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How Hashmap Industrial Plant Inwards Java

HashMap inwards Java plant on hashing principle. It is a information construction which allows us to shop object too retrieve it inwards constant fourth dimension O(1) provided nosotros know the key. In hashing, hash functions are used to link key too value inwards HashMap. Objects are stored past times calling put(key, value) method of HashMap too retrieved past times calling get(key) method. When nosotros telephone phone set method, hashcode() method of the key object is called hence that hash business office of the map tin break a bucket place to shop value object, which is genuinely an index of the internal array, known every bit the table. HashMap internally stores mapping inwards the shape of Map.Entry object which contains both key too value object. When you lot desire to retrieve the object, you lot telephone phone the get() method too ane time again give the key object. This fourth dimension ane time again key object generate same hash code (it's mandatory for it to practice hence to retrieve the object too that's why HashMap keys are immutable e.g. String) too nosotros destination upwards at same bucket location. If at that topographic point is alone ane object too hence it is returned too that's your value object which you lot lead hold stored earlier. Things acquire niggling tricky when collisions occur. It's tardily to reply this enquiry if you lot lead hold read expert mass or course of report on information construction too algorithms like how to attach origin code of JDK inwards Eclipse.


How HashMap plant inwards Java or sometimes how does acquire method piece of work inwards HashMap is a real mutual enquiry on Java interviews nowadays. Almost everybody who worked inwards Java knows nigh HashMap, where to work HashMap too divergence betwixt Hashtable too HashMap too hence why this interview enquiry becomes hence special? Because of the depth it offers.

It has choke real popular Java interview question inwards almost whatever senior or mid-senior marking Java interviews. Investment banks to a greater extent than often than non prefer to inquire this enquiry too sometimes fifty-fifty inquire you lot to implement your ain HashMap based upon your coding aptitude. The introduction of ConcurrentHashMap too other concurrent collections has likewise made this questions every bit starting indicate to delve into a to a greater extent than advanced feature. let's get-go the journey.

How HashMap Internally Works inwards Java

Questions get-go amongst uncomplicated statement:

Have you lot used HashMap before or  What is HashMap? Why practice you lot work it 

Almost everybody answers this amongst yep too and hence interviewee choke along talking nigh mutual facts nigh HashMap similar HashMap lead hold zippo patch Hashtable doesn't, HashMap is non synchronized, HashMap is fast too hence on along amongst basics similar its stores key too value pairs etc. This shows that someone has used HashMap and quite familiar amongst the functionality it offers, but interview takes a precipitous plough from hither too side past times side laid of follow-up questions gets to a greater extent than detailed nigh fundamentals involved amongst HashMap inwards Java. Interviewer strike dorsum amongst questions like:

Do you lot Know how HashMap plant inwards Java or How does acquire () method of HashMap plant inwards Java

And too hence you lot acquire answers like,  I don't bother its measure Java API, you lot amend hold back code on Java origin or Open JDK; I tin break it out inwards Google at whatever fourth dimension etc. But some interviewee definitely answers this too volition say HashMap plant on the regulation of hashing, nosotros lead hold put(key, value) too get(key) method for storing too retrieving Objects from HashMap. When nosotros give Key too Value object  to put() method on Java HashMap, HashMap implementation calls hashCode method on Key object too applies returned hashcode into its ain hashing business office to break a bucket place for storing Entry object, of import indicate to cite is that HashMap inwards Java stores both key too value object every bit Map.Entry inwards a bucket which is essential to empathize the retrieving logic. 

If people neglect to recognize this too say it alone stores Value inwards the bucket they volition neglect to explicate the retrieving logic of whatever object stored inwards Java HashMap. This reply is real much acceptable too does brand feel that interviewee has a fair fleck of noesis on how hashing plant too how HashMap  works inwards Java. But this is merely get-go of story too confusion increases when you lot set interviewee on scenarios faced past times Java developers on hateful solar daytime past times hateful solar daytime basis. Next enquiry could survive nigh collision detection too collision resolution inwards Java HashMap  e.g. 

What volition hap if 2 dissimilar objects lead hold the same hashcode?

Now from hither onwards existent confusion starts, sometime candidate volition say that since hashcode is equal, both objects are equal too HashMap  will throw exception or non shop them ane time again etc, Then you lot mightiness desire to remind them about equals() too hashCode() contract  that 2 unequal objects inwards Java tin lead hold same hashcode. Some volition surrender at this indicate too few volition movement ahead too say "Since hashcode is same, bucket place would survive same too collision volition occur inwards HashMap Since HashMap uses LinkedList to shop object, this entry (object of Map.Entry comprise key too value )  volition survive stored inwards LinkedList. Great this reply brand feel though at that topographic point are many collision resolution methods available  like linear probing and chaining, this is simplest too HashMap inwards Java does follow this. But story does non destination hither too interviewer asks

How volition you lot retrieve Value object  if 2 Keys volition lead hold the same hashcode?

traversal inwards LinkedList until nosotros break the value object , too hence you lot inquire how practice you lot position value object because you lot don't  lead hold value object to compare ,Until they know that HashMap  stores both Key too Value inwards LinkedList node or every bit Map.Entry they won't survive able to resolve this number too volition elbow grease too fail.

But those bunch of people who retrieve this key information volition say that after finding bucket location, nosotros volition call keys.equals() method to position a right node inwards LinkedList too render associated value object for that key inwards Java HashMap. Perfect this is the right answer.

In many cases interviewee fails at this phase because they acquire confused between hashCode() too equals() or keys too values object inwards Java HashMap  which is pretty obvious because they are dealing amongst the hashcode() in all previous questions too equals() come inwards film alone inwards instance of retrieving value object from HashMap inwards Java. Some expert developer indicate out hither that using immutable, final object amongst proper equals() and hashcode() implementation would deed every bit perfect Java HashMap  keys and improve the performance of Java HashMap  by reducing collision. Immutability likewise allows caching their hashcode of dissimilar keys which makes overall retrieval procedure real fast too advise that String and diverse wrapper classes e.g. Integer real expert keys inwards Java HashMap.

Now if you lot clear this entire Java HashMap interview,  You volition survive surprised past times this real interesting enquiry "What happens On HashMap inwards Java if the size of the HashMap  exceeds a given threshold defined past times charge factor ?". Until you lot know how HashMap  works precisely you lot won't survive able to reply this question. If the size of the Map exceeds a given threshold defined past times load-factor e.g. if the charge factor is .75 it volition deed to re-size the map ane time it filled 75%. Similar to other collection classes similar ArrayList,  Java HashMap re-size itself past times creating a novel bucket array of size twice of the previous size of HashMap too and hence get-go putting every quondam chemical factor into that novel bucket array. This procedure is called rehashing because it likewise applies the hash business office to break novel bucket location. 

If you lot deal to reply this enquiry on HashMap inwards Java you lot volition survive greeted past times "do you lot run into whatever job amongst resizing of HashMap  in Java" , you lot mightiness non survive able to alternative the context too and hence he volition elbow grease to give you lot hint nigh multiple thread accessing the Java HashMap and potentially looking for race status on HashMap  in Java. 

So the reply is Yes at that topographic point is potential race condition exists patch resizing HashMap in Java, if 2 thread at the same fourth dimension institute that at nowadays HashMap needs resizing too they both elbow grease to resizing. on the procedure of resizing of HashMap in Java, the chemical factor inwards the bucket which is stored inwards linked listing acquire reversed inwards lodge during their migration to novel bucket because Java HashMap  doesn't append the novel chemical factor at tail instead it append novel chemical factor at the caput to avoid tail traversing. If race status happens too hence you lot volition destination upwards amongst an infinite loop. Though this point, you lot tin potentially scrap that what the hell makes you lot mean value to work HashMap  in multi-threaded surroundings to interviewer :)

Some to a greater extent than Hashtable too HashMap Questions 

Few to a greater extent than enquiry on HashMap inwards Java which is contributed past times readers of blog:

1) Why String, Integer too other wrapper classes are considered expert keys?

String, Integer too other wrapper classes are natural candidates of HashMap key, too String is most oftentimes used key every bit good because String is immutable too final, too overrides equals too hashcode() method. Other wrapper shape likewise shares similar property. Immutability is required, inwards lodge to preclude changes on fields used to calculate hashCode() because if key object returns dissimilar hashCode during insertion too retrieval than it won't survive possible to acquire an object from HashMap. 

Immutability is best every bit it offers other advantages every bit good similar thread-safety, If you lot can  keep your hashCode same past times alone making sure fields final, too hence you lot choke for that every bit well. Since equals() too hashCode() method is used during retrieval of value object from HashMap, it's of import that key object correctly override these methods too follow contact. If unequal object returns dissimilar hashcode than chances of collision volition survive less which later on improve the performance of HashMap.

2) Can nosotros work whatever custom object every bit a key inwards HashMap?

This is an extension of previous questions. Of course of report you lot tin work whatever Object every bit key inwards Java HashMap provided it follows equals too hashCode contract too its hashCode should non vary ane time the object is inserted into Map. If the custom object is Immutable than this volition survive already taken help because you lot tin non alter it ane time created.

3) Can nosotros work ConcurrentHashMap inwards house of Hashtable?

This is some other enquiry which getting pop due to increasing popularity of ConcurrentHashMap. Since nosotros know Hashtable is synchronized but ConcurrentHashMap provides amend concurrency past times alone locking component of map determined past times concurrency level. ConcurrentHashMap is sure introduced every bit Hashtable and tin survive used inwards house of it, but Hashtable provides stronger thread-safety than ConcurrentHashMap. See my post difference betwixt Hashtable too ConcurrentHashMap for to a greater extent than details.

Personally, I similar this enquiry because of its depth too number of concept it touches indirectly if you lot hold back at questions asked during interview this HashMap  questions has verified

• The concept of hashing
• Collision resolution inwards HashMap
• Use of equals () too hashCode () too their importance inwards HashMap?
• The practice goodness of the immutable object?
• Race status on HashMap  in Java
• Resizing of Java HashMap

Just to summarize hither are the answers which practice brand feel for to a higher house questions

How HashMap  works inwards Java

HashMap  works on the regulation of hashing, nosotros lead hold put() and get() method for storing too retrieving object from HashMap.When nosotros give both key too value to put() method to shop on HashMap, it uses key object hashcode() method to calculate hashcode too them past times applying hashing on that hashcode it identifies bucket place for storing value object. While retrieving it uses key object equals method to break out right key value distich too render value object associated amongst that key. HashMap  uses linked listing inwards instance of collision too object volition survive stored inwards side past times side node of linked list. Also, HashMap stores both key too value tuple inwards every node of linked listing inwards the shape of Map.Entry object. 

What volition hap if 2 dissimilar HashMap  key objects lead hold the same hashcode?

They volition survive stored inwards the same bucket but no side past times side node of linked list. And keys equals () method volition survive used to position right key value distich inwards HashMap.

How zippo key is handled inwards HashMap? Since equals() too hashCode() are used to shop too retrieve values, how does it piece of work inwards instance of the zippo key?
The zippo key is handled especially inwards HashMap, at that topographic point are 2 separate methods for that putForNullKey(V value) too getForNullKey(). Later is offloaded version of get() to hold back upwards zippo keys.  Null keys e'er map to index 0.  This zippo instance is dissever out into separate methods for the sake of performance inwards the 2 most usually used operations (get too put), but incorporated amongst conditionals inwards others. In short, equals() too hashcode() method are non used inwards instance of zippo keys inwards HashMap.

hither is how nulls are retrieved 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 damage of usage, Java HashMap is real versatile too I lead hold to a greater extent than often than non used HashMap every bit cache inwards an electronic trading application I lead hold worked. Since finance domain used Java heavily too due to performance argue nosotros demand caching HashMap and ConcurrentHashMap  comes every bit real handy there. You tin likewise banking enterprise jibe next articles from to larn to a greater extent than nigh HashMap too Hashtable inwards Java:

HashMap Changes inwards JDK 1.7 too JDK 1.8

There is some performance improvement done on HashMap too ArrayList from JDK 1.7, which trim down retention consumption. Due to this empty Map are lazily initialized too volition toll you lot less memory. Earlier, when you lot create HashMap e.g. novel HashMap() it automatically creates an array of default length e.g. 16. After some research, Java squad found that most of this Map are temporary too never work that many elements, too alone destination upwards wasting memory. Also, From JDK 1.8 onwards HashMap has introduced an improved strategy to bargain amongst high collision rate. Since a miserable hash business office e.g. which e'er render place of same bucket, tin plough a HashMap into linked list, i.e. converting get() method to perform inwards O(n) instead of O(1) too someone tin convey payoff of this fact, Java at nowadays internally supervene upon linked listing to a binary truthful ane time sure threshold is breached. This ensures performance or lodge O(log(n)) fifty-fifty inwards the worst instance where a hash business office is non distributing keys properly.

Further Reading
If you lot are preparing for Java interviews too hence I advise you lot convey a hold back at Java Interview Guide: 200+ Java Interview questions, an first-class resources for all marking of Java programmers. This mass contains questions from all of import Java topics. You tin likewise bring together Data Structures too Algorithms course of report to brush upwards some of your information construction too algorithm skills. 

Related post:

Difference betwixt Hashtable too HashMap inwards Java

Difference betwixt SynchrnizedHashMap  and ConcurrentHashMap inwards Java

10 examples of Hashtable inwards Java

How acquire method of Hashtable internally plant inwards Java
How to form HashMap inwards Java past times values

Java In-Depth: Become a Complete Java Engineer

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