How to remove element from ArrayList?

Question

I have added data into ArrayList and now want to update that list be deleting some element from it.

I have element something like 1,2,3,4 in ArrayList of type CartEntry.

Code :

ArrayList<CartEntry> items = new ArrayList<CartEntry>();

public void remove(int pId)
{
    System.out.println(items.size());

    for(CartEntry ce : items)
    {
        if(ce.getpId() == pId)
        {
            items.remove(ce);
            //System.out.println(items.get(1));             
        }
    }   
    items.add(new CartEntry(pId));
}

CartEntry Code :

public long getpId() {
    return pId;
}

Constructor :

public CartEntry(long pId) {
    super();
    this.pId = pId;     
}

when I am trying this code it gives me an error:

java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)

Here pId is the argument that specify that item should be deleted from items. Suppose I want to delete item that have 2 data then what will I have to do ?

Answer 1

You are facing ConcurrentModificationException because you are doing two operations on the same list at a time. i.e looping and removing same time.

Inorder to avoid this situation use Iterator,which guarantees you to remove the element from list safely .

A simple example looks like

Iterator<CartEntry> it = list.iterator();
    while (it.hasNext()) {
        if (it.next().getpId() == pId) {
            it.remove();
            break;
        }
    }

Answer 2

There are at least two problems with your code:

1. you call remove on the collection you iterate over, that will cause a ConcurrentModificationException if you continue iterating after the remove.

   There are two ways to fix this:

   • stop iterating after you found the object you want to remove (just add a break or a return) or
   • switch from the enhanced for-loop to using an Iterator and its remove method.

2. you add an element in your remove method, that's probably not what you want.

So I'd use this code (this assumes that there is only ever one CartEntry with a given id in the list):

public void remove(int pId)
{
    for(CartEntry ce : items)
    {
        if(ce.getpId() == pId)
        {
            items.remove(ce);
            return;
        }
    }   
}

If the assumption with the unique id is not correct, then you'll need to use the Iterator approach:

public void remove(int pId)
{
    Iterator<CartEntry> it = items.iterator();
    while(it.hasNext())
    {
        CartEntry ce = it.next();
        if(ce.getpId() == pId)
        {
            it.remove();
        }
    }   
}

Answer 3

you have created an Arraylist of type carEntry. So you need to create an Iterator of type CarEntry

Iterator<CarEntry> it =  items.iterator();    
while(it.hasNext())
{
   if(it.next().getPId == PId)
   it.remove();
}