com.google.common.collect.Sets.SetView bug or feature?

Question

Hello I've this piece of code:

public static void main(String[] args) {
    Set<Integer> set1 = new HashSet<Integer>();
    Set<Integer> set2 = new HashSet<Integer>();
    set1.add(1);
    set1.add(2);
    set1.add(3);
    set1.add(4);
    set1.add(5);

    set2.add(4);
    set2.add(5);
    set2.add(6);
    set2.add(7);
    set2.add(8);

    SetView<Integer> x = Sets.intersection(set1, set2);
    set1.removeAll(x);
    set2.removeAll(x);
}

and it throws

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.HashMap$HashIterator.nextEntry(HashMap.java:841)
    at java.util.HashMap$KeyIterator.next(HashMap.java:877)
    at com.google.common.collect.Iterators$7.computeNext(Iterators.java:627)
    at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:141)
    at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:136)
    at java.util.AbstractSet.removeAll(AbstractSet.java:142)
    at com.Main2.main(Main2.java:30)

is this a normal? or a small bug...

Answer 1

SetView is a view of the intersection of these sets, not a copy. From the Guava docs:

An unmodifiable view of a set which may be backed by other sets; this view will change as the backing sets do.

So, when you call set1.removeAll(x) and pass in the view, you're essentially trying to remove from set1 while looping over part of itself. This is the reason for the ConcurrentModificationException.

To achieve what you're trying to do, have a look at SetView.immutableCopy().

For example:

SetView<Integer> intersectionView = Sets.intersection(set1, set2);
ImmutableSet<Integer> intersectionCopy = intersectionView.immutableCopy();
set1.removeAll(intersectionCopy);
set2.removeAll(intersectionCopy);