java.util.ConcurrentModificationException & iteration?

Question

I'm so very new to Arraylists & iterators & this is the first time I got this exception. I have an ArrayList u & I'd like to do the following algorithm:

for (Character c:u){

    if(k==1){           //base case

        if(isAnswer(s+u.get(0)))

            System.out.println(s+u.get(0)+" is the correct sequence."+ '\n');
        return;
    }

    else{
        u.remove(c);
        puzzleSolve(k-1, s+c , u);
        u.add(c);
        removeLastChar(s);
    }

    } //end of for each

as I searched this exception a little bit i found out I can't remove iterms weth for each on a arraylist & I need to use iterator but i kinna got confused where & how exactly i must put the while(iter.hasNext()) & such stuff for this piece of code. i would be more than grateful if you could help me

PS. s is String ( initially empty) & k is int

Answer 1

how exactly i must put the while(iter.hasNext())

You can use iterator as below:

Iterator<Character> iter = u.iterator();
while(iter.hasNext())
{
  Character c = iter.next();
  .....
}

Initialize your list with generics: List<Character> u = new ArrayList<Character>();

Hint: use iter.remove(), iter.add() wherever applicable instead of u.remove() and u.add().

You need to start here: http://www.tutorialspoint.com/java/java_using_iterator.htm

Answer 2

Try this:

Iterator<Character> iter = u.iterator();
while (iter.hasNext())
{
    Character currentChar = iter.next();

    if(k==1){           //base case

        if(isAnswer(s+u.get(0)))

        System.out.println(s+u.get(0)+" is the correct sequence."+ '\n');
        return;
    }

    else{
         iter.remove();
         puzzleSolve(k-1, s+currentChar  , u);
         u.add(currentChar);
         removeLastChar(s);
    }

}

Answer 3

When you are using foreach loop, you are using iterator implicitly.

ConcurrentModificationException occurs when the collection is modified by "simultaneously" with the passage of an iterator over a collection by any means, except for the iterator.

So, use iterator

Iterator<Character> iter = u.iterator();
while (iter.hasNext())

in cases you need to modify collection in loop.