Instructor (Mehran Sahami): Howdy! So welcome back to yet another fun-filled, exciting day of CS106a. Weíre getting close to that Thanksgiving recess, which is always a good time. In the days of yore, it used not be a whole week. It used to be you got like one or two days off. You got like Thursday and Friday, which means you would have gotten only one day off from this class and now you get a whole week to mellow in style or catch up on all your other work as the case may be.
So thereís one handout today. Itís Assignment No. 6, which is the next assignment, name, circle, watch. They talk a lot about that assignment in this class today. As a matter of fact, weíre very fortunate for coming today because I will give you so much gratuitous information that presents that assignment. Itís just almost laughable.
And then Assignment No. 5, as you know, is due today, so we can do painful to see how much time it actually took you. Just wondering, I heard from my little sources that exist was that last night, a lot of people were at the Lair and the computers at the Lair. Just wondering how many people were at the Lair last night. A fair number. And some of the computers at the Lair just mysteriously didnít have Eclipse anymore. Anyone run into that problem? One person, okay.
Yeah, there was some reimaging that went on, which I had nothing to do with and didnít know about until some of the section leaders said, hey, Eclipse is missing on these machines and, you know, so I sent a very anxious email, I should say, to academic computing, and there had been some mix up and Eclipse is now reinstalled on all the machines, so Iím glad it didnít affect too many people, but if it affected you, I apologize, although we really didnít have anything to do with it, but Iím sorry that you had to experience that pain.
So in terms of the painful, how much time it actually should give you the assignment on the Yahtzee assignment? Just wondering, how many of you actually liked the Yahtzee assignment? Just a quick show of hands. And how many thought it was just more pain than it was worth? Anyone? You folks, all right. And thatís good to know. I always try to Ė actually try to gauge these things.
So anyone in the zero to two-hour category? You just did it and it worked and life was good. Two to four? A couple people; thatís always good to see. Four to six? A reasonable contingent. Six to eight? Good times. Eight to ten? Wow, pretty healthy distribution there. Ten to twelve? Okay, weíre falling off a bit. Twelve to fourteen? You folks. Fourteen to sixteen? Maybe a couple. Sixteen to eighteen? Eighteen plus? Taking a late day? Yeah.
As you may have read Ė I should refresh your memory because it pertains to Assignment No. 6, but as you may have read on Assignment Ė on Handout No. 1 at the very beginning of class, because Assignment No. 7 is due the last day of the quarter, you canít use late days on that assignment. So if you have late days and youíre like, hey, I was just saving them up for No. 7, well, sevenís due the last day so thereís no classes beyond seven and you donít say, hey, Iím taking a late day. That gives me an infinite amount of time to turn it in because there are no more classes. No, it doesnít work like that.
So Assignment No. 7 you canít use late days on. So again, I would discourage you from using your late days, but should you have them and youíre like, hey, I really wanna use my late days, Assignment No. 6 is kind of your last chance to actually use your free late days. Again, not that I would encourage you to use them, but just so you know up here that you donít get Assignment 7 and say, hey, I thought I could use my late days for this. Youíre getting plenty of advanced warning that you actually canít.
One other quick announcement; CS Career Panel is today, so after today, Iíll stop mentioning it because I believe most of you canít actually time travel, so today, 5:30 p.m. in Packard 101. Be there. We have a stellar panel of folks from academia, from industry, a lot of people who actually Ė every one is a computer science major and four of the five people graduated from Stanfordís department. The fifth is actually a faculty member here that didnít graduate from my department, but is a faculty member here.
And youíll actually see thereís a very broad range of stuff to do. Out of those five people, though, actually, I think only one, maybe one and a half who actually do programming as their sort of fulltime job. But all of them are computer science undergraduates, so you get sort of a breadth of what computer sciences can do.
So Iím so happy to see the fifth region. Youíll see the variance gets larger, right. As the quarter goes on, that kind of happens, but itís good to see that the mean is still where we generally want it to be.
So now itís time to talk a little bit about your next assignment. You just turned in your last assignment. Youíre like, no, no, itís not time to talk about the next assignment. Iím still working on the last assignment. Well, just pay attention.
Hereís what youíre going to do for your next assignment. Itís a little program called NameSurfer, okay? And the idea of this program is it turns out that the government actually keeps track of statistics of the popularity of names over time, okay. So every decade when they do the census, they kind of keep track of the number of names and they rank the top 1,000 names in terms of popularity for boysí names and for girlsí names, and they actually make this data available. We thought wouldnít it be cool if you actually were to display names to the user in terms of how that popularity changed over time.
So thatís what NameSurfer does. Hereís the program. What it does is it shows you the decades. You can notice that this is a graphics, or involves graphics, right, has the decades from 1900 up to 2000 and so relatively recent date up to the last census. And what we can do is we can type in a name and then click graph, and what it will do is it will show the rank of that name over time, on a decade-by-decade basis. So letís try Fred, for example. If you graph Fred, you can see Fred real popular at the turn of the prior century and then Fred just had this quick decent into oblivion, where now heís the 974th most popular boys name, at least in the last Census 2000. We can do multiple graphs on here, so if I type in someone else Ė so one thing thatís also interesting is youíll notice popular culture comes in, and so if I type in Britney, the alternative non-traditional British spelling, it didnít exist until 1980, in which case it totally shot up to like the top 100 and maybe with the most recent things that are going on, itís dropped a little bit.
But through this whole region, what we have in our data, and Iíll show you the data in just a second, is that Britney did not rank in the top 1,000 until the 1980ís and then it jumped up. Because it started ranking in the top 1,000 in the 1980ís, it still shows up in our data file because at some point it started ranking higher than that. And it turns out that there are some people who just have names that, well, are not so popular. So we do Mehran and nothing comes up because contrary to popular opinion, Mehran is not in the top 1,000 most popular names. In a few years, you can change that if you have children. Your choice. Not that I would advocate it, man. Itís a rough childhood. Mehran, doesnít that sound like moron? Yeah, I havenít heard that one in the last two hours. Yeah, I know. Iím just Ė rough childhood. Not that it affected me at all. So in terms of actually doing this assignment, you might say, oh, thatís kind of interesting. Now there are a couple of things going on. You might look at that and say, yeah, I could do that, Mehran. I could do that in the zero to two-hour category, right, except for one little thing and the one little thing is check out this bad boy. Whoa. As I resize the window Ė I will continue to resize the window until I get you in a hypnotic trance Ė everything in the graphics display automatically resizes to display for that window size, okay? And what youíre going to find out today is how do you actually do this kind of thing to keep track of as the windows resizing, how do you get events that come to your program that tell you, hey, some resizing happened. I need you to redisplay whatís going on.
The other thing thatís interesting about this is I cannot only when I resize, it keeps track of all the information thatís on there, Fred and Britney. If I ask someone else like, oh, my son, William, whoís always been a popular choice Ė ah ha ha Ė just like in the top ten dips briefly over here because maybe it was getting over used and then it comes back up. Keeps them all. Theyíre in different colors. The color sort of cycles, so if we add someone else, like Ethel Ė Ether, Ethel Ė yeah, Ethel, sorry, not so popular anymore. When Lucille Ball Ė when ďI Love LucyĒ was on or earlier, it was a popular time. The cycles of colors eventually will go back to black, so if we put in something like Bob, it goes back to black. Then youíll see red again, then blue, then magenta. Thatís all given to you in the handout.
If you hit clear, everything goes away, except for the gridlines, and you can sort of go again from there, like hereís George and you can see George has a slight decline. Hereís four back here and now heís like somewhere in the hundreds. Itís okay if it goes off the window occasionally, okay? But the resizing is kind of the key to this and thatís what weíre going to Ė one of the things weíre gonna look at today. Okay.
So thatís what you want to keep in mind for the assignment, resizing, because the demo thatís on the website because it runs in a web browser, doesnít resize and it sort just has it as the little warning down at the bottom. It just shows you how the program actually works so you can try out typing in different names and see the behavior. But it doesnít resize because its size it stuck by the browser and changing the browser window doesnít actually change the size of the applet. Thatís just life in the city, okay? But thatís the way NameSurfer works.
So what we wanna think about is actually building programs that have this kind of property and to understand how programs that have this kind of property work, what we need to think about is something known as a component and related to that is a container. So first of all, Iíll show you all the classes in Java that pertain to the components and containers. Not all of them, but a good number of them. So hereís the classes in the component hierarchy and some of these will look familiar to you. So programs of which Console program and Graphics program that youíve been using the whole time are j-applet, which is an applet, which is a panel, which is a container, which is a component.
So somewhere all the way up the food chain, right, the food chain before we didnít go that high. We just went up to applet when we showed you sort of this picture before. Turns out all your programs are components and containers. Every component is a container, or every container is a component. And so you may ask, well, what are containers and components? So the way you wanna think about this is at some generic level, a component is anything that can appear in a window. Itís anything that can in some sense be displayed. So your programs are appearing in a window thatís getting displayed. Thatís why theyíre components, okay? Now, that also means that a graphics program and a console program are also components because theyíre subclasses of program. Now at the same time, thereís a notion of a container. Whatís a container? A container is simply something that contains other components, so a container just contains a set of components and you might say, hey, Mehran, thatís really weird. A container is a component, but youíre telling me it contains a set of components? Whatís that all about? And the simplest way to think about it is itís this, okay? This is a component, this bag. It turns out this bag is also a container because it contains things and itís perfectly fine for something to be a container and also to be a component.
So hereís what I mean, right. Your programs, as it turns out, are all components, which means they display in the screen, but theyíre also containers because your programs contain other things. For example, in the display of your program, it may contain a button. Right? It may also contain, say, a text field. And the other thing that it may contain, if you have a graphics program besides these kind of components, which buttons and text fields are also components Ė Iíll show you an example of that in just a second Ė it may contain another container. And itís perfectly fine for containers to contain containers. As a matter of fact, youíve been using them for the whole time. Your graphics program, especially if you have a graphics program that has some interactors on it. It could have buttons. It could have text fields. It could have a G-canvas, and gee, guess what? A G-canvas is a container and a component. Because itís a component, it can be contained inside some other container, and because it itself is a container, it may have certain things in it.
Oh, like hereís our friend, the frog image from the mid-term. Yeah, that was just something that was in another container, okay? So thereís no reason why something that is a container canít be a component, which means it can be contained in some other container, thatís fine, and all containers themselves are components, so they can be stuck in somewhere else. Thatís the basic idea. You should think of it as bags and you can put bags inside of other bags, and bags contain things. So even though you see the hierarchy this way, the concept itself is not actually that difficult. Now, the thing you wanna think about this is I just mentioned to you your programs are actually components and containers. G-canvas is a container and a component, but components that we talked about before, j-components, like our friends, say, oh, j-button Ė you actually saw this slide a couple classes ago, j-button or j-label or j-combo box, all these things you saw before Ė are actually j-components. They all are subclasses of j-component, which means by being subclasses of j-component, they were also containers, strangely enough, and they are also components. We just think of them generally as components and we try to put them in other containers, like we put them inside the display for a program and thatís its container. But thatís the general idea thatís kinda going on here. So when you wanna think about containers and components, what you also wanna think about is that these components themselves, before we thought about like action listeners, right, like something happens like I click on some component and some particular event happens, like I clicked on it, something like that. We can think of the whole component as also listening for other events because a component is something that display stuff on the screen.
Stuff can happen to be components, like they can be resized and when a componentís resized, some event can get generated and sent to that component that says, hey, guess what? You got resized. You need to do something. Like the canvas that shows all the graphics, it can get some event that says, hey, canvas, yeah, you were just resized. Do whatever you wanna do. Up until now, you didnít do anything. Someone came and resized your canvas. You said, yeah, youíre resizing my canvas. All of the pieces I have in my canvas stay where they are. Nothingís changing. Now what youíre going to get is the ability to have the canvas be told, hey, you got resized or something else happened to you. You can respond to it however you want, which means you can redraw everything thatís in your canvas, so that, for example, you can have all the layout come out just fine when people resize your window. So what does that actually mean in terms of these components and containers? What it means is that we can have listeners in the same way that you kinda thought about listeners from mouse events and listeners for j-components. You can have listeners for components, okay, like we talked about resizing the window.
Now, hereís a little simple example of something that can go on with one of these components. So Iíll come back to Eclipse, Iíll run a little program here Ė I gotta compile Ė and Iíll show you the very sort of simple program that makes use of this concept of a component having a listener and being able to get certain events. So itís just called My Programming. Hereís what my program does. It draws a square. Thatís all it does is it draws in a filled-in square. Youíre like why are you showing me this, Mehran? I couldíve done this in the first week of class and I would say, no, you were doing terrible in the first week of class. And then you say, no, I couldíve done this in the second week of class and be like, okay, but could you do this? Oh. Yeah, when you did this before, that square just stayed where it was and the size of the window changed. Nothing happened. Now the square always remains in the center of the window no matter how I resize. Itís super small. Itís still in the center. Super big, yeah. Another one of those things good for about 20 seconds. But thatís the idea. How does it know when the windowís getting resized that it needs to do something to update its display? So the way this is going to work, in terms of having this square that recenters itself, is we wanna think about having a program that is a container. And so, up until now, you had console programs and graphics programs, and they were containers and what they did for you automatically for console programming, it created a console for you and put it in your container so that anything you printed showed up in that console.
And for a graphics program, it created a G-canvas for you and put it in the container and anything you drew showed up on that canvas. And now, what youíre going to do is youíre going to create your own program thatís not a graphics program or a console program, but to which youíre going to create your own canvas and add it. And so the way that would look is you would say something like ďPublic ClassĒ and give it the name, whatever name you want, so I call this ďMy ProgramĒ and this is gonna extend just the generic program. Itís not gonna extend console or graphics program. Itís just gonna extend program. And so inside here, itís still gonna have some init method, like youíve come to know and love in other programs. And inside init, what weíre gonna do is say Iím gonna create a new canvas. But the canvas Iím going to create is a canvas that is a new object Iím going to create myself to define what that canvas is, rather than the basic G-canvas. So Iím gonna call this My Canvas. Iím going to do top down design. My canvas is a class that doesnít exist yet, all right? Itís not a built-in class. Itís my canvas. Itís something that Iím gonna write the code for in just a second and Iím gonna have some object called ďcanvas,Ē which is off top My Canvas and I will create a new My Canvas. At this point, you should be getting a little bit worried unless youíve internalized top down design. If youíve internalized top down design, you say, yeah, Iím just gonna go create My Canvas when itís time for me to write that code. For right now, Iím just going to assume it exists.
And then what Iím gonna do is Iím going to add this canvas to my container. When I do an add of something that is a component, in this case, canvas will be a component Ė youíll see why itís a component in just a second Ė it says, hey, program, add this particular element to yourself, and program says, hey, Iím a container, right? I donít already have a canvas. I donít already have a console, but Iím a container. Youíre giving me a canvas, whoosh, the canvas goes up into my container, and if itís the only thing in my container, its size will expand to fill up the whole screen. Kind of like you saw last time. And thatís it. So what Iíve donít in this program is said, hey, Iím gonna have a program. Iím gonna create a canvas where all the action isnít gonna happen in that canvas, and Iím going to add that canvas to the display and then Iím gonna let the canvas do all the work. Is there any questions about this, just creating the canvas? So now we can actually sort of define what is My Canvas and Iíll show you that on the computer because itís a little bit more involved. And you donít need to worry about writing this all down. Actually, on Friday, Iíll give you a handout that has all this code in it. I just didnít want to give it to you now because then you wouldnít pay attention to whatís going on on the screen, okay? So what Iím gonna do is Iím gonna have a class My Canvas. And My Canvas is going to be a G-canvas, so itís going to be an extension of a G-canvas. Itís gonna extend G-canvas. So all My Canvases are G-canvases. Because itís a G-canvas, that means itís a component. Because itís a component, I can add it to a container. So thatís why add works there.
Now, what am I gonna do with this thing? The other thing I wanna do is I wanna say, hey, this is a component thatís gonna listen for stuff, and so the way I do this is rather than doing add mouse listeners like I did before, or add action listeners, which look at actions of things like buttons, the way a component actually listens is it implements the component listener interface. So when you wanna have some component that is actually gonna get events that itís listening for, component events, the way you do that is when you create the class, you say it implements component listener. Thatís just the way you say it, so itís going to implement the interface. Thereís some set of methods it needs to provide that implement that interface and Iíll show you the list of methods real quickly. Theyíre just down here. These four methods here, component resize, component hidden, component moved, and component shown are the methods that comprise that interface, the component listener interface. So anything that implements the component listener interface needs to provide these four methods. In a lot of cases, you wonít necessarily care that something happened and so, for example, if the component is hidden, which means some other window comes over it, the component is moved, or the component is shown, youíre like, yeah, whatever, like I donít care. Those are good things. I need to have methods that correspond to those things because in order to implement the interface, I need to provide methods that correspond to all the methods that the interface is expecting, so I need to have these. But in three of these four cases, Iím not going to do anything, right? Itís just open brace, close brace.
Someone comes along and says, hey, guess what? You know you were occluded. Someone hid you, so your componentís hidden, and youíre like, yeah, thatís cool. Youíre like, arenít you going to do anything? No, I like hiding. And itís like, oh, look, your component was moved. Someone came along and moved your window. Arenít you gonna do something? And youíre like, no. No, Iím cool with that. Thatís fine. You moved me, all right? I wonít get into the music, but if you can think of that, what that reference actually is, itís not that difficult. Louie, Louie. Component shown, again, definitely, so the only time we actually care about doing something is when weíre resized because when weíre resized, we wanna say, hey, our component was resized. Let me move that square to wherever the new center is of that window. So whenever my componentís resized, Iím gonna get the component resized method called and Iím just gonna call the method called Update. And whatís Update gonna do? Updateís right up here. All Update says is remove all, so whoís the remove all message being sent to? Itís being sent to myself because Iím not specifying any particular object to send this message to. So when I say remove all, Iím sending it to My Canvas. Well, My Canvas is a G-canvas, which means remove all, removes everything thatís on the canvas. It clears the canvas, which means wherever the square was before, it just got cleared.
And what Iím gonna do is Iím gonna add some rectangle, and Iíll show you the rectangle. Itís pretty simple. So what I have is I have a private G-rect that I call rect. Itís just gonna be some private variable that I keep track of. In the constructor for my class, what Iím going to do is Iím gonna Ė before I tell you what add component listener this does, Iím going to create a new rectangle, whoís width and height are whatever I specify as constants is the box width and box height. Theyíre just constants, not a big deal. And I set it to be filled. So what I get is a filled square. I created this rectangle as a filled square. I havenít done anything yet with it other than create it, but Iíve created it and I have some variable rect that refers to it. So all Update does is it says, hey, you know what? Remove the square if it was anywhere on the screen before and now add that rectangle back at the center location of the square, right? Whatís the center location of this point? You should be good, real good, with figuring out how to center stuff on the screen. You take the width of the screen, subtract off the width of the object youíre displaying, and divide by two. Thatís the x location. You take the height of the screen, subtract off the height and divide by two. Thatís the y location. And this get width and get height will get the current size of the component, which means when the components been resized, this update method gets called, it closed the screen, and it gets rid of the old box and draws a new box at wherever the new center of the screen is because this gets called, which means Update gets called after the componentís been resized. So that way itís gonna redraw the square always in the center of the screen.
The only other thing about this program that you need to know is in the constructor for My Canvas, My Canvas implements the component listener, so you not only need to say, hey, I implemented the component listener, but you need to let the Ė you need to actually say, hey, you know what? I am a component listener, so add me to the component listener list, and the way it does that is you just call add component listener and you pass this, which means myself. So somewhere, I say, hey, you know what? Add me to the list of component listeners. Iím giving you a parameter to myself so you know who I am, so when something happens to me, you can call my component resize method to get resized. And that part up there, that Add Component Listener, is just a broiler plate, but itís gonna, for example, show up in your NameSurfer assignment because guess what? In your NameSurfer assignment, when your component gets resized, youíre going to do something very similar to this. Youíre gonna find out what the new dimensions of the screen are and redraw everything that should actually be on the screen, which means you need to have some way of being able to keep track of the stuff thatís on the screen. So any questions about this? All righty, so thatís the basic notion of this idea of a component listener, okay, and what we wanna think about next is how do I create a larger program with these kind of things. So this is just the simple hint. Now Iím gonna give you something thatís even a bigger hint to whatís going on in NameSurfer.
And so what Iím gonna do is one thing thatís real popular these days is kind of like music, right, like online music and albums and the whole deal. So what I wanna do is implement a program that keeps track of data related to music and at best, Iím gonna have a little music store, the program for the music score. And the basic idea here is that a lot of what Ė well, I shouldnít say a lot, but so far youíve been Ė I donít know. You did a lot of games in this class and games are great and I totally encourage games, and Iím a big fan of games. I got into computer science because of games. Itís a good time. But a lot of what computers are actually used for as well, like when you go to some .dot com site or whatever to like buy music, and I wonít name a particular one because I might get sued, you know, itís doing data management, right. For them, when you go to their site, all they really care about are what are the songs theyíre providing, do they Ė you know, like youíre buying a physical CD, do they have it stock? Sometimes you just get stuff downloaded thatís all digital. And the other thing they worry about is stuff like prices and inventory management and stuff like that. But thatís really all that whole side is doing is doing management of data.
So weíre gonna write a little program that does management of data, and the basic idea here is someone comes along and gives us a data file called Music Data and the idea of this data file is itís a very simple format. And the basic format Ė Iíll just draw it out here Ė is every line of the file contains information about one album thatís in the inventory and the basic idea is thereís a bracket, hereís the name of the album, right. We canít just use spaces in this case because there might be spaces in the name of the album, but we kind of assume thereís no brackets in the names of albums. So thereís the name. And then thereís a space and a bracket, and then the bandís name that actually produced that album, and then thereís an integer over here, which is the number in stock of that particular album. So we have ten copies of Snakes and Arrows. Any Rush fans in here? No? Thereís like two? Times have changed, Iíve gotta say. All right. Thereís not gonna be any fans of this. Dokken? Anyone? Yeah, I would not advise it. And Smashing Pumpkins. There might be a few Ė yeah, itís a good time. Itís a good time. Old stuff a little better than the new stuff, but thatís not important right now. What is important is that weíre gonna have some data and our program is basically gonna manage this data. Itís gonna read in this data file and allow someone to ask questions about for different kinds of albums, do we have that album in stock, and how many of that album do we have in stock. And do some nice little graphic display on it to make use of some of the graphics concepts that weíve actually shown.
So what I wanna do in this program Ė let me actually show you the program in operation so you can see whatís kinda going on with the whole display and resizing the display and why itís the coolest thing ever. So when we run the program, Iím just gonna call this program Music Shop. When I run Music Shop, what I get is basically a blank screen that asks me for an album name, so I can come along and say, oh, do you have any copies of Plans by Death Cab for Cutie Ė any Death Cab fans? Itís a strange name for a band. What it does, it might be difficult to see, but it writes that album Plans by Death Cab for Cutie, ten in stock, and it writes out little squares to indicate that thereís ten. It writes out ten squares and you might say, okay, Mehran, thatís not the greatest display in the world, and itís not. This display I wrote in a half an hour last night, but whatís kind of cool is, ah, look, it centers this way. And hereís the other cool thing. Look at the size of the boxes. They get smaller and they expand, so they always have the same spacing, but as I expand out or get smaller, they resize. The fonts donít resize. They just recenter. I could, for example, put in perhaps one of the greatest albums of all time, ďSoĒ by Peter Gabriel. Thereís 20 copies in stock as everyone should own a copy, and if you donít, thatís okay. Itís perfectly fine. And sometimes I might type in a band that doesnít exist or an album that doesnít exist, like I might type in Ė I donít know Ė cow, which is Ė thereís probably an album out there called Cow. I donít know why I always pick Cow. I just do. In which case, it definitely disappears. Like we donít have that in stock. Just back off, man.
So thatís whatís going on in this program. So we need to be able to resize the bars and we need to vertically center the stuff we display in the screen. How do we actually do that? So one thing we wanna keep track of is you wanna go back to the data, right? You actually wanna say if I need to do this, in order to display anything, the first thing I need to do is figure out some information about the data that I wanna store. So what data do I wanna store? Well, I probably wanna store some information about albums because thatís what I have in my data file. So if I think about my data file and wanting to encapsulate all the information about one album, okay, I wanna do something on a per album basis because every album is gonna have, for example, an album name. Itís gonna have some band name that produced that album. And some number thatís in stock, like Num Stocked. Oh, and this is a string and this is a string and this is an int and itís good to put semicolons at the end of them and itís probably a real good idea to take this whole thing and turn it into a class thatís gonna encompass all this because what I want to have is modularity in terms of my data. If thereís a whole bunch of data I wanna manage, the first question I wanna ask myself is whatís the basic element of data that I want to have? In this case, the base element is one album. Iím gonna have a bunch of albums, but the basic idea is one album and an albumís gonna contain an album name, a band name, and a Num Stock, which are, for example, gonna be private fields of that album.
So one of the things I might wanna do and Iíll just show this to you real quickly, is I would create album.java, which is just some class. It doesnít extend anything. Itís just gonna be a class to keep track of data and what itís gonna do is itís gonna have some private instance variables, album name, band name, and Num Stock, thatís gonna keep track of the information from one album. And then itís gonna have some other methods. One of the methods itís gonna have is the Constructor. What does the Constructor do? It says, hey, you want an album object? Give me a name for the album, a name for the band, and a number of stock. Iíll set all my internal variables to those things. Iíll do a little annoying pop-up on the screen and what youíll get from your constructor is youíll get an album object that encapsulates that information. Now, I made all this stuff private, right, album name, band name, and Num Stock, because what I wanna do is when you create the album, you say, hey, this albumís in stock. I donít want you to go back and say, hey, no, no, no, the name of that album changed. Thatís not what happens, right? When an albumís released, the name stays the same, generally. We wonít talk about special cases. So in order to get the information out of an album object, I need to have some getters. I can potentially have setters as well, but here I have some getters. Get album name, get band name, and get Num Stock. Pretty straightforward getters. Youíve seen getters in the past; not a whole lot of excitement going on there.
The only other thing I might add, which I told you in the days of yore that you didnít have much need for until now, is to have a 2-string method, just returns a string representation of that particular object if you ask for string representation. So it just writes out ďalbum nameĒ by band name colon the number stock in stock. So it can return to you a string that basically has all the information about the object in some nice little display format. Thatís the whole class, okay? This is just a simple class that keeps track of information about one album because weíre gonna have a bunch of albums that we wanna keep track of. So now that we can keep track of information about one album, it kind of brings up a deeper question. The deeper question is we donít wanna keep track of just information about one album. We, in some sense, wanna keep track of a whole set of albums, a database of albums, and what we wanna do if we think about a running program Ė so Iíll go back to the running program Ė is we wanna allow someone, given a particular name for an album, to be able to go look up all the information on that album. So the way you wanna think about it is given a name, I wanna go and look up the record for that particular thing. And this happens all the time. It happens with your student I.D. records, except we donít do it by name. We use student I.D. reenact records. We say given your Stanford student I.D., letís go grab a record of all of your information. Itís exactly the same problem. This is just the simple version with albums. So if we wanna think about having some mapping, where, for example, from some name, we can go get the whole information about that record. What kind of data structure might we think about? What kind of thing that youíve seen before?
Ah, yeah, the whispered HashMap. Could it be the HashMap? And in fact, yes, it could be a HashMap. And the thing you wanna think about with the HashMap, thatís very concentrated. Itís a low variance event where itís kind of tougher the whole time and youíre like, ah, everyone. Yeah, itís early. Itís before Thanksgiving. Have some sugar. We can think about a HashMap. Now when we think about having a HashMap Ė suddenly, everyone wakes up. Food? Food? Bark, bark. I know, sometimes. Anyway, HashMap, what are we gonna map to? What are gonna be the two types that our HashMap is parameterized by? Whatís the first type? What are we gonna look things up by? Whatís the key? String, right? Weíre gonna look things up by the name, which is a string. When we look something up, what do we wanna get back? An album. So what weíre gonna have is a HashMap that maps from strings to albums, and I think someone said album right over here, although I think I missed you. Sorry. I missed you again. So the basic idea is given some name, Iíll go look up the whole album and at this point, you might say but Mehran, isnít the name inside the album? Yeah, thatís fine. In your student record, it also contains your student I.D. and your name. We just happen to look it up by your student I.D. and itís the same thing here. The userís gonna give us the name to look it up. When we go look it up, itís fine if the nameís also contained in the album, but thereís a whole bunch of other information we care about there as well. So if we have some HashMap, weíll call this particular HashMap ďInventoryĒ and we might say new HashMap thatís gonna map from strings to album, and thereís the constructor. And so thatís how we might actually create this particular HashMap.
So once we have this HashMap, if weíre gonna have some object inventory thatís a HashMap of all these things, we need to load it up. We need to say, hey, all my data is actually sitting in a file somewhere. What I need to do is read the data in from the file and as I read the file line by line, every line Iím gonna create one of these album objects because every line is information about one album, and after I create this album object, Iím gonna add it to my HashMap and so my inventoryís gonna be all of my albums mapped to by their name. Any questions about that basic idea? So let me show you the code for them. What does that actually look like? So hereís album. You saw album. What Iím gonna have is Iím gonna have My Music Shop. Whatís My Music Shop gonna do? Before I show you everything else the Music Shopís gonna do, itís gonna have this HashMap. Itís gonna have a HashMap for inventory thatís a map from strings to albums, and itís initialized just the same way you saw it over here, and the way I set this up is Iím gonna have some method Iím gonna call Ė called Load Inventory. Whatís Load Inventory gonna do? Itís going to have a buffered reader because I need to open a file and Iím just gonna hard code my files called musicdata.txt. I go and I read the file line by line, so I grab a line from the reader. If itís the last line of the file or thereís no more lines left in the file, Iím done. Same thing you did with file crossing before. If there is a line there, then Iím going to write some method thatís top down design called Parse Line, which says, hey, let me give you this string that I just read in. Itís a whole line of the file. You break up the string into all of its fields and create an album object that contains all that information and return that to me and I will assign that to something Iíll call album, which is a type capital A Album. And then what Iím gonna do is I need to put that in my inventory, so in my inventory, Iím going to put it. How am I going to put it in there? Iím gonna put it in by the albums name is the key, so I just say, hey, I have an album object that contains all my information now, so album.getalbumname will give me the name. Thatíll be the key, and the thing that I wanna store relative to that key is the whole album object. So this just line by line reads the line, parses it Ė Iíll show you how to parse it in just a second Ė to create an object, and then adds that object to this HashMap Iím creating thatís gonna store everything. Know this, live this, learn this, love this. Youíll do this for NameSurfer. So after I do all this stuff, I close off my file and Iím done because presumably, I shouldíve read the whole file and put everything into my database and Iím done. I do my little exceptions with file reading, just in case I had exceptions. So the only code you havenít seen so far is parse line. Whatís parse line doing? Parse line is a string manipulation extravaganza. So whatís parse line doing? And Iíll just go through this very simply. Itís a lot easier than it looks. Basically, itís just a lot of indexing for strings. What I do is I say, hey, I got a whole string that has Ė oh, I erased it up here, but basically, starts off with a bracket, has the name of the album, the name of al, has the name of the band, and has some number thatís an integer. So how do I break this up into sub pieces that I can actually store in my structure?
Well, the first thing Iím gonna do is I wanna pull out the name of the album. How do I find the name of the album? The name album starts after the first opening bracket, so I find the index of the first opening bracket and add one to it. Thatís the first character of the album name. How do I find where the end of the album is? I look for the index of the closing bracket, and so basically, if I take a sub string from the start of the album name to the end of the album name, I pull out that piece of text thatís just the album name and Iím gonna store that in a string called Album Name, which is just a local variable. So now Iíve pulled out the album name. How do I get the band name? Band name, I look for the index of the first bracket after the album name. Thatís the critical thing. If I donít look starting after the album name, Iím gonna read the album name again. So I look for the first bracket after the album name, which is ed album name end plus one. That will get me the index of the bracket where the band name starts and I add one to that, which gets me to the first character of the bandís name.
And then I do the same thing over here. Whereís the end of the bandís name? Get the index of the closing bracket starting at the end of the album name so I donít get the closing bracket for the album name. I get the closing bracket for the band name. So I start at the album name plus one. So now I have the boundaries for the bandís name and so the bandís name is just the substring I get in the line from the starting index for the bandís name to the ending index for the bandís name. Now, the final thing I need to do and this is funky thing. Itís probably the funkiest thing of this whole function, of this whole method, and itís pretty simple, which is say that last thing thatís on the line is actually integer, so I donít wanna pull it out as just a string. I need to actually convert that string to a real integer. How do I do that? Well, let me first find out where that number actually lives. Where do I find the number? I look for the number by finding the first space after the end of the band name. The first space after the band name is gonna be this index right here. The number starts on the next location, so if I take that particular location, after the end of the band name, and add one to it, I now have an index thatís the very beginning of the number of that numeric sequence. So what I wanna do is I wanna say pull out that numeric sequence as a string and convert that string to an actual integer. How do I do that?
Well, the way I pulled it out is I say take the substring of the line that starts at the starting position of the number of the number stock. Thatís where I just computed in the last line was where that number starts. Because I donít specify an ending index for this substring, it goes till the end of the line, which means it takes all the characters. If there happens to be like 100 in stock, it starts at the one and takes the 100, it takes that whole substring as this substring right here, and what do I do to that? Thereís a method thatís a static method of our friend the integer class called parse ins, and so if you say integer.parseins, you give it a string, it converts that string into its integer equivalent. So what it does is it says, hey, Iím giving you the number thatís the number stock as a string. It says okay, Iím gonna parse that and turn it into an integer and what I will give you back is something you can store as an int. So thatís how we convert that last portion of line from a string to an integer. We first extract it and we get the integer. And then what are we gonna return to the person who called our function? Weíre gonna return a new album object, weíre gonna create a new album object, where we initialize the album object to have the album name, the band name, and the number stock that we just parsed out of that line. Are there any questions about that? Standard thing you do in files, you pull out a line as a string, you break up that string using some string manipulation operation, however you wanna break it up, and then you potentially create some object out of it so that you can store all of the nice things that you extracted out of the string into nice little name fields.
So this gives you an album object. Then back up here, that album object is getting returned by parse line. We put those album objects into our inventory HashMap indexed by the album name. Are there any questions about that? If thatís clear, nod your head. Good time. All right. And thatís basically the role loading of the database. Now the only other things going on here is we need to figure out our little interactors, right? So in our program, we have a label that asks for the album name where the user can type the album name, so we put in a label. Then we put in a text field that has a maximum size of 20, right? At this point, this should all be sort of old hat. We add the label to the southern region. We add the album name, which is just a text field thatís gonna take in the album name for the southern region so we get those two interactors. We get the label and the text box, okay? And the things we wanna do after we set up the interactors is we say, hey, I need to display the stuff somewhere. Mehran, I remember he told me, oh, about 20 minutes ago, that if I wanted to actually have some canvas that could resize itself automatically when the user changed the window, what I need to do is extend the G-canvas. I need to create in my own version of canvas and make it a component listener. Thatís the same thing Iím gonna do here. Iím gonna create something called a Music Shop display and Music Shop display, which Iíll show you the class for in just a second, is something that Iím going to store as my canvas as a private variable inside my class. And Iíll just go ahead and say, after you put your little interactors on this screen, create your new canvas and add the canvas, just like I did before in the previous program, now load all your inventory, so go read the file, do all that parsing, the funky stuff, set up the inventory. Add action listeners because I wanna be able to listen for events that actually happen on the buttons and also add the text field album name as an action listener. So it basically wires everything up. It says put in the interactors, create the canvas that Iím gonna draw stuff on, load the inventory into data, and get ready to listen for stuff. And then itís ready to go. Itís not gonna do anything until I get some events, but now itís ready to go. So load inventory you saw, parse line you saw. The only thing you havenít seen is what do I do when an actionís performed.
I donít have some button the user compressed. All I have is the text field, so when they hit enter, I check to see if the source is the text field album name. If it is, what Iím going to do is ask the canvas to display the inventory of a particular album. How do I get the particular album? What I do is Iím gonna say what I want you to display, Iím gonna give you an album object thatís gonna contain all the information you need to display, so Iím gonna call display inventory pass in album object. How do I get the album object? I say, oh, text field, give me the text thatís in you. It says, oh, here you go, and hereís what these are typed in. That should be the name of an album. I can use that name for an album in a HashMap to look up the album object, so in my inventory HashMap, I say get on the album name and if there is an album object in my inventory HashMap, thatís what Iíll get back. If there isnít, Iíll get back null and Iíll go ahead and call display inventory with null, so it needs to know how to handle that. So all the action to do the display is gonna happen in display inventory. So let me show you musicshop.display, which kind of pulls this final thing together. These are shock display extends G-canvas, just like you saw in the previous example with the little square, implements component listener, just like you saw in the previous example, and it just has a little bit more complexity. The only additional complexity it has is in its constructor. As before, it needs to add itself as a component listener. Thatís broiler plate. But it says, hey, you know what? Iím gonna keep track of the last album the user actually typed in because when you resize the window, what I need is the information about that album to redraw everything in the window relative to that last album. So when I start off, there was no last album, so I set it to null, but as soon as you give me a real album, thatís what Iím gonna store in last album, is the last album from the user. Iíll show you a few things down here. So last album is basically just album that I keep track of in album object, in last album [inaudible]. I have a few constants that indicate for me how big things are gonna be on my display and I can get all the methods of a component listener. Again, the only one I care about is the resizing event. The other ones I ignore. On the resizing event, Iíll call Update. If I call Update on the net display, display from my inventory the last album that I displayed. So if I had something in my display and my display got resized, basically all I wanna do is redisplay that last album in the same graphics window.
So Update calls display inventory or I can call display inventory directly from my main program to display something. And this is way more complicated than it looks, so Iíll just briefly tell you what it does. It clears everything thatís in the display from G-canvas currently by calling Remove All. That is you just gave me an album to display. The last album that I wanna keep track of, the thing that I wanna keep track of in display next time I get resized or whatever is the album you just gave me because you just saved me a new album to display, so thatís gonna be the album I keep track of to display on resize events. If that album was not null Ė if it was null, Iíd just clear the screen. I wouldnít do anything else. If it wasnít null, it means that you had a real object in your inventory that was an album, so thereís some work for me to do. What am I gonna do? Iím gonna ask that album for the number that are in stock and Iím gonna create a label that has album and the name of the album by and the name of the band, and Iím gonna place it on the screen at a location thatís censored based on the height of the current display. Then Iím gonna have a fore loop that displays squares that indicate Ė shouldnít be dictated; it should be indicated Ė how many are in by inventory. I wonít go through all the math because the math is not interesting. The only thing thatís interesting about this is I have a fore loop that goes through the number in stock and draws one filled in rectangle for every number in stock. Whatís the size of that rectangle? The length of that rectangle is called the broad length that depends on the total size of the window. So I look at the total size of the window and I divide by however many maximum squares I can display in the window, which happens to be 20, to get my size. So as the window gets smaller, the display objectís gonna get smaller and as the window gets larger, broad length will get larger. So it depends on the size of the width of the window.
And I do this in a loop so basically, it just draws all the squares. And you can go through the math on your own if youíre interested in it, but the basic idea is the squares just size depending on the width of the window. Last but not least, I write out another label that says how many are in stock and where I display this on the screen depends on the height of the window because Iím in the center. So as the height of the window changes, this will always recenter as well. And thatís where all the action is, so when I run this program, it starts off not doing anything. And when I type in an album Ė letís use Snakes and Arrows because I think thatís in there Ė I get a bunch of squares and itís just sitting here. If I type in an album that doesnít exist, like Cow, it clears the display and thereís nothing to display. But if I happen to type in Ė Iím gonna do Snakes and Arrows again Ė something actually does display. When I do the resize, itís resizing based on the size of the window because it knows what the last thing displayed was, right. It knows the last thing displayed was the album Snakes and Arrows and itís storing that in its own local variable, so that when I resize, it knows what information to redisplay on the screen to resize. Any questions about that? Weíll do exactly the same thing in NameSurfer and Iíll give you all the code you just saw.
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Duration: 50 minutes