HashCode and Equals - Easy and comprehensive including coding samples

[nitishjain]

Marcus, it's really nice and effective way to understand the equals and hashcode method. I spent entire day understanding the same but things are now clear in just 45 minutes. Thanks for this tutorial.

[avneet]

I have read Josh Bloch's Effective Java - epic book. But unfortunately, I glossed over this part. Marcus has really shined a light at a place few care about since we hit generate in the IDE and that could lead to bad consequences if we aren't careful. Truly great video.

[ggaass]

Outstanding video. Triple thumbs up! As the saying goes, you learn something new everyday.

[simoo]

thanks for taking time out of your personal time to explain this in such a great detail :)

[jvsnyc]

As always cool little things here and there that are necessary to make everything work well in real life. Most people who like A.M. and P.M. instead of 00:00 --> 23:59 are the most likely to make mistakes between them, not the people who choose 0:00 --> 23:59 whenever they can. Similarly, people who are in love with inheritance and casually use it everywhere will probably most likely break symmetry of .equals(). without even realizing or thinking about it.
There are several other pretty good coverages of this in videos -- they generally forget that. I think the tell-tale is that they say instanceof() to check rather than comparing .getClass() return values.

[langeredward]

Marcus, for the overriden hashCode() method in the Car and Engine classes, the first statement is "int result = 1;" but for the SomeClass class, the first statment is "int result=0;". Why the difference? Thanks.

[rangachanakyagade]

Thanks for the Perfect in-depth explanation.

[Cube]

A question regarding 27:40. Is there any real performance gain in splitting "if" blocks compared to making them a one big "or" expression? Clean code and scalability aspects aside.

[MarcusBiel]

Birkhan asked me: Do you mind asking you which IDE your using? I assume your using Intellij but i dont have the option to run 1 method just like you do in your tutorial! How did you manage this, are you using a plugin?

Yes, I am using IntelliJ in the beginning of my course (later I switched to Eclipse to also showcase that IDE).
I am writing JUnit tests. The IDE has a JUnit plugin, which will allow you to run only one test method - but this is something the plugin makes possible for you - it will still have to create a main method in the background, like any kind of program in Java (that does not run within a container which has its own main method again).

Consider starting the course from video 1. While this course is not a course about testing, I show some more details about my IDE and testing in general in the course.

[Twerqt]

Great video. One doubt.
Can you explain why the order in which hashcode is calculated should be same the way equals is implemented.

[punated]

Excellent instruction.  I was having a hard time wrapping my head around the equals() override as well a hashCode() method, but your video really helped to clarify things.  One question:  why is it advantageous to leave the Object type parameter in the equals() override rather than just use the Car object type.

[MarcusBiel]

Hi, and welcome to episode 22 of my free Java Video course. My name is Marcus Biel. In this episode I am going to talk about the two methods hashCode and equals. This tutorial concludes my presentation of all the methods of the java.lang.Object class. hashCode and Equals both follow a contract, that ties them closely together, which is also why I am talking about both methods in just one episode. Knowing all the details about these two methods will make you a better programmer today, so get yourself some popcorn and listen carefully. Let’s start with the equals method. What does it do, what’s it useful for? The Equals method is used to compare two objects for equality, similar to the equals operator that is used for primitive values. But before going into much more detail, let’s first jump into my IDE and see this in action....

[sendacool]

so much information and so great video . I like it

[АлексейПлющов-иы]

Hi from 2019!) Is tthere still a follow up video? Cant open the link, unfortunately...

[fallenIights]

Hi Marcus! Great video. Since java8course.com is not found, where can I find your follow up/in depth HashCode video?

[tnield]

Excellent tutorial! I learned a lot particularly about the hashcode optimizations you mentioned towards the end. What particularly do you dislike about IDE-generated hashcodes? Is it the misuse and devs not discriminating which properties to use? Is it the lack of hashcode optimization which you presented? All the above?

[dilipjain]

Hi Marcus Biel
superb video I got much knowledge
Thankyou :)

[avinmacha]

Hi Marcus, this was really informative; but i couldn't find the link to the followup video on your blog. Is there a direct link for the same please. Thank you !

[PaulusSh]

@Marcus,
Can you please explain, why hello1 and hello2 are equal, even though they are different Objects?

[suman]

First things first, cool stuff - more power to u Marcus.

In Car example following 2 checks :

if(obj == null){
return false;
}

if(getClass() != obj.getClass()){
return false;
}

Can be replaced with this single check right ?

if(!obj instanceOf Car){
return false; // as instanceOf takes care of null check too
}


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