

Or use an explicit Iterator and use that to remove elements.

[Mohamed Sanaulla]

We would like to see the code!
But I think your code is doing what John has mentioned and you can correct it using Rob's suggestion.
This query has come up lot may times before and I am sure its kind of popular exception

[dilip satyaraghava]

This is my code:

/* Linked List Collections Framework */
import java.util.*;
class pqr
{
public static void main(String args[])
{
LinkedList<Integer> ll=new LinkedList<Integer>();

ll.add(10);
ll.add(20);
ll.add(30);

ll.addFirst(5);
ll.addLast(40);

Iterator<Integer> itr=ll.iterator();  //Retriving the data
while(itr.hasNext())
{
Integer obj1=itr.next();
System.out.println(obj1);
}

ll.remove(3);
while(litr.hasNext())
{
Integer obj6=litr.next();
System.out.println(obj6);
}}
}

[dilip satyaraghava]

I missed few lines in the above code.. This is my code...


import java.util.*;
class Linkedlist
{
public static void main(String args[])
{
LinkedList<Integer> ll=new LinkedList<Integer>();
System.out.println("Size="+ll.size());
System.out.println("Content="+ll);
ll.add(10);
ll.add(20);
ll.add(30);
System.out.println("Size="+ll.size());
System.out.println("Content="+ll);
ll.addFirst(5);
ll.addLast(40);
System.out.println("Size="+ll.size());
System.out.println("Content="+ll);
System.out.println("Iterator -FD");
Iterator<Integer> itr=ll.iterator(); //Retriving the data
while(itr.hasNext())
{
Integer obj1=itr.next();
System.out.println(obj1);
}

System.out.println("ListIterator -FD");
ListIterator<Integer> litr=ll.listIterator(); //Retriving the data
while(litr.hasNext())
{
Integer obj2=litr.next();
System.out.println(obj2);
}
System.out.println("List Iterator-BD");
while(litr.hasPrevious())
{
Integer obj3=litr.previous();
System.out.println(obj3);
}
Integer obj=ll.get(1);
System.out.println("Element ="+obj);
ll.remove(2);
while(litr.hasNext())
{
Integer obj7=litr.next();
System.out.println(obj7);
}}
}
Output:
Size=0
Content=[]
Size=3
Content=[10, 20, 30]
Size=5
Content=[5, 10, 20, 30, 40]
Iterator -FD
5
10
20
30
40
ListIterator -FD
5
10
20
30
40
List Iterator-BD
40
30
20
10
5
Element =10
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.LinkedList$ListItr.checkForComodification(Unknown Source)
at java.util.LinkedList$ListItr.next(Unknown Source)
at Linkedlist.main(Linkedlist.java:46)

[Joseph Tulowiecki]

Yea right here:

Integer obj=ll.get(1);
System.out.println("Element ="+obj);
ll.remove(2);
while(litr.hasNext()) 

you can't change values in an object while your iterating through it.
ll.remove(2);

[John Jai]

Either you can reassign the iterator after removing an element

ll.remove(2); 
litr = ll.listIterator();

or remove the element using the iterator

int i = 0;
while(litr.hasNext()){
	if (i == 2){				
		litr.remove();
	}
	litr.next();
	i++;
}
// Bring back litr to point first row - else re-assign litr to ll.listIterator();
while(litr.hasPrevious()){
	litr.previous();
}

[dilip satyaraghava]

the remove function is after the while iteration, so why do i again need to create an object??

[John Jai]

Because the first list iterator you created has knowledge about a list that has 5 elements.

You then go and remove one of the elements from the list.

When you try again to use the same list iterator that still don't know about the removal, you get the exception.

So either
1. Re-assign it to a newly created list iterator which knows about the current 4 element list
2. Or remove the element using the list iterator itself. This way the iterator knows one element is removed from the iterating list.