java - How Iterator's remove method actually remove an object

Question

We all know that the safest "and probably only safe" way of removing an object from a collection while iterating it, is by first retrieving the Iterator, perform a loop and remove when needed;

Iterator iter=Collection.iterator();
while(iter.hasNext()){
    Object o=iter.next()
    if(o.equals(what i'm looking for)){
        iter.remove();
    }
}

What I would like to understand, and unfortunately haven't found a deep technical explanation about, is how this removal is performed,
If:

for(Object o:myCollection().getObjects()){
    if(o.equals(what i'm looking for)){
        myCollection.remove(o);
    }
}

Will throw a ConcurrentModificationException, what does "in technical terms" Iterator.remove() do? Does it removes the object, breaks the loop and restart the loop?

I see in the official documentation:

"Removes the current element. Throws IllegalStateException if an attempt is made to call remove() that is not preceded by a call to next( )."

The part "removes the current element", makes me think of the exact same situation happening in a "regular" loop => (perform equality test and remove if needed), but why is the Iterator loop ConcurrentModification-safe?

Answer 1

How exactly Iterator removes elements depends on its implementation, which may be different for different Collections. Definitely it doesn't break the loop you're in. I've just looked how ArrayList iterator is implemented and here's the code:

public void remove() {
    if (lastRet < 0)
        throw new IllegalStateException();
    checkForComodification();

    try {
        ArrayList.this.remove(lastRet);
        cursor = lastRet;
        lastRet = -1;
        expectedModCount = modCount;
    } catch (IndexOutOfBoundsException ex) {
        throw new ConcurrentModificationException();
    }
}

So it checks for concurrent modifications, removes element using public ArrayList remove method, and increments counter of list modifications so ConcurrentModificationException won't be thrown at next iteration.