You are the exact reason why I continue to make these videos. Thousands of people have taken their time to write the kindest messages to me. When I was making terrible videos many of you reached out kindly and taught me how to get better. I wouldn't be doing this without your kindness and help. I'll do my best to live up to the compliments I receive :)

11 years later, your tutorials are still the best the internet can offer. These 26 bite-sized lectures are a real masterclass. Thank you so much.

Thanks for the video! It explained the concept very simply and clearly.

To be honest you explain codes and DP in the best way! thanks a lot.

Thank you @Derek. Pretty much better than my lecturer.

Thank you Derek! You are AWESOME!

I've read a number of different design patterns books, I've sat through University Lectures and now I sit here studying for my Object Oriented Software Design exam by watching your Design Pattern videos. In each video, you have explained the concepts better than every text and every lecture. You deserve an award. If you accept paypal donations, I will gladly donate.

best to the topic I have ever seen / thank you

Your voice is excellent! Like a British Broadcaster named Jerimy Clarkson.

Thanks ..very nicely explained with a very simple example to understand.

This was super helpful!

its 2022 and still your videos are the best 🙂

Really nice video! I think I've used things like this before without standard names like accept and so on, but never knew the name.

Really Wonderful example to better understand this pattern, Thanks a lot

excellent visitor explanation

A very good explaining of visitor design pattern

I tried at least 5 other sources to understand this design pattern but I failed. Finally, I got it by watching only half of your video while I never did java. Really good job.

Your video's are a godsend

Awesome video, super helpful and concise. Ur a G Derek Bannanas .

Superb awesome explanation You really helped me a lot Thanks feels very tiny word for your work

very well made video. thank you

Thanks for your tutorial.

Must have a donate button !!!!

Very helpful! Thank you

Your words @10:46, " I also don't know, how much cigars cost?" just a beautiful message hidden behind those words. [good message to not to do this] Kudos, and I followed all your DP tutorials and all are worth. Thanks @Darek Banas for making an positive impact to the society. :) :)

Hi Derek! Just wanted to let you know, I passed my Design Pattern course. Because of who? You! Thnx!

You're very welcome :) When I make tutorials I use normally the top 3 to 5 books and at least that many online tutorials along with notes I've taken over the years. I figure that way I'll be certain to cover everything. The GOF book is considered to be the best book on patterns, but it definitely isn't the most useable so I use the best parts of every book. I'm glad it helped

Your videos on design patterns are simply the best!!!!

Very well explained. I heard before coming to this video that this was a potential solution to instanceof being used in code (which I have seen a lot of). I will try to implement it using the concepts you have passed across.

You are great! Thank you sooo much... youve got a fan in germany now :)

THANK YOU! was confusing learning this in class. Really needed a more "real life" example like this video provides! great job.

Great tutorials

Super clear explanation thanks :)

this was extremly helpfull

the clearest video i have ever watched on visitor pattern

really you are doing great help...i learned much better...thank you very much for your great help..:)

Thanks. Useful video

2 words "thank you"

Thank you :) I do my best to cover topics completely that are normally not paid their do respect.

Thank you very much :) For some reason this is a topic that is ignored. I'm happy that people are getting something out of them

Brilliant!

Excellent, thank you very much

thanks a lot very very useful video ;

I agree that I shouldn't and don't do that in the real world. I did it in these videos because I thought it distracted from the code and I had previously mentioned what it was for. Sorry about that. I have learned my lesson.

I LOVE YOU DEREK

Great video!

Thank you!

Such a nice video series!! Plus you have such a cute voice :)

Thanks man, very useful!

I'm very boring in real life. I just write programs all day

You are awesome.. This Visitor pattern, you could only clarify!

this is the simplest explanation of this pattern I've ever seen until now. Thank you.

really great.. !!! Thank you...

Digged my way through the whole series. Several saturdays well spend! Amazing how I only discovered this channel 10 years after this groundbreaking series was first publisched. I remember from those days good tutorials were scarce and weren't always very easy on the ear (not you, Kahn Academy). With the poll at the end it struck me how much of it you did right, despite probably having very little to go on, and figuring things out as you went along.

This is normally used when you want to add functionality, but can't change the code much at all. You can also slap on a bunch of custom methods depending on the class. I made the example simple, but it can get much more complicated if you implement many interfaces. I hope that helps

You're very welcome :) Ill get back into web development and cover those topics, but I have to cover android and game development first. Sorry, but I held a vote and that is what won

Hi, just found out these videos 10 years after. They are the best explanation I've seen for GOF design patterns! Thank you for making this.

Fantastic video, it finally makes sense =)

Hey, I just wanted to let you know that these tutorials were amazing and made it very easy to understand these design patterns. Thank you!

Loved the video :)

10yr old video and helped me a lot Derek. Glad that I was already a sub and found this video from you, you always have the best examples!

Hey Derek. I've just finished your entire design patterns series. Thank you for this. They covered this stuff in university but I finally feel like I get it now.

Very kind

Quite clear.

Hi Derek, Since I didn't know about this pattern before my gut reaction was as follows: Make a class called Necessities and let Liquor, Cigar and Milk extend that. Make another class called Taxes and add GetTaxedPriceFor(Necessities necessities, Date date). This method would take a commodity and the time and returns the corresponding tax for that. This way I could make a call like the following: Taxes.GetTaxedPriceFor(Milk,22/5/2017); Now, here I understand that every time you need to change tax calculations and you want to add new Necessities to the switch you need to open Taxes class and place a bunch of code in there. Soon you'll be hitting so many lines... Oh my god! (Edit: As an addition perhaps it's even better to assign types to necessities so the switch doesn't grow out of proportion and can return the taxed value for all foods for example.) In the Visitor pattern however you'll need to expand the TaxVisitor to handle this situation instead. The more implementations of Necessities you have the more you'll need to handle in the TaxVisitor. So what is the benefit here? Why should I use the Visitor pattern instead? I understand that I'm missing the point here so I kindly ask for your help. (Or any of the commenters for that matter.)

really cool. thanks!

thank you!

You're very welcome :) Yes you could do that, but the goal at least in this tutorial is to make everything an interface over an abstract class if at all possible. The pattern is also defined that way. There are always tradeoffs that you'll have to make decisions on depending on your situation. I always plan on needing to make changes in the future so I use interfaces whenever I can. (Just a personal opinion)

I know this was from forever ago, but I just want to say your style of videos is extremely informing and easy to follow. Well done!

LEGEND

Thank you very much :) I'm not very well known. I doubt the Khan Academy even knows I exist. Mr. Khan was definitely an inspiration for us all!

You're very welcome :) I'm very happy that this series has helped

I doubt many people would disagree with that :) I also don't vote for either party

Godsend video for sure, where did you learn how to do all this? are you just good at reading technical textbooks? or did you have a really good teacher? or how did this happen?, I'm always curious like how things came to be

Yes they will more then likely follow the C tutorial. You'll be amazed by how simple it is

The visitor pattern is used when you have an interface and many subclasses that implement it. What happens when you want to add a function to all of the subclasses while causing minimal changes to the interface? By using the visitor pattern you can change the interface once and then allow an unlimited number of changes in the future to any class that implements it. Does that make sense? It is the ultimate in flexibility!

BOTH!

Hi Sir, I just went through your video. It is very simple and straight forward. I have a question on this content. In client.cs Instead of calling milk.accept(taxCalc); Why cant we write TaxCalc.Visit(milk); I don't understand why we need an IVisitable interface and the accept method at all.

You're very welcome :) I'm sorry, but I don't know of such a source. There have only ever been 3 books written on refactoring ever as far as I know. This is a topic that has been kind of ignored for some reason? I plan on adding quiz functionality to my site and I'll do my best to drive home the power of each pattern when I do that

The thing I don't like about this pattern is that it needs an a really good explanation in order to understand it.

Had no idea how this design pattern worked ... until I saw your video! Great explanation, thanks a lot!

Hi Derek, What about Object Pool, Private Class Data, and Null Object design patterns? BTW, great job! I love your tutorial. Although, you could work a bit on your UML diagrams to stick more to the convention. ;) Regards, Andrzej

I'll incorporate DPs into my next series. Thank you for the request :)

I can't do both because they are both big subjects. Voting starts next week!

Why it is important to have a accept method within each of item? For example: Can't we directly call taxCalc.visit(milk) instead of milk.accept(taxCalc)? In that way, we absolutely have to make no change in the item object.

Thank you :) Im glad you liked it

I followed a course called Object Orientation last semester , and to be honest, I've learned just as much about it watching and implementing your tutorials in the past 3 days as I did in the course which lasted like 4 months..

the legend

Thank you :) Im happy that you have liked them. So, your question is how do you implement this pattern in C++?

I'm watching a 7 year old video because design patterns never gets old or get updated.

Hi there! Just wanted to say a big thanks and thumbs up for all of your design pattern video's! They are much more fun to follow than reading my book, and are often also much clearer. I also love the originality of your examples to find a good real-life illustration, it makes memorizing the patterns much easier. You changed them from boring theoretical lessons to some pretty amazing things for me! Since you mention that you like to hear feedback, I'll mention that I found the examples using a non-standard libraries (like Swing) a tiny bit harder to understand and memorize, because I don't have the necessary experience yet to understand them right-off. But it's a tiny detail and off course those video's were still very helpful. Cheers, a lot!

Which ever side wins will get their tutorial. Hopefully this won't divide everyone, but I can only pick one or the other to do next and do them the right way. I want the next tutorial to be great and perfect. That can only happen if I spend 6 months putting it together. I'll honor which ever group gets the most votes :)

Thank you so so so much for all these videos I 'm currently in a large scale software development course and my professor has spent weeks on explaining some patterns and i've still been lost and in a couple 15 min videos you cleared up a lot Thanks again!

Yes, but right and wrong don't seem to matter in universities. The GOF book states that design patterns are but a guide to work with and not an iron clad recipe.

I just watched all of your "Design Pattern" Videos, it was so helpful. I really appreciate to making this series. Thanks Derek.

more clear than my IT prof. thanks Derek!!

A pattern which seemed very difficult in the Gang of Four, became very clear thanks to your video!!! Love your style...

Hello Derek, thank you for this video. It looks to me that the "Visitable" Interface is unnecessary. With the bellow I can calculate the taxes and I don't need to change the Necessity, Liquor and Tobacco classes. // ... create milk, vodka and cigars objects here... TaxVisitor taxCalc = new TaxVisitor(); double milkPriceWithTax = taxCalc.visit(milk); double vodkaPriceWithTax = taxCalc.visit(vodka); double cigarsPriceWithTax = taxCalc.visit(cigars); Why using this pattern then?

I'll put up the voting page next week on my website. Voting will be open for a week. Thank you for your interest :)

Excellent and simple and I love how you showed the 'with and without' part