Exception in thread main java.util.ConcurrentModificationException

Exception in thread main java.util.ConcurrentModificationException

Answer: If you are getting below exception:

Exception in thread "main" java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
	at java.util.HashMap$KeyIterator.next(HashMap.java:828)
	at com.javahonk.setTest.KeySetTest.main(KeySetTest.java:26)

 

It means you are trying to modify collection while iterating it. By design all collection classes in java.util.package are fail fast and we get exception.

To fix this issue you could do below:

  • Don’t try to modify direct collection itself use iterator remove method if you are removing element form collection. Please see example below:
package com.javahonk.iteratorenumeration;

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

public class AvoidCCMException {

	public static void main(String[] args) {

	List<String> arrayList = new ArrayList<String>();

	arrayList.add("Java");
	arrayList.add("Honk");
	arrayList.add("CCMTest");

	//avoid concurrent modification exception
	Iterator<String> iterator = arrayList.iterator();
	while (iterator.hasNext()) {
		String string = (String) iterator.next();
		System.out.println(string);
		if (string.equalsIgnoreCase("Honk")) {
			iterator.remove();
		}
	}

	System.out.println("\nbelow code will throw exception");
	//throw concurrent modification exception
	iterator = arrayList.iterator();
	arrayList.remove(0);
	while (iterator.hasNext()) {
		String string = (String) iterator.next();
		System.out.println(string);

	}

	}

}

 

Output:

Exception in thread main java.util.ConcurrentModificationException

 

If your requirement is different where iterator method does not support it then go for java.util.concurrent package classes to do modification work. In below example will show you how to use ConcurrentHashMap and CopyOnWriteArrayList class form java.util.concurrent package.

Modify map using ConcurrentHashMap:

package com.javahonk.setTest;

import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

public class ConcurrentHashMapTest {

	public static void main(String[] args) {

	Map<String,String> map = 
			new ConcurrentHashMap<String, String>();

	map.put("Java", "Java");
	map.put("Honk", "Honk");
	map.put("Test", "Test");

	Iterator<String> iterator = map.keySet().iterator();
	while(iterator.hasNext()) {
		String key = (String)iterator.next();
		String val = (String)map.get(key);
	    map.put("New Value", "New Value");
	    System.out.println("Key:" + key + "  Value: " + val);		    
	}

	}

}

Output:
Exception in thread main java.util.ConcurrentModificationException

 

 

Example: Modify list using CopyOnWriteArrayList:

package com.javahonk.setTest;

import java.util.Iterator;
import java.util.concurrent.CopyOnWriteArrayList;

public class CopyOnWriteArrayListTest {

	public static void main(String[] args) {
	CopyOnWriteArrayList<String> coArrayList = 
			new CopyOnWriteArrayList<String>();

	coArrayList.add("Java");
	coArrayList.add("Honk");
	coArrayList.add("Test");

	//remove element while iterating it
	System.out.println("Origional list: \n");
	for (Iterator<String> iterator = 
			coArrayList.iterator(); iterator.hasNext();) {
		String string = (String) iterator.next();
		coArrayList.remove("Test");
		System.out.println(string);

	}

	//Print modifed list
	System.out.println("\nModified list: \n");
	for (String string : coArrayList) {
		System.out.println(string);
	}

	}

}

 

Output:
Exception in thread main java.util.ConcurrentModificationException

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