remove specific object does not work in Java ArrayList

Question

I have an ArrayList of object containing a reference to their parent object.

I am trying to remove every objects within a specific parent (so equals to the parent object).

If I do:

System.out.println(parent);

my console output:

ParentObject@1f8166e5

if I do:

for(int i = 0; i < array.size(); i++){
   if(array.get(i).getParent().equals(parent)){
      array.remove(i);
   }
}

And (test)

for(int i = 0; i < array.size(); i++){
   if(array.get(i).getParent().equals(parent)){
      System.out.println(parent + ":" + array.get(i).getParent());

   }
}

My console output something like:

ParentObject@1f8166e5:ParentObject@1f8166e5

What's wrong with the snippet above? Why array.remove(i) did not work?

What is the indication that remove() doesn't work?

Sotirios Delimanolis
– Sotirios Delimanolis

2014-01-30 21:30:48 +00:00
Commented Jan 30, 2014 at 21:30 — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Jan 30, 2014 at 21:30

Jon Skeet · Accepted Answer · 2014-01-30 21:43:07Z

8

I suspect the problem is that after you've removed element i (which moves everything after it up the list - the element at index n + 1 now has index n etc) you're then skipping the next element. So if you have two consecutive elements to remove, you're missing the second one. The simplest fix is to work from the end:

for (int i = array.size() - 1; i >= 0; i--) {
   if (array.get(i).getParent().equals(parent)) {
      array.remove(i);
   }
}

EDIT: As noted in another answer, it's generally better to use an iterator for removal anyway. In your case, this would look like this:

// We don't know what the Foo type is...
for (Iterator<Foo> iterator = array.iterator(); iterator.hasNext(); ) {
    Foo element = iterator.next();
    if (element.getParent().equals(parent)) {
        iterator.remove(); // Note: *not* array.remove(...)
    }
}

edited Jan 30, 2014 at 21:43

answered Jan 30, 2014 at 21:33

Jon Skeet

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1 Comment

Jon Skeet Over a year ago

@Pier-AlexandreBouchard: Not shuffle in terms of "randomly reassign" but shuffle as in "everything else moves up one place" - if you had an element A with index 10, and you remove the element at index 2, element A now has index 9.

jonasnas · Accepted Answer · 2014-01-30 21:33:47Z

1

You can't remove objects while iterating. You should use Iterator if you want to remove element while iterating

answered Jan 30, 2014 at 21:33

jonasnas

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2 Comments

Hauke Ingmar Schmidt Over a year ago

He is not using an Iterator but an indexed for loop. Removing works, it is just not a good pattern and may lead to problems as @Jon Skeet hinted.

Jon Skeet Over a year ago

I've added an Iterator-based approach to my answer.

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