remove element from ArrayList inside for(Particle p: particles) loop

Question

I know how to remove an element using the remove() function inside a for loop this way :

 // ArrayList<Particle> particles;

    for(int = 0; i < particles.size(); i++){
     if(condition){
     particles.remove(i)
     }    
    }

but I'd like to know how to do the same thing using this alternative for loop syntax that I find more elegant :

// ArrayList<Particle> particles;

for(Particle p:particles){
 if(condition){
// remove particle
 }
}

Does this answer your question? [Calling remove in foreach loop in Java](https://stackoverflow.com/questions/1196586/calling-remove-in-foreach-loop-in-java) — Lev Leontev, Feb 21 '20 at 14:57

Jaeheon Shim · Accepted Answer · 2020-02-21T15:07:59.403

0

You can't do that because you'll get a ConcurrentModificationException. ConcurrentModificationException is thrown when something you're iterating over is modified.

You need to create an iterator and use that to remove your elements. Here's an example:

Iterator iterator = particles.iterator();
while(iterator.hasNext()) {
    iterator.remove();
    iterator.next();
}

edited Feb 21 '20 at 15:07

answered Feb 21 '20 at 15:00

Jaeheon Shim

491
5
14

This will remove every element, though. The OP's first example only removes every other element. – azurefrog Feb 21 '20 at 15:01
@azurefrog How so? He increments the iterator integer by one every time – Jaeheon Shim Feb 21 '20 at 15:03
Lists are not like arrays. When you remove element 0, the rest are "re-indexed" down, so removing 0, then 1, then 2 removes what originally were the 0th, 2nd and 4th elements. – azurefrog Feb 21 '20 at 15:05
@azurefrog Oh I see. I overlooked that. – Jaeheon Shim Feb 21 '20 at 15:07
Well, to be fair, it's possible that the OP did, too... ;-) – azurefrog Feb 21 '20 at 15:07
@azurefrog I edited my answer to correspond with what OP's code would have done, but I guess the question is, is it OP's intent to remove every other element? – Jaeheon Shim Feb 21 '20 at 15:08
it seems though that I loose my class functions when I do so because I'm looping through an iterator and not an object anymore – Kaspie Feb 21 '20 at 15:10
@Asymetr well you could get the object of the type of your class from the return value of iterator.next() – Jaeheon Shim Feb 21 '20 at 15:12
and yes I did completely overlook that, I'm sorry, let me update – Kaspie Feb 21 '20 at 15:12
@Asymetr so what is your intention here? if you're just trying to remove all the elements from the ArrayList, there are easier ways to accomplish that. – Jaeheon Shim Feb 21 '20 at 15:13
I'm almost good I'm just trying to figure out how to get my object back using Particle particle = iterator.next(); – Kaspie Feb 21 '20 at 15:20

score -2 · Answer 2 · answered Feb 21 '20 at 15:30

-2

Use the Iterator class :

Iterator iterator = particles.iterator();
    while(iterator.hasNext()) {
    iterator.remove();
    iterator.next();
}

answered Feb 21 '20 at 15:30

Youness Dominique

19
4

this all works except I get a type mismatch "java.lang.Object" does not match with "Particle" when I try to Particle particle = iterator.next(); do you know how I could resolve this ? Isn't next() suppose to iterate through my array of objects ? – Kaspie Feb 21 '20 at 15:36

remove element from ArrayList inside for(Particle p: particles) loop

2 Answers2