Concurrent modification Exception while iterating over ArrayList

Question

The sort method results in a concurrent modification error when i use temp = iterator.next(). Can you please help me resolve the concurrent modification error. I gave the code for the entire class but I'm only trying complete the sort method. Thanks in advance for your help.

I have to sort all the the arrays in the arraylist.

    package HashSet;

            import java.io.InputStream;
            import java.util.ArrayList;
            import java.util.Collections;
            import java.util.ListIterator;
        import java.util.Scanner;

        public class PhoneBook {
            int capacity = 10;
            private ArrayList<PhoneBookEntry>[] buckets;

            public PhoneBook() {
                this(10);
                load();
            }

            public PhoneBook(int size) {
                capacity = size;
                buckets = new ArrayList[size];
                for (int i = 0; i < buckets.length; i++)
                    buckets[i] = new ArrayList<PhoneBookEntry>();
            }

            public int getSize() {
                int tot = 0;
                for (ArrayList<PhoneBookEntry> x : buckets)
                    tot += x.size();
                return tot;
            }

            public boolean add(PhoneBookEntry entry) {
                if (contains(entry))
                    return false;
                int x = Math.abs(entry.hashCode());
                buckets[x % buckets.length].add(entry);
                return true;
            }

            public void load() {
                InputStream is = getClass().getClassLoader().getResourceAsStream(
                        "phone.txt");
                Scanner scan = new Scanner(is);
                while (scan.hasNext())
                    add(new PhoneBookEntry(scan.next(), scan.nextInt()));
                scan.close();
            }

            public void bucketSize() {
                for (int i = 0; i < buckets.length; i++)
                    System.out.println(i + "    " + buckets[i].size());
            }

            public boolean contains(PhoneBookEntry word) {
                int x = Math.abs(word.hashCode());
                return buckets[x % buckets.length].contains(word);
            }

            public int getCapacity() {
                return capacity;
            }

            public int getLongestList() {
                int numb = 0;
                for (ArrayList<PhoneBookEntry> x : buckets)
                    if (x.size() > numb)
                        numb = x.size();
                return numb;
            }

            public void display() {
                for (ArrayList<PhoneBookEntry> x : buckets)
                    System.out.println(x);
            }

            public int getNumberOfNulls() {
                int numb = 0;
                for (ArrayList<PhoneBookEntry> x : buckets)
                    if (x.size() == 0)
                        numb++;
                return numb;
            }

            public String lookup(String name) {
                String numb = name + "'s number not found";
                for (ArrayList<PhoneBookEntry> x : buckets)
                    for (int i = 0; i < x.size(); i++)
                        if (x.get(i).getN().equals(name))
                            numb = name + "'s" + " number is " + x.get(i).getNr();
                return numb;
            }

            public int internalLookUp(String name) {
                int numb = 0;
                for (ArrayList<PhoneBookEntry> x : buckets)
                    for (int i = 0; i < x.size(); i++)
                        if (x.get(i).getN().equals(name))
                            numb = x.get(i).getNr();
                return numb;
            }

            public void sort() {
                String temp = "";
                ArrayList<String> list = new ArrayList<String>();
                ListIterator<String> iterator = list.listIterator();
                final ArrayList<PhoneBookEntry>[] data = buckets.clone();
                for (ArrayList<PhoneBookEntry> x : buckets) {
                    for (int i = 0; i < x.size(); i++) {
                        list.add(x.get(i).getN());
                    }
                    Collections.sort(list);
                    for (int b = 0; b < x.size(); b++) {
                        temp = iterator.next(); //error line
                        x.get(b).setN(temp);
                        x.get(b).setNr(internalLookUp(temp));
                    }
                }
            }

            public static void main(String[] args) {
                PhoneBook phone = new PhoneBook();
                phone.display();
                System.out.println();
                System.out.println("Capacity is " + phone.getCapacity());
                System.out.println();
                System.out.println("Size is " + phone.getSize());
                System.out.println();
                System.out.println("Get longest list " + phone.getLongestList());
                System.out.println();
                System.out.println("Number of Nulls " + phone.getNumberOfNulls());
                System.out.println();
                System.out.println(phone.lookup("Fish"));
                phone.sort();
            }
        }

Answer 1

It looks like your problem is that you're creating an iterator for a list, then modifying the list (adding + sorting), then attempting to use the iterator.

If you instead created the iterator after this, it should work.

eg.

            for (ArrayList<PhoneBookEntry> x : buckets) {
                for (int i = 0; i < x.size(); i++) {
                    list.add(x.get(i).getN());
                }
                Collections.sort(list);
                ListIterator<String> iterator = list.listIterator(); // Iterator created here
                for (int b = 0; b < x.size(); b++) {
                    temp = iterator.next(); //error line
                    x.get(b).setN(temp);
                    x.get(b).setNr(internalLookUp(temp));
                }

Answer 2

The bug is that you iterate over list while you add to it. you should not do that.

Answer 3

Here is the javadoc for the ConcurrentModificationException .

http://docs.oracle.com/javase/7/docs/api/java/util/ConcurrentModificationException.html

if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception.

As other posters have said, you should do all sorting and adding before or after using your iterator. You can remove an element during iteration, but you must use Iterator.remove() to prevent getting the same exception again.