Removing object from ArrayList in for each loop

Question

I would like to remove an object from an ArrayList when I'm done with it, but I can't find way to do it. Trying to remove it like in the sample code below doesn't want to work. How could I get to the iterator of current px object in this loop to remove it?

for( Pixel px : pixel){ 
[...]
  if(px.y > gHeigh){
     pixel.remove(pixel.indexOf(px)); // here is the thing
     pixel.remove(px); //doesn't work either
  }
}

possible duplicate of [Calling remove in foreach loop in Java](http://stackoverflow.com/questions/1196586/calling-remove-in-foreach-loop-in-java) — DNA, Mar 13 '12 at 20:21
possible duplicate of [Iterating through a Collection, avoiding ConcurrentModificationException when removing in loop](http://stackoverflow.com/questions/223918/iterating-through-a-collection-avoiding-concurrentmodificationexception-when-re) — omerhakanbilici, Oct 31 '16 at 14:16

score 67 · Accepted Answer · answered Mar 13 '12 at 20:13

You can't, within the enhanced for loop. You have to use the "long-hand" approach:

for (Iterator<Pixel> iterator = pixels.iterator(); iterator.hasNext(); ) {
  Pixel px = iterator.next();
  if(px.y > gHeigh){
    iterator.remove();
  }
}

Of course, not all iterators support removal, but you should be fine with ArrayList.

An alternative is to build an additional collection of "pixels to remove" then call removeAll on the list at the end.

Alexis C. · Answer 2 · 2016-03-08T10:10:47.897

24

Using java-8 and lamdba expressions, the method removeIf has been introduced for collections.

Removes all of the elements of this collection that satisfy the given predicate.

So it will only take one line :

pixels.removeIf(px -> px.y > gHeigh);

edited Mar 08 '16 at 10:10

answered Jan 02 '14 at 15:54

Alexis C.

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by the way `removeIf` uses `Iterator` and `while` loop. You can see it at java 8 `java.util.Collection.java` – omerhakanbilici Oct 31 '16 at 14:16
1

@omerhakanbilici This is only the default implementation. You can see it has been optimized for `ArrayList`s for instance. – Alexis C. Oct 31 '16 at 15:14

score 2 · Answer 3 · edited Nov 30 '17 at 23:17

2

you need to create and access the iterator explicitly

Iterator<Pixel> it = pixel.iterator();
while(it.hasNext()){
Pixel.px = it.next();
//...
it.remove();
}

edited Nov 30 '17 at 23:17

Bumptious Q Bangwhistle

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answered Mar 13 '12 at 20:14

ratchet freak

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Jakub Zaverka · Answer 4 · 2012-03-13T21:49:23.707

2

You cannot modify a Collection while someone is iterating over it, even if that someone were you. Use normal for cycle:

for(int i = 0; i < pixel.size(); i++){
    if(pixel.get(i).y > gHeigh){
        pixel.remove(i);
        i--;
    }
}

edited Mar 13 '12 at 21:49

answered Mar 13 '12 at 20:15

Jakub Zaverka

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you have the issue of the next loop needing to be on index `i` again and you need to ensure it is not incremented – ratchet freak Mar 13 '12 at 21:28

score 1 · Answer 5 · edited May 23 '17 at 12:34

1

use a regular for loop, the enhanced for loop maintains an iterator, and doesn't allow for removal of objects, or use the iterator explicitly

Edit: see answer of the this question Calling remove in foreach loop in Java

edited May 23 '17 at 12:34

Community

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answered Mar 13 '12 at 20:13

Alex

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score 0 · Answer 6 · answered Mar 13 '12 at 20:17

0

If Pixel is your own custom object then you need to implement the equals and hashcode method for your Pixel object. The indexOf method also finds the index using equals method. Try implementing that and check it out.

answered Mar 13 '12 at 20:17

raddykrish

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Removing object from ArrayList in for each loop

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