Singleton Design Pattern Tutorial

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Welcome to my Singleton Design Pattern Tutorial. The Singleton pattern is both easy to understand as well as useful. I'll demonstrate first how to implement the Singleton pattern. Then I'll provide and example of its usefulness with a Scrabble example.
Threads can sometimes play havoc with this pattern, so I'll show you how to avoid those problems. I'll also review how to use threads, LinkedLists and more.

[@derekbanas]

Thank you :) I'll definitely cover JAX-RS in an upcoming tutorial. Thank you for the request. I've been holding off on Java networking stuff, but I'll cover it completely soon

[@derekbanas]

Your thanks is all I require. I have no interest in money anymore so don't donate money. If you think others would like the site tell them about it and I'll be happy :)

[@derekbanas]

Thank you for the input :) I'd love to expand on the different pattern flavors available and will very soon when I cover refactoring. This was a fun tutorial to make and I hope it helps people

[@derekbanas]

Thank you :) It is great to hear that I cleared everything up. That is the best compliment I could get.

[@derekbanas]

Thank you :) I greatly appreciate the compliments. I'll try to live up to them. I'm getting very close to doing something very good. I can't wait to cover both games and the math behind them while at the same time cover electronics on a deep level. This is very fun for me.

[@derekbanas]

Thank you very much :) I'm glad you found them useful. I will definitely cover enterprise design patterns, but I haven't covered JEE yet. I decided to make a truly complete java tutorial. i never figured it would take me 2 years to do it. At this rate I'll be making them for another 2 years since Android joined the tutorial.

[@derekbanas]

Thank you :) I'm not done with design patterns. I'll be looking at them further in my OOD and refactoring tutorial. Thank you for the request and I plan on covering that topic

[@derekbanas]

Thank you :) I used them as one source, but also a bunch of others as well as friends for this. I'm glad it helped

[@derekbanas]

Thank you :) I'm not very good at scrabble, but I may make this into a fully functional game. I figured you guys would know how to do that though. Many game tutorials are coming!

[@derekbanas]

Yes you are correct. I sometimes try to cover all of the bases in these tutorials. You got it though. Thanks for pointing that out :)

[@derekbanas]

Thank you :) That is very kind to say. Yes I need to cover some of the topics that demand more attention Java wise. May God bless you as well.

[@derekbanas]

You're very welcome :) I'm using Eclipse right now so I can't test it, but yes I'm pretty sure if you just delete the code block with if(firstThread) you won't any problems. Give it a try. There won't be any hidden errors, you'll see what happens immediately

[@derekbanas]

Thank you :) Yes, I'll cover Android as soon as I'm done with design patterns. Believe me the work you do here will make making android apps extremely easy

[@shibaramsahoo]

Thank you Derek for all your enthusiasm to put it together in a better way... We appreciate your hard work and your support. You Rock!! I just found your tutorials yesterday and been love with your website.. Great job!!

[@derekbanas]

Yes I was just trying to make a fun little program with it

[@derekbanas]

Thank you :) Believe me it makes me very happy to know I'm helping. Good luck on your test

[@derekbanas]

Thank you for pointing that out. I definitely need to make a Java Concurrency tutorial. Ill do my best to keep improving :)

[@derekbanas]

You're very welcome :) I made a little typo there. Sorry about that, but thank you for pointing it out

[@derekbanas]

You're welcome :) you use static classes when you are absolutely positive that every method will only be used in a utility type way. It is normally pretty easy to change a singleton if you require multiple instances down the road. That isn't true with a static class

[@derekbanas]

I use both eager and lazy initialization. I can tell you know the difference from your comment. I didn't bring up the difference because this tutorial is meant to be an easy to digest design patterns tutorial. I haven't really talked about the Java memory model yet, but I'll definitely bring this topic and others when I do.

[@derekbanas]

Thank you :) Yes I return firstInstance to make sure there is only one instance. Maybe I should have used a better name. Sorry about the confusion.

[@xipity]

Subject's explanation is crystal clear. Minor mistakes that mentioned in comments aren't that distracting. Thanks!

[@derekbanas]

I'm glad it helped

[@derekbanas]

Thank you :) That is the first time I've received credit for saving lives :D

[@derekbanas]

Thank you :) It is always nice to hear that I cleared a topic up. No, I don't teach anywhere but on youTube. I plan on staying here for a while and everything will always be free.

[@derekbanas]

Thank you very much :) I try to do the best I can

[@derekbanas]

Are you talking about the thread part? I ran both threads using the same code so there wasn't a need to write separate code for each thread.

[@Laura147HLY]

Love your tutorials, they are quick and concise, and to the point. Excellent!

[@derekbanas]

Laura147HLY Thank you very much :)

[@Karthik-yy6up]

Never thought about the problems that could arise with threading. This is a beautiful tutorial. Thank you Derek.

[@derekbanas]

+Karthik Iyengar Thank you :) I'm glad that it helped.

[@javaidbolaky7817]

Thump up man, your design pattern codes example was really really useful. Its 100% understandable than a reading a lot of books without really see how the design should be. Many Thanks for your dedication.

[@derekbanas]

Thank you for the input. I should have covered that here :)

[@derekbanas]

I'm glad I was able to help :)

[@nehaagrawal1730]

I came across your tutorials on youtube and I feel blessed that I have found you. Your tutorials are so helpful. God bless you!! Keep up the good work!! Please make some tutorials on Java Concurrency.

[@antoniszewczyk5943]

Hello Derek! Thanks for you tutorial again! What are the advantages and disadvantages of employing lazy instantiation? When should I use it and when it is not a good idea?

[@arisapostoloy7267]

Your lessons are irreplaceable and exciting. Do not miss any elements of your personality. Keep calm and educate people.

[@derekbanas]

Thank you for the nice compliment :) I'm happy it helped

[@derekbanas]

Wow I'd like to see your program! I'm very happy if I was able to help :)

[@derekbanas]

I'll have to look into that. Sorry about the error. They slip in every once in a while

[@derekbanas]

It is used just as a precaution. Understand that some things that I do, I do because I was taught that way. We all pick up bad habits on our journey. I tend to always stay cautious :)

[@QLTEQ]

Oh man, these are great. So much that I've seen is making more sense now.

[@derekbanas]

Sorry about that. I'm getting better at avoiding mistakes. Yes, thank you for the short cut. I know them, but I don't use them in the video because I thought they may confuse people

[@derekbanas]

I hope my tutorials were able to help. I wish you the best of luck

[@altapocalypse]

Liked a part about the threads, good explanation of the problem! Thanks

[@derekbanas]

Vadym Ko Thank you :)

[@sambitkabi4617]

And yes just one thing to add , you can also use the Bill Pugh's singleton implementation which avoids the double checked locking and the synchronization is handled by the JVM itself.

[@MrSontiny]

well, I really look forward to seeing the topic. Anyway, the tutorial is easy to understand for beginners like me. Thanks a lot. Gotta wait the tutorial that you mentioned above.

[@derekbanas]

You're very welcome :)

[@gopis1030]

This playlist is really helpfull ... Thank you so much Derek ...can u please tell some examples for singleton pattern

[@ankitajain4194]

Hi Derek nice explanation.Could you please also make one video of Singleton pattern using Enum and pros & cons of both approaches ?

[@Truthiness231]

Sitting here with my copy of Design Patterns (well trust and recommended), and have to say your coverage of usage is better. Thanks much ^.^

[@derekbanas]

I will soon, but I have my hands full at the moment with Android and C

[@sambitkabi4617]

Great one..The intention of Singleton pattern is very well fulfilled.But one thing I would like to suggest is that in multi-threaded environment we need to make the instance variable volatile. Making it volatile will prevent the re-ordering of the instructions and thereby avoiding undesired behavior.

[@heikomeyer2489]

The whole series on design patterns from you is awesome - thank you very much for your efforts on that! However in my opinion I think they would be twice as awesome if you would simply slow down - say about half the speed. No need to change anything else since the content is already great. Just give us 3 or 4 seconds every time you would otherwise flip to the next screen. For now I'm constantly doing play/pause and my mouse may quit service soon ;-)

[@derekbanas]

Thank you :) Sorry about the speed. I hope to revisit this topic soon.

[@fedecacharelli]

in the video you typed the following line: System.out.println("2nd Instance ID: " + System.identityHashCode(newInstance)); so, you were calling the HashCode of the first instance.. (that was obviously going to return the same ID number) but in the source code you bring you've fixed this.. However, just wanted to thank you because you really helped me with these videos! Thanks again, greets from Argentina!

[@rogerf.1758]

Hi Derek, really enjoy your tutorials. Helps an old C-programmer to get in to the modern design patterns. Though, just wanted to point out that it's not so good to use threads and this stuff directly. Where I work that is not allowed. We MUST use the concurrent packages to encapsulate all low level details. Perhaps that's a topic for some tutorials as well? Thaks for very understandable tutorials, I will see them all!

[@FatalRescue]

Whoa, I subscribed within the first 30 seconds....very cool presentation! You've saved lives :D

[@IbrahimAmer92]

no problem at all :) ... but your tutorials are still the best on the internet .You really helped me a lot your tutorials are much more better than any design pattern book . Waiting from you more videos and tutorials ;)

[@xiyuxie5215]

Hi Derek, thank you so much for such an excellent explanation. Could explain in more detail why the second Synchronized method on Singleton.class is preferable?

[@AlexTimification]

Thank you so much, I'm making a ChessGame, this tutorial was so friggin useful for me. Thank you so much one more time^^

[@derekbanas]

Thank you :)

[@sandhyaaa24]

superb way of explaining....about all the program line by line!!! Really sometimes its hard to understand why something is done in certain way ad evething is use in some purpose!!

[@derekbanas]

Thank you :) I'm glad it helped

[@gujaalsmanda]

great tutorial Derek i finally understood what each line does.

[@derekbanas]

+gujaalsmanda Thank you :) I'm glad it helped.

[@ajaisinghkhalsa]

Thanks Derek!!! Your tutorials are the best thing that happened to my life!! That was just some exaggeration, but they are pretty awesome.

[@derekbanas]

Ajai Khalsa You're very welcome :) I'm very happy that I can help

[@derekbanas]

Yes the code will only ever create one instance. Everything else is there to enforce that under any possible circumstance.

[@michaelbrooks6713]

Excellent lesson Derek, Thanks!!!!

[@derekbanas]

If everyone started off the day singing, just think how happy they'd be :)

[@pkamleshkr]

Derek you are really awesome :) Your tutorials are veryyyy gooood !!!

[@derekbanas]

+kamlesh kumar Thank you :) I try to do my best

[@markovukolic8444]

Thank you Derek. You've helped me a lot. So, Thread.currentThread() is not necessary?

[@sanjeevkumaremail]

Hi..Videos are really helpful. Can you cover how singleton handles de-serialization ?

[@dreamer6998]

hello, Derek. once again, thanks for the great tutorials! one quick question about this tutorial: is it necessary in the Singleton class to have this if (firstThread){...}? you mentioned when you typed them in that they were needed so that the other threads had chances to mess with them. in the end of the tutorial, you synchronized the instance.class to stop other threads to mess around, but how about just deleting those if(firstThread){...} code? thank you very much

[@derekbanas]

I mainly just write mods that blow stuff up and make characters and weapons. I use Minecraft Coder Pack and Mod Loader

[@derekbanas]

It would be more like an Inactive Figure :)

[@derekbanas]

I'm sorry. Some times I write out code to explain what is going on. I write much of this out of my head, so I tend to write code like I write sentences. Thread.sleep() says to put the current thread to sleep. So I was saying in code that I wanted the current thread and then I will put it to sleep. Sometimes by explaining to much I guess I can cause confusion. Sorry about that :)

[@arthurmee]

Excellent video Derek. Thank you very much.

[@derekbanas]

Thank you :) I'm glad it helped.

[@mdfarooq7145]

explanation is very neat and clean, Thank you Derek

[@derekbanas]

Thank you :)

[@derekbanas]

It may take a bit to get it. This is pretty high level programming. It definitely isn't a basic tutorial

[@ananth314]

Very good and informative video. Derek- I have a request. Can you please make some videos on Web Services? This is the area I couldnt follow even after reading so many books and articles.

[@XCasperChristensenX]

You are my hero! best supplement for my school book(head first design patterns) - i must say u describe the things better! keep it comming! :D

[@derekbanas]

+Casper Christensen Thank you for the nice compliment :) I'm glad they are helpful.

[@stevenh3574]

Thanks Derek, you break things down really well. I'm glad I found this video. Researching OO Design Patterns just got a bit easier. Are there any OO Design Patterns that are common with mobile app development?

[@derekbanas]

+Steven Harden Thank you :) I pretty much cover patterns you'll use in mobile development. I need to cover enterprise stuff, but that is on the server.

[@shogun301]

Derek! Great videos (all of them)...Do you by chance have anything about the Volley Library in Android..??

[@nicoapinedo]

I have to say, first of all, your tutorials are great, in this particular one there was only one section that had some issues and wasn't reflecting what you were stating during that part of the video (even though your statement was right, just not for that code). Around 11:00 the lines 21-27 for ScrabbleTest.java should be modified to: System.out.println("Instance 2 ID: " + System.identityHashCode(instanceTwo)); System.out.println(instanceTwo.getLetterList()); LikedList playerTwoTiles = instanceTwo.getTiles(7); System.out.println("Player 2: " + playerTwoTiles);

[@vijaykonduri1370]

To complete the circle you may want to stop cloning of ur object and also the serialization and deserialization as well.

[@opkarthik]

Nicely done

[@vaironxxrd]

They asked a lot about design patterns and oop concepts along with logic in my interview

[@chongliu9339]

thank you Derek , your video is great, helps me a lot!

[@derekbanas]

chong liu You're very welcome :)

[@discursion]

There is a mistake in your code when you are demonstrating that both "newInstance" and "instanceTwo" are sharing the same IdentityHashCode. In fact, you are printing twice the same object ID. I know it normally gives the same result, but you should add a note.

[@demetrioreylago6856]

Also the first player tiles are printed twice.... it looks like both players obtain the same set of letters which is not true.

[@RolopIsHere]

I noticed the same!

[@liedebunker1253]

You are right. He made a mistake, although he is very good.

[@omer8836]

Derek Banas buddy you tutorials are helpful but I guess in this tutorial you made a mistake, you should definitely look into it, you are printing out the same object identityHashCode(newInstance) and playerOneTiles

[@mikesaddi]

Great video! BTW, I noticed that you wanted to get 7 letters, but used "

[@derekbanas]

If you find it useful in your design use it. If not don't. What you'll find over time is that there are many people that like to argue about this stuff because they think it makes them look smart :)

[@stepho670]

Can you be my professor? The world needs more people like you!

[@derekbanas]

Thank you for the compliment :) I'm here for free when ever you need me

[@detectivedonaldkimball]

Derek I have some Qs: In Singleton class, in the method: "public LinkedList getLetterList(){ return firstInstance.letterList;}" Can you just return letterList instead of firstInstance.letterList? It seemed to work the same when I tried. Also, in this example couldn't we just make ScrabbleLetters a static field, and thus avoid having to create a singleton class? Finally, instead of having a nullcheck in getInstance method, can't we just instantiate the firstInstance field where it is declared? Would that also make it thread safe rather than using synchronized?

[@15bigdave]

I'm on mobile so I'm not sure if anyone has said anything. in your get tiles loop, it should be strictly less than instead of less than or equal to number of tiles. right now you're returning 8 tiles for getTiles(7).

[@derekbanas]

+15bigdave Thank you for pointing that out. I'll look at it

[@Lesterffx]

Thank you Derek I have learnt some useful methods which I never used before, such as Arrays.asList() and System.identityHashCode(). Also, I learn how to use the keyword/reserved word, synchronized.

[@derekbanas]

+Lester You're very welcome :) I'm glad it helped.

[@StefioaneCy]

Watched a video in a course about singleton. It was 10 minutes long. I watched it five times and couldn't understand how singleton pattern works. I watched your video and got it in 3 minutes. I don't know if you do, but you should be teaching!

[@TheSynergism]

Thank you... It was clearly explained..

[@rptechplex]

Nice explanation. Just an observation at 10:37, hashcode of instanceTwo had to be printed, for clarity sake, isnt it? :)

[@derekbanas]

Raghavendra Prakash Yes I'm using it to show that the object is the same

[@redsnakeintown]

Well you should have printed the hashcode of instanceTwo..you tried to print both places the same "newInstance"

[@kbrendarizo]

yes, that´s right, instancetwo isn´t used

[@abedrazem2260]

Thanks Derek!Very Nice tutorials! just want to ask can inherit Singleton class

[@derekbanas]

Abed Razem Your welcome :) you could, but once you have an instance of a singleton you won't be able to override the derived type

[@rajrocks89]

Hello @Derek Banas, I absolutely love all of your videos! Thanks for creating such great content. Is it possible for you to share the slides? Or, publish all the slides in an ebook format?

[@derekbanas]

Thank you very much :) Sorry, but I don't have the slides. I would gladly give them away for free

[@rajrocks89]

No problem!! I guess making notes instead will help me :P

[@markovukolic8444]

Derek, inside getLetersList method at 6:25, you are returning leterList object like this "return firstInstance.letterList" instead "retun letterList". Why? Is it beacuse you want to be sure that only one object have access? Thank you, you are fantastic.

[@obuseng87]

Thanks derek,u hv been really helpful.r u ever goin to cover android dev?

[@AnmolSingh-yf8ed]

Awesome videos mate! Thanks a bunch! :)

[@derekbanas]

Thank you very much :)

[@586naveen]

best explained...... got the concept of Singleton... thanks

[@derekbanas]

I'm happy it helped :)

[@smuralimohan1]

Thanks Derek for the energetic videos. I have a question related to the concept of Singleton. In a singleton, If we were to maintain only a single 'instance' of a class, why maintain an instance at all? Why can't we just make the constructor private and make the rest of all methods static? What is the advantage of creating a single instance versus using a (static) class with all static methods?

[@utsavprabhakar5072]

Love the question man. I'll think about this and get back to you. I think this has something to do with concurrency and simultanous use of class variables. I'll have to check. Thanks again :)))