In hashing, hash functions are used to link key and value in HashMap. To understand how HashMap works internally in Java, we must know about how the HashMap calculates the index of the bucket. Until now, we know the internal structure of HashMap, that HashMap maintains an array of the bucket. But when we store or retrieve any key-value pair, HashMap calculates the index of the bucket for each and every operation. The Key object is used to calculate the index of the bucket.
By using this key, the hash value is calculated using the hash key private method of HashMap. Note: hash key the method is a private method of HashMap that returns the hash value of the key, also if the hash value is too large then converts it into a smaller hash value. But what will happen, if the hash value of Key Object returns the integer that is greater than the size of the array i. To handle this situation, HashMap reduces the hash value between 0 and n-1 using an expression :.
Now, this index value is generated is used by HashMap to find bucket location and can never generate any Exception as the index value always from 0 to n If the key is null, the value is stored in table[0] position, because hashcode for null is always 0. This hash value is used to calculate the index in the array for storing the Entry objects. Another way to resolve hash collision is by using "Open addressing". If you are going for Java interview, you better know both of them. Anonymous, HahsMap stores both key and value inside Entry object because it's possible to store more than one value in one bucket location, in that case in order to find the correct value corresponding to key, you also need the key object there.
Please explain how remove key method internally works on HashMap?? This was one of my interview questions. Hi Javin, This is a nice article. But I have one question here Why there is only one null key allowed in Hashmap. Why can't we insert many. Anonymous, null handling is very special, I would suggest to check the section about how HashMap handles null keys. To give you answer, multiple null is not an option because HashMap doesn't allow duplicate keys.
Hi Javin, Could you please explain how hava 8 uses balanced tree to solve the collision issue. How they will create the balanced tree, as keys has same hash code. Can anyone explain me please??
Anonymous - test. Size of internal table is always higher than number of entries in the HashMap, and is expanded once number of entries reach certain threshold, which is determined by the load factor. Higher size of internal table provides capacity to accommodate addition of newer entries without having to rehash the entire table. Without any additional capacity in the table, hash-values of each entry has to be recalculated every time a new entry is added. I like your post. You explained it very well.
Hi Paul, superb post.. Result of get is entry of latest key. But how it work internally. As while putting only hashCode of key is calculated. Still a great article, thanks a lot. Unknown, good question, but I have never come across any good reason with the use of null keys, only practical benefit I see is that it doesn't throw NullPointerExcepiton by calling equals and hashcode on null keys. It is a great article, It really helped me. I have faced one more question in an interview and would like to share with you all.
What Hashcode actually represents? Does it represent memory location of an object or it is just an identifier or its a combination of both the options mentioned? Hello Anonymous, if you read the Javadoc of hashCode method you will get your answer. Hashcode is an integer value which is generated by converting internal memory address of object to an integer. Java now internally replace linked list to a binary true once certain threshold is breached. Kindly correct the spelling mistake.
I have been reading it for many years. Hi, Can anyone tell me if 0 index in hash table is reserved for null key, then what will happen if hash key w. In which index of hash table it will be saved.
Hello Unknown, the bucket location index in array is returned by a hash function, which only return 0 if key is null for other keys it return non-zero index. Also, in worst case, if a collision happens then chaining is used, I mean a linked list is used to store both keys into same location. Post a Comment. HashMap in Java works on hashing principles. It is a data structure that 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 keys and values in HashMap. Objects are stored by the calling put key, value method of HashMap and retrieved by calling the get key method. When we call the put method, the hashcode method of the key object is called so that the hash function of the map can find a bucket location to store the value object, which is actually an index of the internal array, known as the table.
HashMap internally stores mapping in the form of Map. Entry object which contains both key and value object. When you want to retrieve the object, you call the get method and again pass the key object. This time again key objects generate the same hash code it's mandatory for it to do so to retrieve the object and that's why HashMap keys are immutable e.
String and we end up at the 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 a little tricky when collisions occur. It's easy to answer this question if you have read a good book or course on data structure and algorithms like these data structure course s and books.
If you know how the hash table data structure works then this is a piece of cake. Since the internal array of HashMap is of fixed size, and if you keep storing objects, at some point in time hash function will return the 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 a new entry is stored as the next node. If we try to retrieve an object from this linked list, we need an extra check to search for the correct value, this is done by the equals method.
Since each node contains an entry, HashMap keeps comparing the entry's key object with the passed key using equals and when it returns true, Map returns the corresponding value.
Since searching inlined list is an O n operation, in the worst case hash collision reduces a map to a linked list. This issue is recently addressed in Java 8 by replacing the linked list to the tree to search in O logN time. By the way, you can easily verify how HashMap works by looking at the code of HashMap.
Almost everybody who worked in Java knows about HashMap, where to use HashMap, and the difference between Hashtable and HashMap then why this interview question becomes so special? Because of the depth, it offers. It has become a very popular Java interview question in almost any senior or mid-senior level Java interview.
Investment banks mostly prefer to ask this question and sometimes even ask you to implement your own HashMap based upon your coding aptitude. The introduction of ConcurrentHashMap and other concurrent collections has also made these questions as a starting point to delve into a more advanced feature.
Questions start with a simple statement:. Why do you use it? Almost everybody answers this with yes and then the interviewee keeps talking about common facts about HashMap like HashMap accepts 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 the person has used HashMap and is quite familiar with the functionality it offers, but the interview takes a sharp turn from here and the next set of follow-up questions gets more detailed about the fundamentals involved with HashMap in Java.
Interviewer strike back with questions like:. But some interviewee definitely answers this and will say HashMap works on the principle of hashing , we have put key, value and get 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, the important point to mention is that HashMap in Java stores both key and value object as Map.
Entry in a bucket is essential to understand the retrieving logic. If people fail 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 the interviewee has a fair bit of knowledge on how hashing works and how HashMap works in Java. But this is just the start of the story and confusion increases when you put the candidate on scenarios faced by Java developers on day by day basis.
The next question could be about collision detection and collision resolution in Java HashMap like:. What will happen if two different objects have the same hashcode? Now from here onwards real confusion starts, sometime candidate will say that since hashcode is equal, both objects are equal and HashMap will throw an exception or not store them again, etc, Then you might want to remind them about equals and hashCode contract that two unequal objects in Java can have the same hashcode.
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 uses LinkedList to store object, this entry object of Map. Entry comprise key and value will be stored in LinkedList. Great this answer makes sense though there are many collision resolution methods available like linear probing and chaining, this is the simplest and HashMap in Java does follow this.
But the story does not end here and the interviewer asks. How will you retrieve the Value object if two Keys will have the same hashcode?
The 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 the 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. 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 i n all previous questions and equals come in the picture only in case of retrieving value object from HashMap in Java.
Some good developers point out here that using an immutable, final object with proper equals and hashcode implementation would act as perfect Java HashMap keys and improve the performance of Java HashMap by reducing collision. Immutability also allows caching their hashcode of different keys which makes the overall retrieval process very fast and suggests that String and various wrapper classes e.
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.
Similar to other collection classes like ArrayList , Java HashMap re-size itself by creating a new bucket array of size twice the 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 the hash function to find a 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 a hint about multiple threads 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. Though this point, you can potentially argue that what the hell makes you think to use HashMap in a multi-threaded environment to interviewer :. String , Integer, and other wrapper classes are natural candidates of the HashMap key, and String is the 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. Immutability is required, in order to prevent changes on fields used to calculate hashCode because if a key object returns different hashCode during insertion and retrieval then it won't be possible to get an object from HashMap.
Immutability is best as it offers other advantages as well like thread safety, If you can keep your hashCode the same by only making certain fields final, then you go for that as well. Since the equals and hashCode method is used during retrieval of value objects from HashMap, it's important that key objects correctly override these methods and follow contact.
If an unequal object returns different hashcode then the chances of collision will be less which subsequently improves the performance of HashMap. This is an extension of previous questions. Of course, you can use any Object as a 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 the custom object is Immutable then this will be already taken care of because you can not change it once created. This is another question which getting popular due to the increasing popularity of ConcurrentHashMap. Since we know Hashtable is synchronized but ConcurrentHashMap provides better concurrency by only locking a portion of the map determined by concurrency level. ConcurrentHashMap is certainly introduced as Hashtable and can be used in place of it, but Hashtable provides 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 the number of concepts it touches indirectly if you look at questions asked during the interview this HashMap question has verified. Just to summarize here are the answers which do make sense for the above questions.
How HashMap works in Java. HashMap works on the principle of hashing, we have put and get method for storing and retrieving objects from HashMap. When we pass both key and value to put method to store on HashMap, it uses key object hashcode method to calculate hashcode and them by applying hashing on that hashcode it identifies bucket location for storing value object.
While retrieving it uses the key object equals method to find out the correct key-value pair and return the value object associated with that key. HashMap uses a linked list in case of collision and objects will be stored in the next node of the linked list. Also, HashMap stores both key and value tuples in every node of the linked list in the form of Map. Entry object. What will happen if two different HashMap key objects have the same hashcode?
They will be stored in the same bucket but no next node of the linked list. And keys equals method will be used to identify the 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 the case of the null key? Later is an 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, the equals and hashcode methods are not used in the case of null keys in HashMap.
In terms of usage, Java HashMap is very versatile and I have mostly used HashMap as a cache in an electronic trading application I have worked on. Since the finance domain used Java heavily and due to performance reasons we need caching HashMap and ConcurrentHashMap comes as very handy there.
Due to this empty Maps are lazily initialized and will cost you less memory. Earlier, when you create HashMap like new HashMap it automatically creates an array of default length e. After some research, the Java team found that most of these Map are temporary and never use that many elements, and only end up wasting memory.
Also, From JDK 1. Since a poor hash function e. This ensures performance or order O log n even in the worst case where a hash function is not distributing keys properly. This book contains questions from all important Java topics. You can also join these best Data Structures and Algorithms courses to brush up on some of your data structure and algorithm skills. Related post:. Difference between Hashtable and HashMap in Java.
Share to Twitter Share to Facebook. Labels: collections interview questions , core java , core java interview question , java collection tutorial. Hi Dude Can we use hashMap in multithreaded scenario? February 7, at AM Gautam said Good explanation. Some pictures and flowcharts will be good. February 7, at AM Foudres said February 7, at AM Rod said February 8, at AM Pallavi said February 13, at AM 0x1e said It generates the hash code as Now calculate the index value of by using index formula.
The index value will be 4, as we have calculated above. It compares the first element Key with the given Key. If both keys are equal, then it returns the value else check for the next element in the node if it exists.
In our scenario, it is found as the first element of the node and return the value The hash code of the Key "Sunny" is The calculated index value of is 4, as we have calculated for put method.
Go to index 4 of the array and compare the first element's Key with the given Key. It also compares Keys. In our scenario, the given Key is the second element, and the next of the node is null.
It compares the second element Key with the specified Key and returns the value It returns null if the next of the node is null. JavaTpoint offers too many high quality services. Mail us on [email protected] tpoint. Please mail your requirement at [email protected] Duration: 1 week to 2 week.
Reinforcement Learning. R Programming. React Native. Python Design Patterns. Python Pillow. Python Turtle. Here I am taking key of my own class so that I can override hashCode method to show different scenarios. My Key class is. So whenever the first character of key is same, the hash code will be same. You should not approach this criteria in your program. It is just for demo purpose.
As HashMap also allows null key, so hash code of null will always be 0. Definition of hashCode method is public native hashCode. It indicates the implementation of hashCode is native because there is not any direct method in java to fetch the reference of object.
It is possible to provide your own implementation of hashCode. In HashMap, hashCode is used to calculate the bucket and therefore calculate the index. This method is provided by Object class. You can override this in your class to provide your own implementation.
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