Exception in thread "main" java.util.ConcurrentModificationException

package test;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class Test {

    public static void main(String[] args) {

        List list = new ArrayList();
        list.add("1");
        list.add("2");
        list.add("3");

        Iterator iterator = list.iterator();
        while(iterator.hasNext()){

            Object next = iterator.next();
            if(next.equals("2")){
                list.add("5");
            }else{
                System.out.println(next);
            }
        }
        System.out.println(list);
    }

}