Question

I’m suffering on this exception. What’s the problem on my code?
I just want to separate Person’s duplicate name in another ArrayList

public class GlennTestMain
{

    static ArrayList<Person> ps;

    static ArrayList<Person> duplicates;
    public static void main(String[] args)
    {
        ps = new ArrayList<GlennTestMain.Person>();

        duplicates = new ArrayList<GlennTestMain.Person>();

        noDuplicate(new Person("Glenn", 123));
        noDuplicate(new Person("Glenn", 423));
        noDuplicate(new Person("Joe", 1423)); // error here


        System.out.println(ps.size());
        System.out.println(duplicates.size());
    }

    public static void noDuplicate(Person p1)
    {
        if(ps.size() != 0)
        {
            for(Person p : ps)
            {
                if(p.name.equals(p1.name))
                {
                    duplicates.add(p1);
                }
                else
                {
                    ps.add(p1);
                }
            }
        }
        else
        {
            ps.add(p1);
        }
    }

    static class Person
    {
        public Person(String n, int num)
        {
            this.name = n;
            this.age = num;
        }
        String name;
        int age;
    }



}

Here’s the stacktrace

Exception in thread "main" java.util.ConcurrentModificationException
at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at hk.com.GlennTestMain.noDuplicate(GlennTestMain.java:41)
at hk.com.GlennTestMain.main(GlennTestMain.java:30)

Answer 1

You cannot modify the collection you are iterating on. That might throw a ConcurrentModificationException. Though it might work sometimes, but it is not guaranteed to work everytime.

If you want to add, or remove something from your list, you need to use an Iterator, or ListIterator for your list. And use ListIterator#add method to add anything in your list. Even if in your iterator, if you try to use List.add or List.remove, you will get that exception, because that doesn’t make any difference. You should use the methods of iterator.

See these posts to understand how to use it: –

• Java : ConcurrentModificationException while iterating over list
• Iterating through a Collection, avoiding ConcurrentModificationException when removing in loop