Thread qs

http://www.journaldev.com/1162/java-multi-threading-concurrency-interview-questions-with-answers

Setup-and-gotchas-of-the-hibernate-second-level-and-query-caches

http://blog.jhades.org/setup-and-gotchas-of-the-hibernate-second-level-and-query-caches/

concurrent hashmap

http://javarevisited.blogspot.sg/2011/04/difference-between-concurrenthashmap.html

How to use ConcurrentHashMap in Java - Example Tutorial and Working

ConcurrentHashMap in Java is introduced as an alternative of Hashtable in Java 1.5 as part of Java concurrency package. Prior to Java 1.5 if you need a Map implementation, which can be safely used in a concurrent and multi-threaded Java program, than, you only have Hashtable or synchronized Map because HashMap is not thread-safe. WithConcurrentHashMap, now you have better choice; because, not only it can be safely used in concurrent multi-threaded environment but also provides better performance over Hashtable and synchronizedMap. ConcurrentHashMap performs better than earlier two because it only locks a portion of Map, instead of whole Map, which is the case with Hashtable and synchronized Map. CHM allows concurred read operations and same time, maintains integrity by synchronizing write operations. We have seen basics of ConcurrentHashMap on Top 5 Java Concurrent Collections from JDK 5 and 6 and in this Java tutorial, we will learn :

Ø       How ConcurrentHashMap works in Java or how it is implemented in Java.

Ø       When to use ConcurrentHashMap in Java

Ø       ConcurrentHashMap examples in Java

Ø       And some important properties of CHM .

How ConcurrentHashMap is implemented in Java

ConcurrentHashMap is introduced as an alternative of Hashtable and provided all functions supported by Hashtable with additional feature called "concurrency level", which allows ConcurrentHashMap to partition Map. ConcurrentHashMap allows multiple readers to read concurrently without any blocking. This is achieved by partitioning Map into different parts based on concurrency level and locking only a portion of Map during updates. Default concurrency level is 16, and accordingly Map is divided into 16 part and each part is governed with different lock. This means, 16 thread can operate on Map simultaneously, until they are operating on different part of Map. This makes ConcurrentHashMap high performance despite keeping thread-safety intact.  Though, it comes with caveat. Since update operations like put(), remove(),putAll() or clear() is not synchronized, concurrent retrieval may not reflect most recent change on Map.

In case of putAll() or clear(), which operates on whole Map, concurrent read may reflect insertion and removal of only some entries. Another important point to remember is iteration over CHM, Iterator returned by keySet of ConcurrentHashMap are weekly consistent and they only reflect state of ConcurrentHashMap and certain point and may not reflect any recent change. Iterator of ConcurrentHashMap's keySetarea also fail-safe and doesn’t throw ConcurrentModificationExceptoin..

Default concurrency level is 16 and can be changed, by providing a number which make sense and work for you while creating ConcurrentHashMap. Since concurrency level is used for internal sizing and indicate number of concurrent update without contention, so, if you just have few writers or thread to update Map keeping it low is much better. ConcurrentHashMap also uses ReentrantLock to internally lock its segments.

ConcurrentHashMap putifAbsent example in Java

ConcurrentHashMap examples are similar to Hashtable examples, we have seen earlier,  but worth knowing is use of putIfAbsent() method. Many times we need to insert entry into Map, if its not present already, and we wrote following kind of code:

synchronized(map){

  if (map.get(key) == null){

      return map.put(key, value);

  } else{

      return map.get(key);

  }

}

Though this code will work fine in HashMap and Hashtable, This won't work in ConcurrentHashMap; because, during put operation whole map is not locked, and while one thread is putting value, other thread's get() call can still return null which result in one thread overriding value inserted by other thread. Ofcourse, you can wrap whole code in synchronized block and make it thread-safe but that will only make your code single threaded. ConcurrentHashMap provides putIfAbsent(key, value) which does same thing but atomically and thus eliminates above race condition.

When to use ConcurrentHashMap in Java

ConcurrentHashMap is best suited when you have multiple readers and few writers. If writers outnumber reader, or writer is equal to reader, than performance of ConcurrentHashMap effectively reduces to synchronized map or Hashtable. Performance of CHM drops, because you got to lock all portion of Map, and effectively each reader will wait for another writer, operating on that portion of Map. ConcurrentHashMap is a good choice for caches, which can be initialized during application start up and later accessed my many request processing threads. As javadoc states, CHM is also a good replacement of Hashtable and should be used whenever possible, keeping in mind, that CHM provides slightly weeker form of synchronization than Hashtable.

Summary

Now we know What is ConcurrentHashMap in Java and when to use ConcurrentHashMap, it’s time to know and revise some important points about CHM in Java.

1. ConcurrentHashMap allows concurrent read and thread-safe update operation.

2. During update operation, ConcurrentHashMap only lock a portion of Map instead of whole Map.

3. Concurrent update is achieved by internally dividing Map into small portion which is defined by concurrency level.

4. Choose concurrency level carefully as a significant higher number can be waste of time and space and lower number may introduce thread contention in case writers over number concurrency level.

5. All operations of ConcurrentHashMap are thread-safe.

6. Since ConcurrentHashMap implementation doesn't lock whole Map, there is chance of read overlapping with update operations like put() and remove(). In that case result returned by get() method will reflect most recently completed operation from there start.

7. Iterator returned by ConcurrentHashMap is weekly consistent, fail safe and never throw ConcurrentModificationException. In Java.

8. ConcurrentHashMap doesn't allow null as key or value.

9. You can use ConcurrentHashMap in place of Hashtable but with caution as CHM doesn't lock whole Map.

10. During putAll() and clear() operations, concurrent read may only reflect insertion or deletion of some entries.

That’s all on What is ConcurrentHashMap in Java and when to use it. We have also seen little bit about internal working of ConcurrentHashMap and how it achieves it’s thread-safety and better performance over Hashtable and synchronized Map. Use ConcurrentHashMap in Java program, when there will be more reader than writers and it’s a good choice for creating cache in Java as well.

Related Java Concurrency and Collection Tutorials from this blog

How HashMap works in Java

Difference between HashMap and HashSet in Java

Difference between Synchronized Collection and Concurrent Collection in Java

When to use CopyOnWriteArrayList in Java

How to solve producer consumer problem using BlockingQueue in Java

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Read more: http://javarevisited.blogspot.com/2013/02/concurrenthashmap-in-java-example-tutorial-working.html#ixzz3gj5h1tdv

How HashMap works in Java

http://javarevisited.blogspot.sg/2011/04/difference-between-concurrenthashmap.html


HashMap in Java works on hashing principle. It is a data structure which allows us to store object and retrieve it in constant time O(1) provided we know the key. In hashing, hash functions are used to link key and value in HashMap. Objects are stored by calling put(key, value) method of HashMap and retrieved by calling get(key) method. When we call put method, hashcode() method of key object is called so that hash function of map can find a bucket location to store value object, which is actually index of internal array, known as table. HashMap internally store mapping in form of Map.Entry object which contains both key and value object. When you want to retrieve the object, you call get() method and again pass key object. This time again key object generate same hash code (it's mandatory for it to do so to retrieve object and that's why HashMap keys are immutable e.g. String) and we end up at same bucket location. If there is only one object then it is returned and that's your value object which you have stored earlier. Things get little tricky when collisions occurs. Since internal array of HashMap is of fixed size, and if you keep storing objects, at some point of time hash function will return same bucket location for two different keys, this is called collision in HashMap. In this case, a linked list is formed at that bucket location and new entry is stored as next node. If we try to retrieve object from this linked list, we need an extra check to search correct value, this is done by equals() method. Since each node contains an entry, HashMap keep comparing entry's key object with passed key using equals() and when it return true, Map returns corresponding value. Since searching in lined list is O(n) operation, in worst case hash collision reduce a map to linked list. This issue is recently addressed in Java 8 by replacing linked list to tree to search in O(logN) time. By the way, you can easily verify how HashMap work by looking at code of HashMap.java in your Eclipse IDE, if you know how to attach source code of JDK in Eclipse.


How HashMap works in Java or sometime how get method work in HashMap is a very common question on Java interviews now days. Almost everybody who worked in Java knows about HashMap, where to use HashMap and difference between Hashtable and HashMap then why this interview question becomes so special? Because of the depth it offers. It has become very popular Java interview question in almost any senior or mid-senior level Java interviews. Investment banks mostly prefer to ask this question and some time even ask you to implement your own HashMap based upon your coding aptitude. Introduction of ConcurrentHashMap and other concurrent collections has also made this questions as starting point to delve into more advanced feature. let's start the journey.

How HashMap Internally Works in Java

Questions start with simple statement :

Have you used HashMap before or  What is HashMap? Why do you use it 

Almost everybody answers this with yes and then interviewee keep talking about common facts about HashMap like HashMap accept null while Hashtable doesn't, HashMap is not synchronized, HashMap is fast and so on along with basics like its stores key and value pairs etc. This shows that person has used HashMap and quite familiar with the functionality it offers, but interview takes a sharp turn from here and next set of follow-up questions gets more detailed about fundamentals involved with HashMap in Java . Interviewer strike back with questions like :

Do you Know how HashMap works in Java or How does get () method of HashMap works in Java

And then you get answers like,  I don't bother its standard Java API, you better look code on Java source or Open JDK; I can find it out in Google at any time etc. But some interviewee definitely answer this and will say HashMap works on principle of hashing, we have put(key, value) andget(key) method for storing and retrieving Objects from HashMap. When we pass Key and Value object  to put() method on Java HashMap, HashMap implementation calls hashCode method on Key object and applies returned hashcode into its own hashing function to find a bucket location for storing Entry object, important point to mention is that HashMap in Java stores both key and value object as Map.Entry in bucket which is essential to understand the retrieving logic. If people fails to recognize this and say it only stores Value in the bucket they will fail to explain the retrieving logic of any object stored in Java HashMap . This answer is very much acceptable and does make sense that interviewee has fair bit of knowledge on how hashing works and how HashMap  works in Java. But this is just start of story and confusion increases when you put interviewee on scenarios faced by Java developers on day by day basis. Next question could be about collision detection and collision resolution in Java HashMap  e.g. 

What will happen if two different objects have same hashcode?

Now from here onwards real confusion starts, Some time candidate will say that since hashcode is equal, both objects are equal and HashMap  will throw exception or not store them again etc, Then you might want to remind them about equals() and hashCode() contract  that two unequal object in Java can have same hash code. Some will give up at this point and few will move ahead and say "Since hashcode is same, bucket location would be same and collision will occur in HashMap, Since HashMap use LinkedList to store object, this entry (object of Map.Entry comprise key and value )  will be stored in LinkedList. Great this answer make sense though there are many collision resolution methods available  like linear probing and chaining, this is simplest and HashMap in Java does follow this. But story does not end here and interviewer asks

How will you retrieve Value object  if two Keys will have same hashcode?

Interviewee will say we will call get() method and then HashMap uses Key Object's hashcode to find out bucket location and retrieves Value object but then you need to remind him that there are two Value objects are stored in same bucket , so they will say about traversal in LinkedList until we find the value object , then you ask how do you identify value object because you don't  have value object to compare ,Until they know that HashMap  stores both Key and Value in LinkedList node or as Map.Entry they won't be able to resolve this issue and will try and fail.

But those bunch of people who remember this key information will say that after finding bucket location , we will call keys.equals() method to identify correct node in LinkedList and return associated value object for that key in Java HashMap . Perfect this is the correct answer.

In many cases interviewee fails at this stage because they get confused between hashCode() and equals() or keys and values object in Java HashMap  which is pretty obvious because they are dealing with the hashcode() in all previous questions and equals() come in picture only in case of retrieving value object from HashMap in Java. Some good developer point out here that using immutable, final object with proper equals() and hashcode() implementation would act as perfect Java HashMap  keys and improve performance of Java HashMap  by reducing collision. Immutability also allows caching there hashcode of different keys which makes overall retrieval process very fast and suggest that String and various wrapper classes e.g. Integer very good keys in Java HashMap.

Now if you clear this entire Java HashMap interview,  You will be surprised by this very interesting question "What happens On HashMap in Java if the size of the HashMap  exceeds a given threshold defined by load factor ?". Until you know how HashMap  works exactly you won't be able to answer this question. If the size of the Map exceeds a given threshold defined by load-factor e.g. if load factor is .75 it will act to re-size the map once it filled 75%. Similar to other collection classes like ArrayList,  Java HashMap re-size itself by creating a new bucket array of size twice of previous size of HashMap , and then start putting every old element into that new bucket array. This process is called rehashing because it also applies hash function to find new bucket location. 

If you manage to answer this question on HashMap in Java you will be greeted by "do you see any problem with resizing of HashMap  in Java" , you might not be able to pick the context and then he will try to give you hint about multiple thread accessing the Java HashMap and potentially looking for race condition on HashMap  in Java. 

So the answer is Yes there is potential race condition exists while resizing HashMap in Java, if two thread at the same time found that now HashMap needs resizing and they both try to resizing. on the process of resizing of HashMap in Java , the element in bucket which is stored in linked list get reversed in order during there migration to new bucket because Java HashMap  doesn't append the new element at tail instead it append new element at head to avoid tail traversing. If race condition happens then you will end up with an infinite loop. Though this point you can potentially argue that what the hell makes you think to use HashMap  in multi-threaded environment to interviewer :)

Some more Hashtable and HashMap Questions 

Few more question on HashMap in Java which is contributed by readers of Javarevisited blog  :

1) Why String, Integer and other wrapper classes are considered good keys ?

String, Integer and other wrapper classes are natural candidates of HashMap key, and String is most frequently used key as well because String is immutable and final,and overrides equals and hashcode() method. Other wrapper class also shares similar property. Immutabiility is required, in order to prevent changes on fields used to calculate hashCode() because if key object return different hashCode during insertion and retrieval than it won't be possible to get object from HashMap. Immutability is best as it offers other advantages as well like thread-safety, If you can  keep your hashCode same by only making certain fields final, then you go for that as well. Since equals() and hashCode() method is used during reterival of value object from HashMap, its important that key object correctly override these methods and follow contact. If unequal object return different hashcode than chances of collision will be less which subsequently improve performance of HashMap.

2) Can we use any custom object as key in HashMap ?

This is an extension of previous questions. Ofcourse you can use any Object as key in Java HashMap provided it follows equals and hashCode contract and its hashCode should not vary once the object is inserted into Map. If custom object is Immutable than this will be already taken care because you can not change it once created.

3) Can we use ConcurrentHashMap in place of Hashtable ?

This is another question which getting popular due to increasing popularity of ConcurrentHashMap. Since we know Hashtable is synchronized but ConcurrentHashMap provides better concurrency by only locking portion of map determined by concurrency level. ConcurrentHashMap is certainly introduced as Hashtable and can be used in place of it but Hashtable provide stronger thread-safety than ConcurrentHashMap. See my post difference between Hashtable and ConcurrentHashMap for more details.

Personally, I like this question because of its depth and number of concept it touches indirectly, if you look at questions asked during interview this HashMap  questions has verified

• Concept of hashing
• Collision resolution in HashMap
• Use of equals () and hashCode () and there importance in HashMap?
• Benefit of immutable object?
• Race condition on HashMap  in Java
• Resizing of Java HashMap

Just to summarize here are the answers which does makes sense for above questions

How HashMap  works in Java

HashMap  works on principle of hashing, we have put() and get() method for storing and retrieving object form HashMap .When we pass an both key and value to put() method to store on HashMap , it uses key object hashcode() method to calculate hashcode and they by applying hashing on that hashcode it identifies bucket location for storing value object. While retrieving it uses key object equals method to find out correct key value pair and return value object associated with that key. HashMap  uses linked list in case of collision and object will be stored in next node of linked list. AlsoHashMap stores both key and value tuple in every node of linked list in form of Map.Entry object. 

What will happen if two different HashMap  key objects have same hashcode?

They will be stored in same bucket but no next node of linked list. And keys equals () method will be used to identify correct key value pair in HashMap .

How null key is handled in HashMap? Since equals() and hashCode() are used to store and retrieve values, how does it work in case of null key?
Null key is handled specially in HashMap, there are two separate method for that putForNullKey(V value) and getForNullKey(). Later is offloaded version of get() to look up null keys.  Null keys always map to index 0.  This null case is split out into separate methods for the sake of performance in the two most commonly used operations (get and put), but incorporated with conditionals in others. In short, equals() and hashcode() method are not used in case of null keys in HashMap.

here is how nulls are retreived from HashMap

   private V getForNullKey() {

        if (size == 0) {
            return null;
        }
        for (Entry<K,V> e = table[0]; e != null; e = e.next) {
            if (e.key == null)
                return e.value;
        }
        return null;
    }

In terms of usage Java HashMap is very versatile and I have mostly used HashMap as cache in electronic trading application I have worked . Since finance domain used Java heavily and due to performance reason we need caching HashMap and ConcurrentHashMap  comes as very handy there. You can also check following articles form Javarevisited to learn more about HashMap and Hashtable in Java :

HashMap Changes in JDK 1.7 and JDK 1.8

There is some performance improvement done on HashMap and ArrayList from JDK 1.7, which reduce memory consumption. Due to this empty Map are lazily initialized and will cost you less memory. Earlier, when you create HashMap e.g. new HashMap() it automatically creates array of default length e.g. 16. After some research, Java team founds that most of this Map are temporary and never use that many elements, and only end up wasting memory. Also, From JDK 1.8 onwards HashMap has introduced an improved strategy to deal with high collision rate. Since a poor hash function e.g. which always return location of same bucket, can turn a HashMap into linked list, i.e. converting get() method to perform in O(n) instead of O(1) and someone can take advantage of this fact, Java now internally replace linked list to a binary true once certain threshold is breached. This ensures performance or order O(log(n)) even in worst case where hash function is not distributing keys properly.

Related post:

Difference between Hashtable and HashMap in Java

Difference between SynchrnizedHashMap  and ConcurrentHashMap in Java

10 examples of Hashtable in Java

How get method of Hashtable internally works in Java
How to sort HashMap in Java by values

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• What is Iterator and ListIterator in Java Program with Example - Tutorial Code Sample
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Read more: http://javarevisited.blogspot.com/2011/02/how-hashmap-works-in-java.html#ixzz3gj3squhQ