Question

I get Concurrent Modification Exception when i try to remove an item from HashMap. I know that removing items during iteration through HashMap will trigger this exception , but i need to replace older item with new one. How can i do this ? Maybe to create a copy of countNumberOfEachCharacter HashMap , and whe iterate through original HashMap to remove item from copy HashMap ?

countNumberOfEachCharacter = new HashMap<Character,Character>();
if (countNumberOfEachCharacter.containsKey(word.charAt(i))) {
    System.out.println("This character already exists");                     
    for (Iterator it = countNumberOfEachCharacter.entrySet().iterator(); it.hasNext();) {

      Map.Entry entry = (Map.Entry) it.next();

      Object key = entry.getKey();
      Object value = entry.getValue();

      if (key.equals(word.charAt(i))) { 

        int toIncrease = Integer.parseInt(value.toString());
        toIncrease++;

        System.out.println("key  "+key);                                                     
        System.out.println("increased  "+toIncrease);                                                       
        countNumberOfEachCharacter.remove(word.charAt(i));

        char c = Character.forDigit(toIncrease, 10);                                                     
        countNumberOfEachCharacter.put(word.charAt(i),c);                                                                                                                                                                               
    }                                                                                                                                                                                                           
  }                                                                           
}
else {    

   System.out.println("First time found this character");

   char c = Character.forDigit(1, 10);                                
   countNumberOfEachCharacter.put(word.charAt(i),c);                            
   System.out.println("Stored "+word.charAt(i)+" with count "+c);                                                               
}

Answer 1

…but i need to replace older item with new one

I take it from “replace” (and from the code you’ve quoted) that the key remains the same, it’s just the value that differs. If so, I don’t believe calling setValue on Map.Entry objects causes a ConcurrentModificationException, so you could do that.

Update: Just tested it, and indeed, it works:

import java.util.*;

public class ReplaceMapEntryValue {

    public static final void main(String[] args) {
        Map m;
        Iterator<Map.Entry> it;
        Map.Entry entry;

        // Create
        m = new HashMap();
        m.put("a", "alpha");
        m.put("b", "beta");

        // Update
        it = m.entrySet().iterator();
        while (it.hasNext()) {
            entry = it.next();
            if (entry.getKey() == "b") {
                entry.setValue("bravo");
            }
        }

        // Show
        it = m.entrySet().iterator();
        while (it.hasNext()) {
            entry = it.next();
            System.out.println("key = " + entry.getKey() + ", value = " + entry.getValue());
        }

        // Done
        System.exit(0);
    }
}