Improper use of foreach in java?

Question

I'm trying to remove an item of type SubClass from an ArrayList using a foreach, and removing it right when it finds it.

The code:

for (SuperClass item : list)   
{     
     if (item instanceof SubClass)     
     {
        list.remove(item);     
     }   
}

I don't really know how the Iterator works in this case, but what I'm asking is: is this safe? Or should it throw an out of bounds exception?

Any help is appreciated!

Even if you could do this (and some concurrent lists will allow you) it would be very inefficient i.e. `O(n^2)`. I suggest using your IDE to transform the loop into an Iterator loop and using `Iterator.remove()` — Peter Lawrey, Aug 26 '13 at 07:04

score 7 · Accepted Answer · edited May 23 '17 at 12:13

7

You cant remove items from a list while using foreach statement. You will get ConcurrentModificationException

You need to use Iterator.remove() method

for(Iterator<SuperClass> i = list.iterator(); i.hasNext(); ) {
     SuperClass s = i.next();
     if(s instanceof SubClass) {
        i.remove();
     }
}

edited May 23 '17 at 12:13

Community

1
1

answered Aug 26 '13 at 06:48

sanbhat

17,522
6
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score 0 · Answer 2 · answered Aug 26 '13 at 06:50

0

Try ListIterator:

List<SuperClass> sampleList = new ArrayList<SuperClass>();
ListIterator<SuperClass> listIterator = sampleList.listIterator();
//
listIterator.remove();
//

answered Aug 26 '13 at 06:50

newuser

8,338
2
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33

[Here](http://docs.oracle.com/javase/6/docs/api/java/util/ListIterator.html) it writes that: `remove() Removes from the list the last element that was returned by next or previous (optional operation).` I suspect that the comment lines represent the lines that should contain the actual foreach statements that use the Iterator? – Vlad Ilie Aug 26 '13 at 07:02

score 0 · Answer 3 · answered Aug 26 '13 at 07:11

As sanbhat say it is not safe. but what you want to do can be solve by 2 diffenret way.

1) you can store all the objects & after foreach loop you can remove by index. but it is not good way.

2) use for loop (not foreach loop) & ittrate till length. then if object found remove it. thats it.

(make sure you have written condition like i

Improper use of foreach in java?

3 Answers3