CODEWITHSUNDEEP
javaloopsforeachset
d-b
d-b

Reputation: 971

Not allowed to manipulate TreeSet in for-loop

I have a TreeSet that contains integers. I loop through this set and "consume" the integers one by one. During each loop I need to increase the remaining, uncomsumed integers with 1.

For example, I start with a set with four values: 2, 3, 5, 8. After the first loop where I consume 2 the result should be a set with this content 4, 6 , 9. After the second loop, where 4 is consumed, it should be 7, 10 and so on.

This is my code

    for (Integer i : positionSet) {
        TreeSet <Integer> tmpSet = new TreeSet<>(); 
        //use i for something
        //positionSet.remove(i); //I tried with this both on and off, but it made no difference
        for (Integer j : positionSet) {
            tmpSet.add(j + 1);
        }
        positionSet.clear();
        positionSet.addAll(tmpSet);
    }

It crashes on the second round with a java.util.ConcurrentModificationException, which I presume is caused by me modifying the set that is used in the loop-header.

How do I modify the contents of the set while looping over it? I tried copying the set back and forth in a couple of different ways but the code constantly fails with the same error message. I must not modify the set while looping, however modifying the set is the whole purpose of this loop.//

Upvotes: 2

Views: 259

Answers (1)

Mark Jeronimus
Mark Jeronimus

Reputation: 9541

You cannot modify a data structure during iteration, except for the allowed provisions. For an iterator this is only Iterator.remove(), but a for-each doesn't have such provision, nor can you affect other elements.

The best you can do is create a while loop that's independent of the data structure:

while (!positionSet.isEmpty()) {
    Integer i = <take one element out of positionSet somehow>;
    //positionSet.remove(i); //remove if the previous line didn't already remove it

    //use i for something
    TreeSet <Integer> tmpSet = new TreeSet<>();
    for (Integer j : positionSet) {
        tmpSet.add(j + 1);
    }
    positionSet.clear();
    positionSet.addAll(tmpSet);
}

Upvotes: 2

Related Questions