Internal Working and implementation of hashmap and hashset | Java Interview Questions

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Internal Working and implementation of hashmap and hashset | Java Interview Questions | Code Decode

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2 жыл бұрын

In this video of Java Interview Question and Answer series we have explained internal working of hashmap and hashset which is important question in Java interview questions and answers series
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We need to cover 3 min points in High level for this question:
It works on the principle of hashing
How Put Method works Internal working
How Get method Works internally
Hash Map internally works on the principle of Hashing.
Hashing means using some function or algorithm to map object data to some integer value, hashCode() method return you that hash code. Hence Its necessary to write hashCode() method properly for better performance of HashMap.
If you override hashCode() method, it’s necessary to fulfill Equals and Hashcode Contract.
MAP is an object that maps keys to values”
So, there must be some mechanism in HashMap to store this key-value pair.
Everything in Hashmap is stored in a bucket internally (of hash table - underling DS)
Firstly hash value is calculated using the key’s hash code by calling its hashCode() method. This hash value is used to calculate the index in the array for storing Entry object. JDK designers well assumed that there might be some poorly written hashCode() functions that can return very high or low hash code value. To solve this issue, they introduced another hash() function and passed the object’s hash code to this hash() function to bring hash value in the range of array index size.
With Hash Code in place , we put the newly created Entry object K,V in the bucket of Hash Table.
So, in case of collision, Entry objects are stored in linked list form. When an Entry object needs to be stored in particular index, HashMap checks whether there is already an entry?? If there is no entry already present, the entry object is stored in this location.
If there is already an object sitting on calculated index, its next attribute is checked. If it is null, and current entry object becomes next node in linkedlist. If next variable is not null, procedure is followed until next is evaluated as null.
What if we add the another value object with same key as entered before. Logically, it should replace the old value. How it is done? Well, after determining the index position of Entry object, while iterating over linkedist on calculated index, HashMap calls equals method on key object for each entry object.
All these entry objects in linkedlist will have similar hashcode but equals() method will test for true equality. If key.equals(k) will be true then both keys are treated as same key object. This will cause the replacing of value object inside entry object only.
We know that two unequal objects can have the same hash code value. This is a case of collision.
Hash collisions have negative impact on the lookup time of HashMap. When multiple keys end up in the same bucket, then values along with their keys used to be placed in a linked list. In case of retrieval, linked list has to be traversed to get the entry. In worst case scenario, when all keys are mapped to the same bucket, the lookup time of HashMap increases from O(1) to O(n).
Java 8 has come with the following improvements/changes of HashMap objects in case of high collisions.
The alternative String hash function added in Java 7 has been removed.
Buckets containing a large number of colliding keys will store their entries in a balanced tree instead of a linked list after certain threshold is reached.
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Пікірлер: 151

@rahulagrawal3675 2 жыл бұрын

Really liked the idea of explaining through slides also. And thanks for explaining the functionality for Java 7 and Java 8 separately; insightful indeed.

@CodeDecode 2 жыл бұрын

Thanks Rahul 🙂👍

@sanyasee17 2 жыл бұрын

The Best Video to understand Internal working of HashMap. Thanks a lot ❤

@vijayavinayaktandur9599 2 жыл бұрын

Thank you so much @code decode guys for your support. I cleared multiple interviews with your valuable videos. Hats off

@CodeDecode 2 жыл бұрын

thanks and all best Vijay for your future

@shreyashachoudhary480 Жыл бұрын

Best video I've ever seen about HashMap's working!

@CodeDecode Жыл бұрын

Thanks Shreyash 🙂🙂

@smitchaudhari9783 Жыл бұрын

This channel is underrated! Great work.

@CodeDecode Жыл бұрын

Thanks for nice words

@rajatgoyal2812 Жыл бұрын

I was searching for this type of video for a long time. This video is very informative and helped me better understand the concepts. Thanks for sharing such insightful content.

@CodeDecode Жыл бұрын

Thanks 🙏🙏👍

@ssbunny111 2 жыл бұрын

Hi Mam, All you videos are excellent and very helpful to clear interviews. Your are doing this video at 1 AM... Hatsoff to your dedication

@CodeDecode 2 жыл бұрын

Thanks a lot 🙂🙂. Yeah that's when we get spare time after office ends 😃. We all are working IT professionals. Btw nice observation 👏👏

@user-gr3ry5et1h Жыл бұрын

You delivered a nice explanation. It was needed for interview because it is important. I found this video and it helped me out. Thanks.

@CodeDecode Жыл бұрын

Thanks Devanshu 🙂👍

@kaushalchandra9824 27 күн бұрын

Thanks Mam for such à deep knowledge. God Bless you. Keep posting 🎉

@saivamsi4811 2 жыл бұрын

Hi Mam! You are helping us a lot. Please keep teaching like this And also if possible please make a complete session on Collections frame work end to end. There are many tutorials on youtube, but nobody can teach us the way you do. I found your tutorials very helpful and easiest way to understand.

@CodeDecode 2 жыл бұрын

Sure Sai. Thanks a ton for the nice words. We do have videos on collection framwork. Can u plz tell what all topics u need apart from what are uploaded.

@saivamsi4811 2 жыл бұрын

@@CodeDecode Linked List, Stack, Vector, HashSet Vs LinkedHashSet Vs TreeSet..Thanks in Advance..

@anison1111 8 ай бұрын

Simple and accurate explaination - good job

@CodeDecode 8 ай бұрын

thanks

@shivamanand9836 11 ай бұрын

13:32 “while converting the list to binary hashcode is used as branching variable” - couldn’t get this part as hashcode is same then they are getting converted to linked list and then equals method is being used to add the values in the linked list. So the question is if it has reached a certain threshold and then how different values of hashcode can come to get converted into tree?

@shekhar_sahu 2 жыл бұрын

Thank you for taking us through the source code of these classes. I used to be afraid of checking such files. But this is how we get to learn good coding practice also.

@CodeDecode 2 жыл бұрын

Very true Shekhar. Very glad to see u are going through them. 👏👏👏

@aswinkumar6796 5 ай бұрын

Hi I have one doubt In bucket, all elements with same hascode only? Is it possible, different hashcodes in same bucket as you mentioned in video@13:59?

@anisha9709 Ай бұрын

I guess it should be when 2 keys that are different but having same hashcode?

@ArunSharma-hu4td 2 жыл бұрын

Excellent explanation of internals of hashmap as well as hasheet with proper proof

@CodeDecode 2 жыл бұрын

Thanks Arun 🙂👍

@ArunSharma-hu4td 2 жыл бұрын

@@CodeDecodeone video for arrayList internals

@vickybhoir3017 3 ай бұрын

nice explaination

@CodeDecode 2 ай бұрын

😊

@raghuakuthota4900 2 жыл бұрын

Too good of explanation :) thanks lot - great work - keep going

@CodeDecode 2 жыл бұрын

Thanks Raghu 👍🙂

@divyavegoti4609 2 жыл бұрын

Have been checking all your videos..Inspiring work and content!!!

@CodeDecode 2 жыл бұрын

Thanks a ton Divya 🙂

@yasirakhn Жыл бұрын

The content is really great, explaining everything in details which makes the internal working of HashMap and HashSet very clear. However, I have one question. Do we need this understanding for programming in real projects as generally we need to use HashMaps and HashSets to store and retrieve collection data and can also iterate using iterators, or is this just for interview purpose ?

@GauravSharma-up9gs 2 жыл бұрын

Thanks, Your channel is very helpful...

@CodeDecode 2 жыл бұрын

Thanks a ton Gaurav. It means a lot 👍🙂🙂👍

@sravanreddyreddy3562 11 ай бұрын

Just a feedback - There is too much of juggling btw screens...

@CodeDecode 11 ай бұрын

Understood. We will try to reduce it. 👍👍

@akankshasinha3352 Жыл бұрын

Thankyou so much dear❤ your videos are good to go for interview topics.. crisp and perfect .. Interviewer bhi khush hojaye😂

@CodeDecode Жыл бұрын

Haha Thanks a lot Akanksha🙂🙂 and we will be happy when u land at awesome job Girl ❤❤. Keep learning keep rocking girl 🎊🎊👍👍👍👍🎂

@vinayuddagiri 2 жыл бұрын

Very good Explanation.

@CodeDecode 2 жыл бұрын

Thanks Vinay 🙂👍

@sandiyr1989 Жыл бұрын

you r an amazing teacher..

@CodeDecode Жыл бұрын

Thanks 🙂

@aniketkalamkar227 2 жыл бұрын

Wonderful video explaining exactly what changed with Java8. Can you please create one for ConcurrentHashMap as well

@CodeDecode 2 жыл бұрын

Thanks 🙂Sure Aniket 🙂👍

@abhishekjain6559 2 жыл бұрын

Nice explanation.. Great help .. thanks

@CodeDecode 2 жыл бұрын

Thanks Abhishek 🙂👍

@sreejak6776 2 жыл бұрын

Thankyou for the video mam

@CodeDecode 2 жыл бұрын

You're welcome sreeja

@krishnarohit3166 2 жыл бұрын

Awesome Explanation

@CodeDecode 2 жыл бұрын

Thanks Krishna 🙂👍

@reshusinghal8298 2 жыл бұрын

Ur videos are too good to prepare for interview. I have a question here. Whe linked list is converted into binary tree then which hash code is used to determine the left or right node? As per my understanding hash code of all keys is same because of which collision is happening so now which hash code to use for node calculation?

@shashwatidash8524 3 ай бұрын

I have a doubt if anyone could help. Since all entries within a bucket index have the same hash code (due to being placed in the same bucket), how can the hash code determine an Entry object's placement to left or right. Instead, the Comparable interface or custom comparator only will be used to maintain the ordering within the binary tree. Am I getting it right? Since not the whole bucket is changed to a Binary tree, only a particular bucket index is!

@chinmayd4093 Ай бұрын

@shashwatidash8524 as per my understanding, the first element(for particular bucket) which you will be adding , will be acting as root node of that tree.

@MHK958 Жыл бұрын

Awesome explanation

@CodeDecode Жыл бұрын

thanks

@aneelakar400 2 жыл бұрын

Great explanation Mam.

@CodeDecode 2 жыл бұрын

Thanks 🙂👍

@anonymousxyz3856 Жыл бұрын

excellent explanation

@CodeDecode Жыл бұрын

Thanks 🙂🙂

@rajyalakshmi3077 2 жыл бұрын

Thanks! Could you please do a video of executor service future get and completablefuture from java how to handle if anyone executor service tasks takes too long

@CodeDecode 2 жыл бұрын

Nice topics Rajya, we will surely put video on these

@akashkarn8429 2 жыл бұрын

After finding such great videos, I am feeling blessed :D Thanks a lot !

@akashkarn8429 2 жыл бұрын

This is pure Quality Content !!

@CodeDecode 2 жыл бұрын

Thanks a lot Akash 🙂👍

@mahi2082 Жыл бұрын

Nice explanation

@CodeDecode Жыл бұрын

Thanks 👍

@jeebamvarghese3542 2 жыл бұрын

Great explanation

@CodeDecode 2 жыл бұрын

Thanks Jeeba

@jerinxavier5380 6 ай бұрын

How can we use hashcode to compare elements while adding to the binary tree. Unless the hashcode was the same we wouldn't push in the same bucket. Right.?

@maartensnels3804 2 жыл бұрын

Keep up the good work!

@CodeDecode 2 жыл бұрын

Thanks Maarten 🙂👍

@amarthyaseshu683 2 жыл бұрын

Thanks for sharing!

@CodeDecode 2 жыл бұрын

🙂👍

@rasolutions8676 2 жыл бұрын

Dose Linked List in the bucket (specific index calculated after hashing) is going to convert to tree or is it the complete bucket is getting converted into tree a) if it is linked list at specific bucket index then how the hashcode less or greater then calculation will happen as per me at that point hashcode would be same for both element isn't it? b) if the complete bucket is getting converted into tree then why we are saying linked list will get convert to tree...if i heard right! 🤔🤔🤔 Please help me understand the concept

@vaibhavjain8939 2 жыл бұрын

Next Level

@CodeDecode 2 жыл бұрын

Thanks Vaibhav 🙂🙂👍👍

@vndprasadgrandhi7024 2 жыл бұрын

Thank you very much.. Could you please do if possible tree set and tree map

@CodeDecode 2 жыл бұрын

Nice topic. Sure we will do that

@chiragshah9171 Жыл бұрын

14:29 how can we have different hash values if there is hash collision, it's bit confusing there 😮

@rahulingole4923 3 ай бұрын

still usefull video

@CodeDecode 3 ай бұрын

😊

@ganeshahiwale4899 Жыл бұрын

Really liked the explanation can you please create a telegram channel so that we can have communication and polling advantage and discuss our doubts

@kamallochannayak2706 2 жыл бұрын

Great mam...

@CodeDecode 2 жыл бұрын

Thanks 👍🙂

@salmanpatel2968 Жыл бұрын

nice explanation keep it up we will support you and my request to you please make a video on time complexity and space complexity

@CodeDecode Жыл бұрын

Yeah that's s tough one to understand. We will create video on that soon 👍👍

@salmanpatel2968 Жыл бұрын

@@CodeDecode thanks pl create on that topic

@kanchankatkar5226 5 ай бұрын

If same hashcode is there then what will happen in binary tree fornat how key can be comparable will you give example, it's confusing for me

@krishnan6201 Жыл бұрын

can you plz explain about memory allocation enhancement in 1.8 feature.

@phanimc11211 Жыл бұрын

neatly explained what changed with Java8

@CodeDecode Жыл бұрын

Thanks

@rajatgoyal2812 10 ай бұрын

At 14:12 you mentioned that in tree the main comparison will be on the basis of hashcode, But in a bucket when there are several entries and all of them have same hashCode then how are we comparing on the basis of hashCode.

@vengateshm2122 2 жыл бұрын

Thank you!

@CodeDecode 2 жыл бұрын

🙂👍

@Sanjaykumar-nz4tf 10 ай бұрын

Hi Mam, I have a doubt in the put() method of hashmap.. If the hashCode of few keys are same means all those will be stored inside same bucket in linked list format (if threshold is increased by 8 means it converts to balanced tree) * Right branch will be higher value of hashCode and left will be lower than that.. My Doubt is all the variables present in same bucket will have same hashCode.. Then how they will be compared while storing in balanced tree?

@AshishKumar-vj7fq 7 ай бұрын

I also have the same doubt. How different hashcode can be present in same bucket because different objects will land on same bucket only when hashcode for them is same.

@noorahameds8 Жыл бұрын

How will it work if the value is a list Map m = new hashmap();

@satyajeethukkire6099 11 ай бұрын

Maam, most of the comment section has the doubt of collision at 14:29 please explain the doubt, it is really confusing

@tejasnerkar1330 Жыл бұрын

do you have link for all the presentation slides that we can access? It will be really helpful to go through it as a revision before interview.

@simplegirl2218 2 жыл бұрын

Mam, please make a video on multi threading concepts..with basic and advanced ..

@CodeDecode 2 жыл бұрын

kzfaq.info/sun/PLyHJZXNdCXsdUXzeeBZIADof_U-40jBJO

@kudumulasivaramakrishnared6379 2 жыл бұрын

Please continue all data structures (linear,non linear in graphs, hash table)

@CodeDecode 2 жыл бұрын

Sure 🙂👍

@9-1939 5 ай бұрын

👌👌👏👏👏🙏

@arulantony2137 2 жыл бұрын

traversing linked list and tree logn and O(n) then how hashmap complexity isO(1) ?

@CodeDecode 2 жыл бұрын

It's said best case complexity o(1) worst case o(logn)

@suvch5842 2 жыл бұрын

Do you have any java course from beginner level to advanced in Core to Advanced java

@shivachanda2438 Жыл бұрын

Very good explanation, but I have a doubt: while explaining handling collisions in java8 nd above, u mentioned that if the hash code is same in a bucket, then we go with comparing keys , but collision is caused because of same code na, my doubt is all the entries in a bucket will having same hashcode ryt that is how they are placed in a bucket , keys may be different but hascode remains same based on hashcode only we are choosing the bucket na, please clear my doubt.

@CodeWithCB Жыл бұрын

All entries in a bucket may not have same hash value though they will have same index. Remember we first calculated hashcode which again converted to some hash value. This hash value is then mapped to some index. And this is the point where two different hash values may result in same index. So while doing get operation first hash values are equated and then keys are equated.

@shivachanda2438 Жыл бұрын

@@CodeWithCB sry what do u mean by hash code and hash value both are same ryt , for a given key we will find hashcode , what is hash value. I didn't get u, please elaborate

@CodeWithCB Жыл бұрын

@@shivachanda2438 internal implementation of Hashmap frist calculates hashcode of key as per hashcode() given for key object. Then internally it converts this hashcode to hashvalue h using formula (h = key.hashCode()) ^ (h >>> 16) . This is done to spread keys across the array causing less collisions. And this hashvalue is used further to find actual index using formula index = (n - 1) & hash.

@hackstreet781 Жыл бұрын

From where do you learn Java ? I also want to read from there.

@CodeDecode Жыл бұрын

Mostly docs helps us a lot in understanding the concepts

@djinthemix33 11 күн бұрын

I hope to be that fortunate winner... Plz Like✓ SHARE✓ SUBSCRIBE✓

@arinbose6366 2 жыл бұрын

Thanks but do you have video of say passing hashmap as parameter etc

@CodeDecode 2 жыл бұрын

What do u need? M unable to understand the requirement. Can you plz elaborate?

@arinbose6366 2 жыл бұрын

@@CodeDecode I was looking for below found it ,in future can you show some examples like say taking hashmap as parameter, taking hashmap returning list etc, no need for video you can post in github that would help, for you these might seem very simple and common task but for starters this would help a lot,thanks again for all your videos. // returning hashmap public HashMap asHashMap( K[] keys,V[] values ) { HashMap result = new HashMap(); if (keys == null || values == null || keys.length != values.length ) throw new IllegalArgumentException(); for (int i =0; i

@arinbose6366 2 жыл бұрын

@@CodeDecode I had just given a suggestion,otherwise learnt a lot from your video series 👍

@rohitkapade1130 Жыл бұрын

14:29 in second point how come is this possible that in one bucket will have keys with different hashcode as we assigned bucket on basis of hashcode itself. I guess binary search tree is based upon keys.

@RANDOMGAMER-nq6jf Жыл бұрын

I'm also having the same doubt. The hashcode enters the bucket only if it is same and the keys are used to compare amongst the elements in that bucket. Im confused here..please explain here....

@RANDOMGAMER-nq6jf Жыл бұрын

14:29 im having doubt on why there will be different hashcodes in same bucket??

@chiragshah9171 Жыл бұрын

Yea.. same doubt. Linked list gets created when there is hash collision then how can be there different hash code values

@kanikanarwat826 2 жыл бұрын

Hi what will happen if we have overriden equals method but it always returns true.

@MHK958 Жыл бұрын

Just do it in eclipse, if equal returns true every time then i think hashmap will store only 1 key if same has code come

@PankajPatil-kw9ic 24 күн бұрын

1:27 Explain slowly

@malaiarasi4400 2 жыл бұрын

Hashmap concept was awesome.. Hashset is not much clear.

@CodeDecode 2 жыл бұрын

How can we help Malai? What is unclear can u plz tell us so that we can clarify that to u?

@malaiarasi4400 2 жыл бұрын

Mam i went through the video again now it's clear. Thanks

@CodeDecode 2 жыл бұрын

Awesome Malai 🙂👍

@lakshmaiahyannagiri291 2 жыл бұрын

please share the document of this class

@CodeDecode 2 жыл бұрын

Which document are you asking for Lakshmaiah?

@BaluKompalli Жыл бұрын

Explanation fine but i am unable to catch that flow. Listened more than 3times. Little bit confusion. Could you please explain in less technical way. Atleast one simple example

@CodeDecode Жыл бұрын

Sure we will do that👍👍

@BaluKompalli Жыл бұрын

@@CodeDecode I listened in very slow motion and put a diagram in a paper. Then i understood the concept. I think it is fine for me. No need to do any other video with examples. Another thing is , I tried to donate thank you, but not redirecting to payment page. Will check and update you again.

@suhilirshad Ай бұрын

very confusing explaination . please make it short and understandable for interview pont of view purpose

@koushikpan1320 Жыл бұрын

why is your voice shivering. Looks like you are worried of something. 👀

@CodeDecode Жыл бұрын

It might be cold here or mic u Issues. Not sure. We will check 🙂👍

@prasadsatpute5197 2 ай бұрын

Didi please please please please Me and You not going anywhere. If we learn well or quickly, it does not mean that you speak so fast. Please explain with some breath at normal speed or read calmly bs itna kahen hai please mam please

@harshwardhanshetty4683 Ай бұрын

use chatgt thats way more clear