Question

Could you please tell is there any way in which concurrent modification exception could occur in a single threaded environment also the below application which I am posting consists of two threads , please tell me can I see the same exception in a single thread also..please advise

package concurrentmodificationError;

import java.util.*;

class ItrDemo
{
    public static void main(String arg[])
    {
       Vector v=new Vector();
       v.addElement("Amit");
       v.add("Rahul");
       v.add(1,"Nitin");
       v.addElement("Ankit");
       System.out.println("There are "+v.size()+"elements in the vector ");

       final Iterator itr=v.iterator();
       Thread th=new Thread()    {
               public void run()
               {
                   System.out.println("New Thread started,traversing     elements of vector...");
                   System.out.println("Contents of vector are...");
                   while(itr.hasNext())
                   {
                     System.out.println(itr.next());
                     try
                      {
                          Thread.sleep(2000);
                       }
                      catch(Exception e1)
                      {
                      }
                   }
              }
         };// end of annomyous class
      System.out.println("Suspending main thread and starting a new thread       for traversing the contents of vector...");
      th.start();
      try
       {
           Thread.sleep(1000);
       }
       catch(Exception e1)
       {
       }
       System.out.println("main thread resumed,modifying vector...");
       v.remove("Ankit");
       v.add("Puneet");
       v.add("Raman");
       System.out.println("Vector Modified , Ankit removed and Puneet &        Raman added.");
  }

}

yeah I got it folks that in the single threaded enviorement this error can comes.. as shown in below piece of code..

System.out.println("Content of list are : ");
          ListIterator itr1=list.listIterator();
          while(itr1.hasNext())
          {
              list.add(new Emp("Anand","Manager",56000)); //
            Emp e=(Emp)itr1.next();  
            e.display();
          }

Please advise what are the ways to overcome it..so that does not get this error..!!Please advise

Answer 1

A ConcurrentModificationException can be thrown in a single thread environment. It is used whenever an object is modified in a context where it shouldn’t, not necessarily in another thread.

Example:

public class CME {
    public static void main(String...args) {
        HashSet<Integer> coll = new HashSet<Integer>();
        coll.add(1);
        coll.add(2);
        coll.add(3);

        for(Integer i : coll) {
            coll.remove(i); // Throws ConcurrentModificationException
        }
    }
}