The below mentioned  stand alone java applications namely TestConcurrentModificationException and EscapeConcurrentModificationException  demonstrate the cases when java.util.ConcurrentModificationException exception is thrown and when not. And how to fix it.

import java.util.Iterator;

import java.util.LinkedHashMap;

/**

*

* @author shyam

*/

public class TestConcurrentModificationException {

/**

* @param args the command line arguments

*/

public static void main(String[] args) throws Exception{

LinkedHashMap<string,String> processedMap=new LinkedHashMap<string,String>();

System.out.println("processedMap.size() Before population ->"+processedMap.size());

for(int i=0;i<1000;i++){

processedMap.put(i+"",i+"");

}

System.out.println("processedMap.size() After population ->"+processedMap.size());

//Incorrect use

Iterator<string> processedItr = processedMap.keySet().iterator();

while(processedItr.hasNext()){

String key=processedItr.next();

//below mentioned case is valid, which resets the value to an existing key.

processedMap.put(key,"any value");

//below mentioned cases cause java.util.ConcurrentModificationException

processedMap.remove(key);

processedMap.put("newkey","any value");

}

System.out.println("processedMap.size() After processedMap.remove() ->"+processedMap.size());

}

}

If u need to iterate through and modify a collection then use the following program which escapes from java.util.ConcurrentModificationException with a newly constructed replica of the original collection to iterate through and modifies(remove/put a new key value mapping) the original collection, when needed.

import java.util.Iterator;

import java.util.LinkedHashMap;

/**

*

* @author shyam

*/

public class EscapeConcurrentModificationException {

/**

* @param args the command line arguments

*/

public static void main(String[] args) throws Exception{

LinkedHashMap<string,String> processedMap=new LinkedHashMap<string,String>();

System.out.println("processedMap.size() Before population ->"+processedMap.size());

for(int i=0;i<1000;i++){

processedMap.put(i+"",i+"");

}

System.out.println("processedMap.size() After population ->"+processedMap.size());

//correct use

LinkedHashMap<string,String> processedMapReplica=new LinkedHashMap<string,String>(processedMap);

Iterator<string> processedItr = processedMapReplica.keySet().iterator();

while(processedItr.hasNext()){

String key=processedItr.next();

//below mentioned cases cause java.util.ConcurrentModificationException

processedMap.remove(key);

processedMap.put("newkey","any value");

//below mentioned case is valid, which resets the value to an existing key.

processedMap.put(key,"any value");

}

System.out.println("processedMap.size() After processedMap.remove() ->"+processedMap.size());

}

}

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-Shyam.