Using JAVA Iterator

Question

I'm trying delete strings which have even index numbers in this ArrayList, using "Iterator".remove.

    import java.util.*;
import static java.lang.System.out;

class MAIN
{
    public static void main(String args[])
    {
        ArrayList<String> al = new ArrayList<String>();
        Iterator i = al.iterator();
        int len;
        Scanner sc = new Scanner(System.in);
        out.printf("Size:");
        len = sc.nextInt();

        while( len!=0 )
        {
            al.add( sc.next() );
            len -= 1;
        }

        Object o;
        for( len = 0 ; len < al.size() ; len +=1)
        {
            out.println(al);
            if( len%2==0 )
            {
                o = i.next();
                out.println(o);
                i.remove();
                i.next();
            }
        }


        return;
    }

}

I'm getting the "ConcurrentModificationException" at i.next();. What's wrong ?

Hovercraft Full Of Eels · Accepted Answer · 2014-11-24 04:37:40Z

5

You're not using the iterator to iterate through your correction, and so you cannot use it to remove elements. To use it correctly, you must loop with the iterator itself.

i.e.,

for (Iterator<E> iter = list.iterator(); iter.hasNext(); ) {
    E element = iter.next();
    // 1 - can call methods of element
    // 2 - can use iter.remove() to remove the current element from the list

    // ...
}

Maybe something like:

int count = 0;
for (Iterator<String> iter = al.iterator(); iter.hasNext(); ) {
   String element = iter.next();
   System.out.print(element);
   if (count % 2 == 0) {
       iter.remove();
       System.out.print(" -- has been deleted");
   }
   System.out.println();
   count++;
}

Another option is to use a for loop to loop backwards through your collection. This can work since removing an item will not change the ordering or indexing of the items before the removed item.

edited Nov 24, 2014 at 4:37

answered Nov 24, 2014 at 4:30

Hovercraft Full Of Eels

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1 Comment

kesari Over a year ago

Didn't understand your last line.

Saurabh Arora · Accepted Answer · 2014-11-24 04:49:44Z

0

You are getting error because iterator checks for the modification, check for its implementation which is present in AbstractList class where an int variable modCount is defined that provides the number of times list size has been changed.

Check this link for detailed answer:

http://www.javacodegeeks.com/2011/05/avoid-concurrentmodificationexception.html

answered Nov 24, 2014 at 4:49

Saurabh Arora

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Comments

Abhishek · Accepted Answer · 2014-11-24 07:07:09Z

0

Use an Iterator and then call remove() over it:

Iterator<String> itr = list.iterator();

while (itr.hasNext()) {

    String s = itr.next();
   if (Condition)
     itr.remove();
}

answered Nov 24, 2014 at 7:07

Abhishek

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