Deleting while iterating in java! but deleting other then the current object

Question

iterator.remove(). is good however my question: if I want while to iterate, for a kind of condition, to delete another object from the array List.

Example (al being the arraylist)

  for (Iterator i = al.iterator(); i.hasNext(); ){
 IEvent event =(IEvent) i.next();
   if (nbSendingWTS > 0 || nbSendingCTS > 0){
                    i.remove();
                    al.remove(swtsee);
                    al.remove(sdctsee);
                    System.out.println("dropping evtg");
                }

This is giving me an error: Exception in thread "main" java.util.ConcurrentModificationException

Also the normal iteration:

          for(IEVEnt event:al){}

is giving an error

To be more clear the swtsee a d sdctsee are taken from previous iterations on the arraylist and saved so i can delete if I have the new condition. So is there a way when I detect them to shift them to higher indexes and then I use a reverse iteration?

What to do?

If you really need to do this, consider ArrayBlockingQueue, ConcurrentLinkedQueue, or other collections designed for concurrent use. Their iterators are guaranteed not to throw this exception. But they're probably overkill unless you're really using multiple threads. — Vance Maverick
– Vance Maverick, Commented Mar 18, 2011 at 4:53

Zimbabao · Accepted Answer · 2011-03-18 04:35:47Z

1

You can't remove element as discussed by you.

Do not delete while iterating.

Keep a Hash for all the objects you want to delete.
Do a second iteration which delete using .remove() if the object is in Hash.

answered Mar 18, 2011 at 4:35

Zimbabao

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Comments

Kanagaraj M · Accepted Answer · 2011-03-18 04:30:45Z

1

You can't delete if you are using for each style or iterator.

Use normal for loop like following

for(int i=0; i<al.size ; i++){
   if(something){
      al.remove(i)  
      i--;
    }

}

This will work.

answered Mar 18, 2011 at 4:30

Kanagaraj M

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4 Comments

Vix Over a year ago

yes but i want to remove many objects when i find the condition. So if it was in lower indexes i will have the problem

Kanagaraj M Over a year ago

i-- will do. for example your list has 10 elements(size=10). currently you are in 5th element, so i=4; now you are deleting 2nd element. al.get(i) will return you the next element. (i--, i++, the result would be i)

user unknown Over a year ago

Do you mean for (int i = al.size; i > 0; --i) if (something) al.remove (i)

user unknown Over a year ago

Ah, yes, now I see. you decrement on deletion. Yes, that works, but you can delete with for-each-style iterator, not just directly.

user unknown · Accepted Answer · 2011-03-18 05:03:37Z

1

To remove with an Iterator, you collect your stuff in a new Collection, and remove in a final step, for example:

    // list := List (1, 2, 4, 3, 4, 9, 6, 5, 7, 8);
List <Integer> toRemove = new ArrayList <Integer> ();       
for (int i : list)
    if (i % 2 == 0) 
        toRemove.add (i);
list.removeAll (toRemove);

I can't see how a1 is connected to your i. As long as it isn't iterated over, it should be secure to call those 2 a1.remove (...)-ings while iterating.

answered Mar 18, 2011 at 5:03

user unknown

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Comments

Dead Programmer · Accepted Answer · 2011-03-18 05:32:35Z

For your reference Java Collections

your code should work fine commenting the following lines

for (Iterator i = al.iterator(); i.hasNext(); )
{
    IEvent event =(IEvent) i.next();
   if (nbSendingWTS > 0 || nbSendingCTS > 0)
   {
          // You have got the iterator for the underlying array list(al)
          **only remove the elements through iterator.**   
          i.remove();

          // after remove thru iterator 
          // you are structurally modifiying arraylist directly(al.remove()) 
          // which gives u concurrent modification
          // al.remove(swtsee);  
          // al.remove(sdctsee);
          System.out.println("dropping evtg");
   }
}

and the best way to do is

 List<Integer> l = new ArrayList<Integer>();
    List<Integer> itemsToRemove = new ArrayList<Integer>();
    for (int i=0; i < 10; ++i) {
    l.add(new Integer(1));
    l.add(new Integer(2));
    }
    for (Integer i : l)
    {
        if (i.intValue() == 2)
            itemsToRemove.add(i);
    }

    l.removeAll(itemsToRemove);
    System.out.println(l);

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