Overriding equals and hashCode in Java - 039

How can I tell if two Java instances represent the same instance? And what does class equality mean in Java?
The question here is, what does it mean to be the same or different in Java. If you modeled me, maybe two of me had blue shirts and the third had a red shirt. So their shirt color attribute would be the blue for two DJ instances, and red for a third DJ instance. Are they equal because they have the same color shirt, or are they the same me? We're going to clear that up in this lesson as we learn about what it means in Java for two instances to be equal.
#java #programming #tutorial
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Full transcript:
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Concepts: Java, classes, equals, hashcode
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[@jonesmm3]

"Equality is something that should be easy but has many nuances." -DJ 2015 Wisdom for the ages...

[@ngndnd]

finally someone who explains this. Every video i see just explains the basics

[@Zydico]

Incredibly helpful, with very straightforward explanations!

[@catzrulez5233]

Nice visuals and editing, and more importantly good and concise teaching too. Thank you so much for sharing.

[@Allalilacias]

Thank you for the speed and clarity of the explanations. Great video!

[@sanjup100]

Really nice way. I wasted two hours to understand this. Finally got it. Thanks Deege U

[@deegeu]

+Arvind Pant Glad it helped!

[@ri5hipat]

Your approach to teach stuff is unique, Thanks for this video!

[@delucabruno]

I just discovered your videos and I love them pleaase keep doing them It helps a lot! gratings from Argentina!

[@deegeu]

+Bruno De Luca Thank you! Thanks for watching.

[@Triviorito]

Hey thanks for making this topic so fun to understand! Thumbs up!

[@yogeshadsare3150]

Thanks Deege U , the way of teaching is very good and effective. :)

[@dateneyoutubekanaal3667]

Thanks, really helped me out, great video!

[@jennab8837]

This video saved my grades. Thank you!

[@zombiezombie1977]

Just found this channel...great job thanx :)

[@mr_karamazov246]

Genuinely good video.

[@user-xb7rh1sd4f]

Great examples. Thank U

[@mertyertugrul]

Your tutorials are great, nice work.

[@Buuhuuda]

Thank you for explaining something in 6 Minutes while my Professor needs 2 hours to overcomplicate this topic.

[@JD-kx1oe]

Very good explanation and funny as well. Good Job!

[@Michael10145]

Best explanation I've seen. Cheers

[@terran008]

wow this was a great explanation! thanks a lot for this video

[@proalexv]

Thank you so much, great video would recommend that when talking about the overriding you noted when as many for the attributes, would be helpful if you're example included multiple to see how it would be extended. Either way, great video!

[@yasminfatahi6361]

Great explanation, THANK YOU!

[@simplet3306]

The equal equal sign checks for value in the memory address of two objects not their fields that’s why we should override equals and hashcode. True or not?

[@amarsingh-cd8wn]

greatly explained ... u rockk>>

[@sayemsss560]

so much quality video. thank you !!!!

[@Ravikumar-gj6qw]

Nice and good explanation

[@computerscientologist]

so helpful, thank you!

[@Martin-wx5ch]

Nicely explained

[@ErwinCorzo]

Te ganastes un suscriptor mas, Saludos!!!.

[@deegeu]

Muchas gracias!

[@muhammadhamedkamal1716]

Thanks for explanation...

[@Joni67sinix]

excellent video!! congratss and thanks!!

[@punerealestatebuilder]

Amazed why this video has few likes even after 5 years. Great video and great style to explain.

[@Utshaw]

Hi, 5:22 you showed us a chart for creating overriden hashCode() method . Is it conventional / necessary to use that chart ?(Is it Oracle specific)

[@Ztenam976]

You are a good explanator

[@_sudipidus_]

Informative short and succinct

[@kshitijgupta6976]

very good video

[@muresanmarius7746]

I like how you make you're videos:)

[@TheGazanews]

funny and effective thank you

[@ameyapatil1139]

superb keep up

[@Unbreak00]

very nice, thank you.

[@pritamkumarbehera5423]

Ohh... it's still helpful after 6years also

[@aneukmie]

Great video! But where did you get the hash chart formula? Is it available somewhere in java documentation?

[@deegeu]

+Mi Husna Krung I think I got it from the Effective Java book, but it's a standard strategy for creating hash codes. Most will have some form of this, but the prime number varies.

[@dhanatontanahal557]

Sir what does the code inside hashcode mean? why we started with 7 and then with some formula? what exactly is the idea? thank you

[@smugless191]

Where did you get the table at 5:30 from?

[@keithscull8288]

Great video THANK you! Been trying to understand why the Oracle documentation kept saying that if equals returned true hashcode must return true, what if I have 2 different objects with the same shirt color then that wouldnt be the case but what the docs fail to say in detail is that it's a conventional "must" not a technical default, n that you should override hashcode when overriding equals

[@h3w45]

Thnk you.

[@TheGazanews]

can you please cover more topics on java . this s the best way to lean period thank you a lot

[@deegeu]

+mohamed elghannay I'll be doing Java for a long time. There's still many topics on just Core Java, and I'll hopefully get back to doing more Enterprise Java stuff soon!

[@TheGazanews]

thanks

[@anaskala1196]

could you possibly explain the meaning of (this==obj) in the overriding Methode code of equals. does it refer to obj1 in the call: obj1.equals(obj)??. if this is true, how does this work???. Thank you in advance.

[@egecant]

YOU ARE AWESOMEEEEEEE

[@louaykhammar7268]

thanks

[@budsyremo]

Don't 41*hash + myObject.hashCode() create hashcode which is diff for all ? I mean two shirts o same color always produce diff hashcode ?

[@mayankparihar1988]

very great video.. I understand what you teach. but I have small question. why we using this process to generate hash.. what is reason behind it.

[@deegeu]

+Mayank Parihar The hashcode provides a mostly unique number for your class instance of any size. You can think of it as a fingerprint of your class instance. It's not 100% unique, but most of the time it is. It's a quick way to determine if two instances are equal. If the hash is not equal, the instances are not equal. If the hashs are equal, it's likely the instances are equal.

[@Kirmo13]

3:47 Why do you need to cast the instance if you just made sure that it's part of the DJ class?

[@amyjen6934]

So that you can compare the specific parameters or use methods of that specific class. If you don't cast the Object parameter to the specific type of your class, you would be limited to using methods and fields defined in the Object class itself.

[@glueee2621]

*Stupid question time By every instance is equal to each other, does that mean the traditional == way in which both instances are pointing towards the same object?

[@deegeu]

If you use ==, you are testing if the two variables point to the same instance. If you use equals(), that tests if the values contained by the two instances are the same.

[@rahulmangulkar298]

To generate hashCode Eclipse is using one prime number where as Netbeans is using two prime numbers. Which one is correct? Below code is auto generated using Eclipse IDE. @Override public int hashCode() { final int prime = 31; int result = 1; result = prime * result + ((shirtColor == null) ? 0 : shirtColor.hashCode()); return result; } Note: 1 is not prime number.

[@gautamprusty5336]

Hi Deeg U..I can't understand which circumstance we have to use hashcode and euals method ..If u give me answer ,It will make happy......

[@deegeu]

Equals is when ever we want to determine two object values are equal. You could create two instances with the same classs values, but if you compare the references they will be different. One use of hashcode is when you want to store the values in a hashtable. If you don't provide equals and hashcode, you will get collisions in your hashtable.

[@maximusdeisadze5744]

why type cast "DJ" to obj if it passes the if statement?

[@adriansalabanzi2250]

Like it!

[@MouradBENKADOUR]

Great job thanks, but what's the difference if I use in equals() method (this.getClass()==obj.getClass()) instead of (this instanceOf ObjClass), for me it's equal

[@deegeu]

+Mourad BENKADOUR Good question! Testing getClass tests if the two types are identical. InstanceOf tests if the type on the left is an instance of the type or subtype of the type on the right.

[@MouradBENKADOUR]

+Deege U ok thanks

[@GurleenKaur-ty4iy]

You said equals method just calls the equal operator (==) , then how come we have different results for both in case of string?

[@jvsnyc]

The default inherited from Object class just calls the == operator. Most classes you will find override it to something more appropriate, including String.

[@gautamprusty5336]

Thanks,,,,,,,,,,,,,,,'

[@KUROYUKI48]

is it possible to have multiple overriding equals and hashCode in Java?

[@deegeu]

You should override it in each subclass. If you mean overloading, then no.

[@KUROYUKI48]

Deege U thanks

[@fayyu5506]

👌

[@HungryBirdStephenHaokip]

Why start with prime non zero number .... plis some body explain me

[@junaidmuhammad4091]

he said equal instances always produce the same hascode but i cant get why equal hashcodes do not mean equal instances. Any body can explain please?

[@deegeu]

The hashcode is the output of a function that is usually unique. It's possible to have the same output for two different class instances. Think of it this way. There are a finite number of possible hashcodes since it's contained by an integer. There are more possible strings than integers, but all strings have hashcodes. This means some of the hashcodes are repeated for different strings. So having the same hashcode does not guarantee uniqueness.

[@junaidmuhammad4091]

Deege U thank you so much you helped me alot. Keep it up!

[@pinological]

2:59

[@supremereseller5953]

2020 gang, thank you

[@quickmaths4762]

what do you mean by "once we have done this for every attribute in our class"? No idea what you mean. Write another function for every attribute? Add Objects.hashCode(this.attr) for every attribute we have to the fourth line in your code? or even sth else?

[@SreenathN92]

Why "DJ is cool" == "DJ is cool" is true? What does the compiler do?

[@deegeu]

It's because Java interns the strings. So the two strings really do point to the same instance. This tutorial might help make it clearer. kzbin.info/www/bejne/o4SbgmuqdtWJpLc

[@Dante3085]

Same memory address.

[@Dante3085]

Do "System.out.println(object1.toString());" "System.out.println(object2.toString());" for both objects. "toString()" gives you the memory adress of you'r object if it's not overwritten. So it should be the same for both objects.

[@amarsingh-cd8wn]

String string1=new String("java is cool"); String tring2=new String("java is cool"); if(string1==tring2) System.out.println("true"); else System.out.println("false"); now the output will be false becouse isolated memory is allocated for both string objects

[@Dante3085]

Yeah, different memory adresses, right?

[@rebeccabecky2229]

Why a non zero prime number

[@I3uzzzzzz]

yourHead.contains(hair)==false

[@gautamprusty5336]

Hi Deeg, can u tell me ,why we are overriding equals and hashcode methods in our class,,,urgent required

[@robinwenzel7743]

Triggered

[@Ravikumar-gj6qw]

Not explained clearly