--- title: Null Piece + refactoring - Implement Chess with Ruby for beginners slug: null-piece-refactoring-implement-chess-with-ruby-for-beginners published_at: 2021-06-03 17:00:07 +0000 updated_at: 2026-03-04 20:14:09 +0000 summary: description: In this series, you'll learn how to implement a simple terminal based Chess game with Ruby. In this episode, we finally talk about the null object pattern and create a NullPiece object that is a singleton for representing the absence of a piece. Playlist: https://www.youtube.com/playlist?list=PLS6F722u-R6LZxh7mvEUvZKshcH6KSNhr Code: https://replit.com/@CJAvilla/chess#main.rb tags: [Null object pattern, nil object, null object, nil object pattern, Ruby Singleton, Singleton pattern, Implementing Chess with Ruby for beginners] views: 660 author: CJ Avilla url: https://www.cjav.dev/videos/null-piece-refactoring-implement-chess-with-ruby-for-beginners youtube_url: https://www.youtube.com/watch?v=NfDUpT2U3b0 youtube_id: NfDUpT2U3b0 embed_url: https://www.youtube.com/embed/NfDUpT2U3b0 thumbnail_url: https://i.ytimg.com/vi/NfDUpT2U3b0/hqdefault.jpg type: video --- # Null Piece + refactoring - Implement Chess with Ruby for beginners *Published: June 03, 2021* *Views: 660* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=NfDUpT2U3b0) [![Null Piece + refactoring - Implement Chess with Ruby for beginners](https://i.ytimg.com/vi/NfDUpT2U3b0/hqdefault.jpg)](https://www.youtube.com/watch?v=NfDUpT2U3b0) ## Description In this series, you'll learn how to implement a simple terminal based Chess game with Ruby. In this episode, we finally talk about the null object pattern and create a NullPiece object that is a singleton for representing the absence of a piece. Playlist: https://www.youtube.com/playlist?list=PLS6F722u-R6LZxh7mvEUvZKshcH6KSNhr Code: https://replit.com/@CJAvilla/chess#main.rb ## Transcript what's up friend welcome back um thanks so much for sticking with me uh as we go through this process for building out an object-oriented version of chess in ruby in the previous episode we went through and added all the game logic for figuring out if we're in check or in checkmate or um and then we also added some logic to duplicate the board because we all we had we wanted to add this tool so that we could duplicate the board try a move see if we're in check and then bail out if we weren't in check so that that's what we were working on last time in this episode what i wanted to do was go back to our game class and improve our play method so that when someone is in check we tell them that they're in check and then the game or and then if the game is in checkmate then the game is over so right now we have a very simple game loop we just say like wow well not over so while the game is not over we're going to uh we're going to print out the board we're going to say whose turn it is we're going to take the turn and the taking the turn is like receiving in which piece they want to take pick up and where they want to place it and then we swap the player so we change who the current player is and then we keep going but there is no there is no like way to figure out who like is the game over yet so that's the thing that we want to figure out today is whether or not the game is over and i think what we want to do is we have reference to the current player so in here we could check to see like if the so we're going to swap the player down here and then if the the current players this is going to be a new current player at the end so after this turn happens so it's like let's say it's black's turn black makes a move then we're going to swap then it's going to be white's turn and then white is going to we're going to check to see if white is in checkmate and if white is in checkmate then it's going to be over so if um board dot check mate of current player dot color um i think so that's that's just like one way to say that something is over right like um that the current player is in checkmate the other way that it could be over is if we're in a stalemate so i don't actually know how does a stalemate work you don't have any um let's just say that this is kind of like where we'll draw the line for the functionality of our of our chess game is like it just goes until someone is in checkmate and then if you if you get to a point where there's a stalemate then you'll just have to know that it's a stalemate uh okay so then that was actually pretty fairly simple um so let's go back into our main class and let's like move our so right now we've sort of set up the board so that we can easily put black into checkmate um but let's move it around just a little bit so that um the rooks are in different positions and then we can kind of just move we can move the king around and try to get into checkmate so let's and let's start up our game here and again this is so like such a nice feature of being able to pass in the board into the game is we can sort of like set up the board how we want it before we start the game so that we don't have to play through the entire thing or every single scenario to figure out whether or not we're in check or checkmate at the end the other thing that i was thinking about doing was in the game when we when we do play right here what we could do is check to see like if um board dot in check of the current player.color then we could we could say like you're in check basically like um uh puts like um you are in check or maybe like the the actual color of the person uh or the of the player so current player.color is in check so it's going to say like it's white's turn white is in check or something like that right and then we'll say take turn swap player um i think that's it so uh where do we need to put the pieces so for thinking about the chess board if the king is in the corner let's move the king out of the corner a little bit let's put him at one one and then let's move the rook to like three two and four two so three two and four two and then let's run this and see if we can get a game printed out over here okay fantastic all right so we've got we've got our two rooks here and we have the king there and it's black's turn so black is this piece so if we picked up the black piece which is at one one one one and we tried to move him to zero let's actually try to move him to the right and we shouldn't be able to so we want to go to one one two it should say uh wait are we allowed oh we're allowed to do that because these are on three right oh wait no we should not be able to do that because we're moving into check okay so something is wrong something is wrong we should not have been able to move into check so when we do the move so on the board when we do the move what are we looking at so we're moving the piece so does the available if the available moves does not include the end position i wonder if we can use safe moves here now because um yeah so i think we can because we're using move bang inside of safe moves i think we might be able to use safe moves here without getting into an infinite loop so let's stop and then restart and we're going to go one one to one two so we're gonna restart the game and we're gonna say one one two one two and that should fail right great okay so we are not allowed to move into check now okay so that that was like a little bug that we had that we fixed there so we need to select a position that we want to move to and we can move to all of these so we are at 1 1. why don't we move to 2 2 0 here so we'll say 2 0 okay and now it's white's turn so white is going to move this rook and let's say we want to move the rook up here so i think the rook is at three where are we at three two so we're going to say three two wanna select three two and we want to move the rook let's move the rook to two two two two okay now it is black's turn and black is in check that's fantastic okay so now we're able to see it down here in the bottom we do see that black is in check great so we want to select the black piece which i think is at two zero we want to move it let's move it to three zero and okay it's the white white pieces turn so let's move this uh let's move this rook up one and then we'll move the king down then we'll move this rook way over yeah that should be fine so we want to go from four three so four three two oh so we didn't select a white piece so it's four two four two and we wanna go to three two and then we've moved up here okay great it's a black's turn black again is in check because we just moved here so that black is in check so let's move the black piece down which i think is at three zero we'll move to four zero and okay now it's white's turn let's move this piece from zero one two from two two two two zero i think that's allowed great okay so now we're here now it's black's turn black again is in check okay so if we move black to this position down here which i think so we're at two three four zero should pick up the black piece and then we want to move to five one that should be allowed okay and now what we want to do is move the yeah i think we want to move this rook over one this way so we're at 0 1 2 0 2 0 we want to move to 2 1 and then black is in check okay great black is not able to move let's move this piece back over this way and then we'll put them into checkmate so then we can go from [Music] what is this 0 1 2 3 4 5 1 2 5 0. okay it's whites or yeah it's the white piece is turned now we're going to like put him into checkmate so we're going to move this rook to here and then the king won't be able to get out because both both moves will be in checkmate i think so we want to move this piece from zero one two three uh is that three two i think two three zero and that's it oh gosh come on how anticlimactic right like the game just ended but it knows that uh it knows what checkmate is so if we go back to the game over here we probably want to say like uh print out the board one more time and then say like who the winner is so um puts game over and then we can say uh the winner is and then i think it's like the opposite color um so uh yeah let's actually just swap we'll swap the players back again uh so we don't have to like add the logic here for figuring out who the other player was that's that's that lost okay so i think that should work great um and yeah so maybe like for our for the grand finale here we can just put the king at well no yeah i think i think that's where we'll stop it it's just like it's kind of a pain to like move everything around one at a time like that so um now we have a working chess game right so this this totally works everything about this works uh well enough that you could actually go and play this with a friend right now we only have one concept of a player it's very simple class that just takes in the color and allows you to get the move from standard input with a comma separated list of like coordinates of where you want to pick the piece up and where you want to put it down but let's go through and make some improvements so let's do some refactoring now so in our main class here we have a bunch of requiring relative stuff and we're kind of like pulling in weird pulling in the pieces from like different places we have these pieces um we have a pieces dot rb file that just pulls in all the pieces but now we have like quite a few files that we're managing so the very first thing i want to do in terms of organization is add a new folder called pieces and now i want to move like the bishop into pieces and the king into pieces the knight into pieces the pawn into pieces and the like queen into pieces um the rook into pieces and steppable and slideable can probably also go into pieces and let's also move piece into pieces okay so now we have we have this pieces dot rb and pieces as a directory this is actually a common pattern that i've seen a lot where we have we'll have like a ruby class and its job is purely to import the the classes that are in some sub directory so here we want to just go in and add like the pieces directory here so we're going to say pieces slash pieces slash um okay that cleans it up just a bit and then we have our board renderer and we have a board class the board renderer really is actually just printing this is actually printing out we're using put statements here so an alternative that would be a little bit better would be to actually just return the string value and then allow the game to actually render things back the other thing that i've talked about a lot is the null object pattern so the null object pattern is is a tool for avoiding having to pass in um to pass around or to to check to do nil checking everywhere right so let's do some refactoring here for the nil the null object pattern instead of using null or nil on our board when we when we construct the board here right so we're setting up the board and this is initializing the array with an eight by eight array of nils right um but instead of nils what we want to do is pass in a special class whose responsibility is to represent the absence of something so here what we might do is add a new file called like the null piece dot rb and this is going to be a class just like every other piece class null piece except that um we can do special things on the null piece that represent the absence of a piece so if we look at knight right when you look at knight there's a couple methods that are implemented here moveders we've got 2s it's got it's including the steppable thing with available moves but this this 2s one is interesting because right now if you call 2s on the nil you get back the empty string which um right now in our board renderer when we're checking to see if the piece is nil we're printing out an empty string actually two spaces so what we want to do the very first thing we probably want to do is add a 2s here that will return just a space okay so that that'll represent like a space on the board that does not have a piece and then what we can do is go back into our board renderer and instead of checking to see if the piece is nil we should be able to just remove all of this and it should work just the same now the other thing that we need to do is when we actually construct the board rather than constructing it with nils here we want to pass in the null piece right we want to put pass in a null piece so there is a um rather than creating new instances of these nil null pieces are these the null ob typically with the null object pattern you will use a singleton object and a single tin will give you an instance so what is a singleton i think we can include this so we want our null piece to include the singleton and the way that this works is instead of calling dot new so we're not going to call dot new we're going to call dot instance and dot instance is going to give us back the one and only instance it's kind of um the singleton pattern is sometimes referred to as the highlander pattern if you've watched that show from the 90s where like the guys are fighting with swords um or the people are fighting with swords it's the uh there can only be one right like there's something about like the highlander where there's only ever one person who's like uh invincible or uh yeah immortal and um the same thing is true with a null piece right so we're gonna include singleton here which means that we want to use the instance of the null piece there's only going to be one in memory ever and that'll keep things nice and nice and snappy and convenient and then we can call nullpiece.instance here and then we should i think maybe okay uninitialized constants we've got to go back to our pieces class and import the null piece so we can do that here null piece okay notice the convention also that the name of the file the ruby file is snake case and then it this matches exactly what the class name is but the the class name is like uh can't like pascal case or something um okay we might need a library for this do we need a library for singleton singleton do we have to require yeah we do require singleton okay i think this is built in but it's probably just like not part of stuff that you get by by default so let's see if we have it in replit okay undefined method color for null piece great so now we can just uh wait where is this being called line 56 on board 56 on board do we actually want to check the color um [Music] color should the piece have a color so okay so instead of instead of worrying about this logic here what we want to do is worry about our pieces logic because pieces right now is rejecting pieces that are nil so we actually want to reject the pieces that are dot is a null piece i think and i believe that would get rid of that maybe peace on line 28 so peace on line 28 okay all right so now we don't have to check if it's nil anymore um and instead of having the color be black or white we can just go back to our null piece object and make a method def color and that'll just return nil which now we're just going to be comparing nil to black and nil to white but now we are able to play our game so this kind of refactoring is a little bit gnarly because we don't have any automated tests and in fact it's especially gnarly because this isn't even actually stored in inversion control or anything so um i've been recording this whole series on replit trusting that replace just gonna rock it and like be fine but um it's been good so far so let's just keep keep driving on so we've got um let's try to find any other places where we're checking for nil oh right so here instead of putting nil at that position we want to put a null piece so null piece dot instance nil okay is it empty so instead of checking is it nil here we want to check is it does it equal null piece dot instance i think that should work the same and then if king is nil meaning yeah we didn't find it so that one's a good nil okay that's the only nil that's left in board what about game uh nil let's see all right so here we no longer need this nil check and recall that we had a bug where we weren't checking to see if it was nil and so we had an issue there before we no longer need that nil start position is nil that's the only one that's left there pieces player great okay so we are we're looking good so that is the null object pattern that's a handy pattern that you can use anytime that you want a class to represent the absence of something okay i'm trying to recall if there were any other major refactorings that we wanted to do let's go through and just take a look at all of our code here and see if there's any ways that we can tighten this up a bit so we have the start chess method here and we're iterating over the board and we're creating all of our pawns and then here we're saying rook rook knight knight bishop bishop king queen king queen so i'm trying to think of ways that we might be able to clean this up one thing that comes to mind is rather than like having all the initialization on one line i wonder if we can make this simpler by just having a list of classes so like rook um knight uh bishop because we can just pass the class name right and now we can work with the classes directly um and that that's really like one row sort of of the pieces and then we can say each do class um when you're creating a a variable that refers to a class like a in a class an instance of a class like this or the class name directly sometimes we use this k class um by convention i actually saw this over here right in singleton in their example they called their class class with a k which is kind of cool um so we're going to iterate over all of these classes and then i think um we need to do like uh zero zero or seven or maybe we need the same the same sort of thing where we get like the row and column um so that should give us like the back the black row and the white row for each of these classes and then i think we might be able to do something like class.new or like let's see board at um so we need we need to know the the column and the column is going to be defined by the index of the class so we can say each with index this will give us the class and the column and we'll make this row and we're going to say board at row column is equal to class or maybe we call it like piece class dot new and then we need to pass in actually yeah let's make it location is equal to this thing just trying to make it clear like so if anyone came back to this and was trying to read through it they would see that this refactoring really cleaned it up and made it easy to read and we want to pass in the board and we want to pass in the location and we want to pass in the color so i think that might actually work instead of all of this stuff and then if we go back to our main class and we comment out our test board there and we make a new b equals board dot start chess and then we run it we should see like a fully set up thing all right peace on line 28 peace on line 28 we got a bug we've got a bug piece on line 28 if the board location's color okay so somewhere we're still adding nils to somewhere we are still adding nils pawn in available moves so pawn pawn in available moves board dot empty hmm so in checkmate in check each the block so in check gosh it looks like it looks like we do we're still getting nils that are slipping in here somewhere so oh you know what it is is when we're duplicating the board um we did not update the board duplication logic to work with nil yeah we didn't update this to work with the nil pieces but board dot new board dot new should have initialized the array with null piece objects hmm okay what where did this come from in available moves on pawn so we're on pawn and we're in available moves and we're on line 29 on line 29 we're checking to see if it's an enemy at the diagonal left is it an enemy at the diagonal left an enemy is a method on piece and here we're checking okay let's just print out um uh board at location because it should not be nil it should be the null piece instance ah what why do we have an extra one done here that's whack okay p oh you know what p location i wonder if we need that null check because it's out of bounds five eight is out of bounds all right so where we're checking enemy um let's say let's just check to make sure that it's on the location or it's like in inbound so board in bounds location and the color is not the same color okay great all right so we were able to print this out and it looks like yep our king and our queen are across from each other so that looks cool um so that was kind of a fun that's kind of a fun refactoring like we got rid of these lines by doing this fancy sort of loop over all of the different classes here let's see if we can get these all on one line yeah that's that's neat that's neat okay cool i can't remember where i saw this sort of pattern all right what else what other refactorings could we potentially make to clean this up a bit so this one this one is interesting right where the row is less than the grid length and greater than the grid length i think that there is a method between that we could use to clean this up so let's see if we go to irb is is i wonder if is it i think it might be on number between uh 0 and 5. is 2 between 0 and 5 cool is 2 between 0 and 1 false is 2 between 0 and 2 true okay so i think we might be able to say something like row and column dot all are um [Music] they're all between the grid.length and grid.first.link okay so this ended up being longer or like less clear maybe than this one so i think i'm just going to keep the second one there all right the other thing here okay that's good that's good uh empty okay we've got our instance fantastic okay all right we're moving our piece i'm gonna remove some of these comments from the bottom just to clean things up our pieces are organized into a pieces directory we have these pieces this top level pieces thing we covered a ton of a ton of concepts here so in the board class i'm just going to like do a quick recap of everything we talked about we talked about the factory method pattern we talked about these macros and what adder reader writer assessor do we talked about how to plan out your object-oriented code if you're using like the noun is the classes and the verbs or the methods that's like one approach we also talked about the messages that you're sending between things being the methods and then the actors and the systems being sort of secondary to those messages we talked about operator overloading we talked about the null object pattern we talked about the singleton pattern which is a design pattern here where we have uh this null piece instance um we talked about initializing instances of nested arrays but uh with a specific number of elements we talked about using question marks and methods as a convention for returning a boolean value we did dig in quite a bit here to we talked about yeah the flatten method for flattening out nested arrays into a single a single flat array we talked about creating your own error instance or like your own error classes so that you can handle those we also went into we went into the the concept of duplicating or deep duplication of objects so that we could create a new board and temporarily like attempt to make a move to see if we ended up in check that was kind of fun what else do we got here we talked about dependency injection with our game right at the very top rather than using the board class directly in our initialize method we're taking in the board the players and the renderer so and this is actually passing in the class for the renderer which we're using to initialize a new instance of the renderer so we can print stuff out we talked about keeping your instance variables in the initialize method like the actual at sign versions of those and instead using the adder reader writer assessors so that we can use bare words and it makes it easier to refactor in the future if we wanted to extract these into more [Music] sophisticated methods we looked at this swap player method where we're doing a ternary operation to check like who the current player is and then returning either one or the other what else did we do uh we yeah we got we got some user input and we were doing a little bit of validation we printed out some things um let's see within our player class this is where we were doing get s or get the string from the terminal we talked about modules so with with our pieces like queen which is slidable we talked about including the slidable module which again is similar to a class but you can't actually initialize an instance of this but we did want a method that was shared across several several classes and so we defined it inside of a module here that we called available moves and then what we did on the slideable one is we like sort of uh iterated and grew the possible moves or the available moves in a certain direction until we either ran off the board or ran into an enemy or whatever um what else did we dig into here uh we used it we used a ternary here also for rendering out symbols we also talked about the symbol versus the string and why we would use a symbol and that's because we don't want to reallocate instances of new strings every time we go technically you can use like the dot freeze so we could do like black here dot freeze or something that that would achieve the same thing but i like using symbols so that's what we ended up doing there what else did we dig into uh gosh there's just so much to this the pawn was actually surprisingly complex right we we figured out our available moves but those were sort of dependent on whether it's at the start and the direction of the pawn and then we can you know you can move one forward or you can move two forward if there's nothing in front of you um and we also talked about like building up the moves for uh if you if there's an enemy this method here is particularly long i would probably go back and refactor this to break this out and to like get give me just the forward moves and then combine those with like the diagonal moves or something and then we yeah we just selected the ones that were inbound there we talked about yeah we talked a little bit about inheritance so we have this parent class piece that all of our our specific concrete classes inherit from um and yeah i think i think i'm gonna wrap it there we've got a working chess game and i'm really happy hopefully you learned a ton um yeah if you're if you're still just learning ruby and uh you found this useful i'd really appreciate a like maybe even a subscribe if you will and i do i do like to publish videos about web development with ruby and ruby on rails so if this was something that you found useful do consider subscribing i plan to put out more content around web development and i do especially love ruby so i think that's where i'll call it thank you so much for sticking with me through this whole thing i really appreciate your time and attention until next time i'll see you [Music] later --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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