Java - Exception in thread “main” java.util.ConcurrentModificationException

Question

Is there any way I can modify the HashMap values of a particular key while iterating over it?

A sample program is given below:

public st         


        

Answer 1

Although not related to the question, but just adding

ConcurrentModificationException can also occur if you get the iterator over a collection first and then add some more elements over it and then iterating over the collection will throw this exception.

For example :

package got;

import java.util.*;

public class GotCharacters {


    public static void main(String... args){

        Person p1 = new Person("TL", 40, "Tyrion Lannister");
        Person p2 = new Person("JM", 50, "Jorah Mormont");
        Person p3 = new Person("AS", 20, "Arya Stark");

        //Defining the collection and adding some elements
        ArrayList<Person> al;
        al = new ArrayList<Person>();
        al.add(p1);
        al.add(p2);
        al.add(p3);

        //Getting the iterator
        Iterator<Person> itr = al.iterator();

        Royalty r1 = new Student("DT", 25, "Daenerys Targaryen", "DragonMother", "Targaryen");
        Royalty r2 = new Student("JS", 28, "Jon Snow", "Lord Commander", "Targaryen");
        Collection<Royalty> c = new ArrayList<Royalty>();
        c.add(s1);
        c.add(s2);

        //Adding more elements after getting the iterator
        al.addAll(c);

        while(itr.hasNext()){
            System.out.print(itr.next());
        }
    }
}

Outcome :

Exception in thread "main" java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
    at java.util.ArrayList$Itr.next(ArrayList.java:851)
    at myArrayList.ArrayList1.main(ArrayList1.java:34)