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

This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.

Below peice of code is causing the problem.

for(String s:hm.get(1)){
        hm.get(1).add("hello");
    }

You are iterating and modifying the same. Avoid this by creating new ArrayList

  ArrayList ar1 = new ArrayList();

for (String s : hm.get(1)) {
            ar1.add("hello");
        }

have a read here