HashMap vs ConcurrentHashMap – Example and exploring Iterator

If you are a Java Developer, I am sure that you must be aware of ConcurrentModificationException that comes when you want to modify the Collection object while using iterator to go through with all its element.

Java 1.5 has introduced java.util.concurrent package with Collection classes implementations that allow you to modify your collection object at runtime.

ConcurrentHashMap is the class that is similar to HashMap but works fine when you try to modify your map at runtime.

Lets run a sample program to explore this:

package com.journaldev.util;

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

public class ConcurrentHashMapExample {

	public static void main(String[] args) {

		//ConcurrentHashMap
		Map<String,String> myMap = new ConcurrentHashMap<String,String>();
		myMap.put("1", "1");
		myMap.put("2", "1");
		myMap.put("3", "1");
		myMap.put("4", "1");
		myMap.put("5", "1");
		myMap.put("6", "1");
		System.out.println("ConcurrentHashMap before iterator: "+myMap);
		Iterator<String> it = myMap.keySet().iterator();

		while(it.hasNext()){
			String key = it.next();
			if(key.equals("3")) myMap.put(key+"new", "new3");
		}
		System.out.println("ConcurrentHashMap after iterator: "+myMap);

		//HashMap
		myMap = new HashMap<String,String>();
		myMap.put("1", "1");
		myMap.put("2", "1");
		myMap.put("3", "1");
		myMap.put("4", "1");
		myMap.put("5", "1");
		myMap.put("6", "1");
		System.out.println("HashMap before iterator: "+myMap);
		Iterator<String> it1 = myMap.keySet().iterator();

		while(it1.hasNext()){
			String key = it1.next();
			if(key.equals("3")) myMap.put(key+"new", "new3");
		}
		System.out.println("HashMap after iterator: "+myMap);
	}

}

When we try to run the above class, output is

ConcurrentHashMap before iterator: {1=1, 5=1, 6=1, 3=1, 4=1, 2=1}
ConcurrentHashMap after iterator: {1=1, 3new=new3, 5=1, 6=1, 3=1, 4=1, 2=1}
HashMap before iterator: {3=1, 2=1, 1=1, 6=1, 5=1, 4=1}
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.test.ConcurrentHashMapExample.main(ConcurrentHashMapExample.java:44)

Looking at the output, its clear that ConcurrentHashMap takes care of any new entry in the map whereas HashMap throws ConcurrentModificationException.

Lets look at the exception stack trace closely. The statement that has thrown Exception is:

String key = it1.next();

It means that the new entry got inserted in the HashMap but Iterator is failing. Actually Iterator on Collection objects are fail-fast i.e any modification in the structure or the number of entry in the collection object will trigger this exception thrown by iterator.

So How does iterator knows that there has been some modification in the HashMap. We have taken the set of keys from HashMap once and then iterating over it.

HashMap contains a variable to count the number of modifications and iterator use it when you call its next() function to get the next entry.

/**
     * The number of times this HashMap has been structurally modified
     * Structural modifications are those that change the number of mappings in
     * the HashMap or otherwise modify its internal structure (e.g.,
     * rehash).  This field is used to make iterators on Collection-views of
     * the HashMap fail-fast.  (See ConcurrentModificationException).
     */
    transient volatile int modCount;

Now to prove above point, lets change the code a little bit to come out of the iterator loop when we insert the new extry. All we need to do is add a break statement after the put call.

if(key.equals("3")){
				myMap.put(key+"new", "new3");
				break;
			}

Now execute the modified code and the output will be:

ConcurrentHashMap before iterator: {1=1, 5=1, 6=1, 3=1, 4=1, 2=1}
ConcurrentHashMap after iterator: {1=1, 3new=new3, 5=1, 6=1, 3=1, 4=1, 2=1}
HashMap before iterator: {3=1, 2=1, 1=1, 6=1, 5=1, 4=1}
HashMap after iterator: {3=1, 2=1, 1=1, 3new=new3, 6=1, 5=1, 4=1}

Finally, what if we won’t add a new entry but update the existing key-value pair. Will it throw exception?

Change the code in the original program and check yourself.

//myMap.put(key+"new", "new3");
myMap.put(key, "new3");

If you get confused (or shocked) with the output, comment below and I will be happy to explain it further.

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