Is removing multiple elements from a set in the middle of a foreach safe?

Question

Is the following code safe? Assume its single-threaded.

for(String s : set){
    set.remove(s);
    set.remove(randomElementFromSet());
}

I'm pretty sure this should be fine, but I want to double check that I run no risk of iterating over an element that has already been removed.

Also if possible please provide reference so I can look it up next time.

No its not. You can use failsafe iterators for the same. Refer the [link](http://www.journaldev.com/378/how-to-avoid-concurrentmodificationexception-when-using-an-iterator) for more information — Vivek Singh, Oct 24 '15 at 19:00
No, it probably throws ConcurrentModificationException. Use iterators as stated in last comment. — Forkmohit, Oct 24 '15 at 19:02
Why don't you test it? With a standard HashSet or TreeSet, you'll get a ConcurrentModificationException, as documented. — JB Nizet, Oct 24 '15 at 19:02
Did you create a test scenario and executed the code? That would've answered your question already. — Jeroen Vannevel, Oct 24 '15 at 19:02

score 1 · Accepted Answer · edited Oct 24 '15 at 22:27

1

Have you tried it? You would get ConcurrentModificationException.

One safe way to remove elements from a Collection while iterating over it is by using an Iterator explicitly:

for(Iterator<String> iter = set.iterator(); iter.hasNext();) {
    String s = iter.next();
    iter.remove();
}

edited Oct 24 '15 at 22:27

Louis Wasserman

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answered Oct 24 '15 at 19:03

Eran

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score 1 · Answer 2 · answered Oct 24 '15 at 19:18

You should not filter a collection using a for-each loop as you can get a ConcurrentModificationException.

To do it correctly you can use an Iterator, or removeIf. Both of these pieces of code print [Foo].

Approach using Iterator:

Set<String> set = new HashSet<String>(Arrays.asList("Foo", "Bar", "Baz"));
for (Iterator<String> i = set.iterator(); i.hasNext();)
    if (i.next().contains("B"))
        i.remove();
System.out.println(set);

Approach using removeIf (Java 8):

Set<String> set = new HashSet<>(Arrays.asList("Foo", "Bar", "Baz"));
set.removeIf(str -> str.contains("B"));
System.out.println(set);

Is removing multiple elements from a set in the middle of a foreach safe?

2 Answers2