Java - Collection.remove() behaves differently in different conditions

Question

This is a follow up to my previous question :

Collection - Iterator.remove() vs Collection.remove()

The below two pieces of code , which apparently differs only by a single line , but one throws exception and other don't . Can you please explain the difference ?

 List<String> list = new ArrayList<String>
(Arrays.asList("noob1","noob2","noob3"));


System.out.println(list);

for (String str : list) {
    if (str.equals("noob2")) {
        list.remove(str);
    }
}

runs fine , but if i change the condition to

if (!str.equals("noob2"))

the code throws exception !

Is there any general rule which determines it ? Or it is undefined ! — AllTooSir, Jan 08 '13 at 15:29
general rule for what? approach is wrong, use Iterator instead, and debugger to see where exception is thrown. — Nikolay Kuznetsov, Jan 08 '13 at 15:32

Peter Lawrey · Accepted Answer · 2013-01-08T15:32:08.243

What happens in this situation is you are removing the second list element.

List<String> list = new ArrayList<String>
        (Arrays.asList("noob1", "noob2", "noob3", "noob4"));

System.out.println(list);

for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); ) {
    String str = iterator.next();
    if (str.equals("noob3")) {
        System.out.println("Checking "+str);
        list.remove(str);
    }
}
System.out.println(list);

prints

[noob1, noob2, noob3, noob4]
Checking noob1
Checking noob2
Checking noob3
[noob1, noob2, noob4]

By removing the second last element you have reduced the size to the number of elements which you have iterated over.

// from ArrayList.Itr
    public boolean hasNext() {
        return cursor != size;
    }

This causes the loop to exit early before the concurrent modifcation check is performed in next(). If you remove any other element next() is called and you get a CME.

BTW Something which also bypasses the check is

for (Iterator<String> iterator = list.iterator(); iterator.hasNext(); ) {
    String str = iterator.next();
    System.out.println("Checking "+str);
    if (str.equals("noob2")) {
        list.remove("noob1");
        list.remove("noob3");
    }
}

as long as the size of the collection is the same as the index it is up to, the check is not performed.

score 1 · Answer 2 · answered Jan 08 '13 at 15:23

The for loop is just a simplified syntax for an iterator scan of the list. The iterator may throw an exception if the list is modified under it, but it is not guaranteed. Because of hasNext, iterators are often working one element ahead, making the first case less likely to be affected by list modification. By the time "noob2" is removed, the iterator already knows about "noob3".

davioooh · Answer 3 · 2013-01-08T15:29:57.317

1

I suppose the exception is thown because you are trying to change a collection you are looping on... and not because the if condition.

I suggest you to create a new list only containing the items that verify the condition. Add them to the new list and avoid to change the original collection.

edited Jan 08 '13 at 15:29

answered Jan 08 '13 at 15:23

davioooh

23,742
39
159
250

score 1 · Answer 4 · answered Jan 08 '13 at 15:29

Actually you should never remove collections' elements during "casual" iterating. When you have to modify your collection in some loop you have to use iterator to make these operations.

    public class Test {
        public static void main(String... args) {
            List<String> list = new ArrayList<String>(Arrays.asList("noob1", "noob2", "noob3"));

            System.out.println(list);

            for (Iterator<String> iterator = list.iterator(); iterator.hasNext();) {
                String str = iterator.next();
                if (!str.equals("noob2")) {
                    iterator.remove();
                }
            }
            System.out.println(list);
        }
    }

score 1 · Answer 5 · answered Jan 08 '13 at 15:32

It's because you are trying to remove from a Collection you are currently iterating through. Making a minor alteration you can do what you want to do:

 String[] strValues = {"noob1","noob2","noob3"}; // <<< Array
 List<String> list = new ArrayList<String>(Arrays.asList(strValues));

 System.out.println(list);

 for (String str : strValues) { // << List is duplicate of array so can iterate through array
     if (!str.equals("noob2")) {
         list.remove(str);
     }
 }

That should work. Hopefully

PermGenError · Answer 6 · 2013-01-08T15:41:06.460

Well, your first case doesn't throw the Exception because, the iterator returns false for Iterator.hasNext() at index 2 as you remove the element at index 1.

Iterator<String> itr = list.iterator();
    while(itr.hasNext()){
        String s= itr.next();
        if(s.equals("noob2")){
            list.remove(s); // size of the list is 2 here 
            System.out.println(itr.hasNext());// this returns false as it doesn't have anything at index 2 now.(on 2nd iteration )
        }
    }

You can test it clearly using a simple for-loop:

for (int i=0; i<list.size(); i++) {
        if (list.get(i).equals("noob2")) {
            System.out.println(list.get(i));
            System.out.println(list.size());
            list.remove(list.get(i));
            System.out.println(list.size());
        }
    }

Output:

[noob1, noob2, noob3]
noob2
3
2

Notice the size of the list after you remove the element, which fails after incrementing. 2<2 which is false

Java - Collection.remove() behaves differently in different conditions

6 Answers6