为什么这段代码会引发ConcurrentModificationException？

Question

Write a method removeEvenLength that takes a Set of strings as a parameter and that removes all of the strings of even length from the set.

My solution:

public static void removeEvenLength(Set<String> set) {
    for(String word : set) {
        if(word.length() % 2 == 0) {
            set.remove(word);
        }
    }
}

Input: [foo, buzz, bar, fork, bort, spoon, !, dude]

Output:

ConcurrentModificationException on line 2:
java.util.ConcurrentModificationException
    at java.util.TreeMap$PrivateEntryIterator.nextEntry(TreeMap.java:1115)
    at java.util.TreeMap$KeyIterator.next(TreeMap.java:1169)
    at removeEvenLength (Line 2)

So I can solve it by creating an Iterator . But I want to know why the above code doesn't work?

EDIT:

Iterator doesn't work either:

public static void removeEvenLength(Set<String> set) {
    Iterator<String> i = set.iterator();
    while(i.hasNext()) {
        String word = i.next();
        if(word.length() % 2 == 0) {
            set.remove(word);
        }
    }
}

Same error.

Answer 1

In this iteration iterator object is implicitly created. When you have iterator you can change collection just from iterator. In this case you are removing object directly, that's why this exception is thrown.

Create iterator, and remove object with iterator:

iterator.remove(); // removes current element

Answer 2

To understand why ConcurrentModificationException occurs, you will have understand the concept of fail-fast iteration. If a thread is iterating over a collection, and it realizes that the collection is being modified as the iteration is going on, the iterator will throw an exception rather than "probably" cause any integrity problems later on in the code.

Of course, not all iterators follow this approach, and using the Java Iterator will almost always ensure that the iteration never fails on modification.

To remove an element using the iterator, use this code

Iterator<String> iter = list.iterator();
    while(iter.hasNext()) {
        String obj = iter.next();
        if(<removal_condition_here>) {
            iter.remove();
        }
    }