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This presentation is delivered by the Stanford Center for Professional 

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Development. 
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Today we're actually gonna go over some of the graphic stuff you saw, talk a little 

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bit more about objects and classes and get into variables and values and all kinds 

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of goodies. 

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Any questions? 

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All righty, then let's just dive right in. So 

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one of the main topics for today is this thing called a variable. 

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And a variable, you know, like variables come up in mathematics and it's like, oh X and Y are 

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variables, right and there are these things and we do all these manipulations on variables. In the 

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computer science world, 

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they're really friendly, right, and you don't have to worry about integration or 

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differentiation or you know, those kind of variables. 

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Variables are kind of your friend, 

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and basically all the variable is in the computer science worlds 

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is it's a box. 

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It's a box where we stick stuff 

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and the stuff we stick into that box can change. That's why we call it a variable 

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because it's a box that has a variable contents, 

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and you think back you know, in the days of yore in math, and you know, oh yeah, it's kind of like X 

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can have different values, yeah, it's basically just like that. 

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So in computer science, what we think of as a variable, is each variable has three 

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things associated with it. 

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It's got some name, 

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and that's just how we refer to that particular box. 

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It has a type, 

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which is something a little bit different in mathematics, but the 

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type basically says 

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what kind of thing does this box store? Some boxes store 

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numbers; some boxes store letters; some boxes will store other things, like little objects 

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in the world. 

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But a type is just what's stored in that box. 

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And then there's a value, and the value, as you can imagine is just what's in the 

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box. What is the actual thing that's in there, right? If it stores the number, then it might 

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store, for example, the Value 3 and that's just the value, and it may have 

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some name associated with it. 

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And how do we actually name these? There's actually a rule and it's not a very 

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complicated rule, but a very simple rule you need to remember for what are valid names 

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for variables in Java. So a valid name, 

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so this is how you actually name these puppies, has to start with a letter or 

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an underscore. So it starts with a letter 

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like one of the alphabetic letters and can be upper or lower case or the underscore 

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character. Okay, 

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and that's kind of you know, 

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underscore. It's down at the bottom of the line, okay? 

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And then after you have that initial letter or underscore, 

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then 

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after that, you can have any number of letters, numbers, that's like the number 

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digits, like 1, 2, 3, 4, you know, 0, etc. or underscores, 

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okay? 

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So you can't start with a number. You have to start with a letter or underscore, but 

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for most purposes in this class, just thing about them as letters. When not actually using 

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underscores, you might occasionally use numbers. You can actually have numbers 

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after the first letter. 

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There is one slight caveat to this rule which you can't have any variables 

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that's name is the same as some which known as a reserved word in Java, which 

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means its name can't be the same as some special word in Java, like the word 

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class is a special name in Java, and there's actually a page in your book, I think in 

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Chapter 2, that lists all the special names. 

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It's like out of the English language it has about 

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127,000 words. 

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I think there is like 40 in Java that you can't make a variable name. Okay, you have 

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lots of other choices. 

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As a matter of fact, there's lots of things that don't have to be valid words 

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in English. They can just be any name that follows this rule. 

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The important thing to think about in terms of a name and this is one of the 

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good software engineering principles, is make your name descriptive. 

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If you have a program that's maintaining for example, the balance in the bank 

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account, a real good name for the place where you store the value of the balance 

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would be something like "Balance." 

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A real bad name would be something like "A," 

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because no one knows what A is. It's like hey it's A - yeah, I know what A is. And someone says, "Yeah, A is balance," and 

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someone says, "No, no, no, in my program, A 

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is actually how many miles I bicycle today," and you're like, "No, no, no, man, A was balanced." 

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Well, if it's called Balance, there's just no 

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ambiguity, so give them descriptive names. 

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So that's kind of a name part of this. 

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The next thing is what is this type all about. What are the different types that you 

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can actually have. And there are some things that we refer to as primitive types. 

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These are the types that have the smaller developed brains and use 

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knuckles drag on the ground. 

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No, there just the types that are built into Java, okay and some of the basic 

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types we have is something we call an INT, 

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which is short for 

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an integer, 

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but we actually write INT, so INT, I-N-T, is the name of the type, okay. 

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And this is just an integer of value. It's just gonna store some whole number 

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basically. It stores a number between 

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minus 2 billion and plus 2 billion, 

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but for all intents and purposes, you can just imagine you can store any 

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integer in there, okay? 

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There's also besides integers what other kinds of numeric data do we have? 

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People already know, it's like 

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I would think it would be like real values, but a bunch of people are already 

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saying double because you've read ahead and you've done the assignment the way you should. And that was kind 

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of a mini social, but both went 

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to the same person. There's this thing called a double 

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and a double is actually some real valued 

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numeric 

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value, right. It's something like 2.3 is a double or even 2.0 

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can be a double, okay? 

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Why is this thing called a double - anyone know, as opposed to like a real? Uh-huh? 

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[Inaudible]. Yeah, there's this wonderful verb people call the I triple E, which is 

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like the Institute for 

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Electrical and Electronic Engineers and 

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they come up with all these standards for things. Is anyone a member of I Triple 

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E here? 

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No one? Oh, man, join and pay your dues. It's a good time. 

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I'm not actually a member myself. 

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But what those folks actually do is they come up with standards for things and 

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one of the standards they came up with is how you represent 

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numbers that are real valued numbers inside a computer, right, because remember a 

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computer only understands ones and zeros, so how do you actually represent a real valued number. 

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And so there's a standard, 

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and part of that standard has to do with a precision of the number, how many 

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digits in some sense and a rough approximation you store and double stands 

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for a double precision real number. 

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And so for the purpose of this class, all real valued numbers that we're gonna 

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use are just of type double. Okay, 

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there's a couple of other types that I'll just mention now and we'll actually go into them in much 

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more detail in a couple of future classes. One is called Boolean. 

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And if you've ever heard of Boolean logic, this is a logical value, so this is 

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just essentially true or false, and we'll talk about that in excruciating 

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detail next time, but I'll just let you know that there's a type called Boolean. 

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There's also a type called care or Car 

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and as you can imagine it's because we like sort of the first syllable of most 

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things and this is the first syllable of character, 

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okay, and so we'll also talk about this in a couple of weeks time when we 

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actually get into a some things with characters, but that's just a character. 

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It's a variable; it's a box, but 

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still is a character. It's a box that stores an integer, box that stores a real value, 

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box that stores true or false and a box that stores a character. Those would be the different 

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types of them. 

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So one thing people - so that's kind of types, at least some of the basic types. 

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And then when you think about value that we actually store in this box, people always 

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get uncomfortable when they see INTs and they see double, right, and they sort 

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of say, "But Meron, like 2 is an integer, right?" And I'm like, "Yeah." 

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And they're like, 

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"But you just told me 2.0 is a double, right?" And I'm like, "Yeah, 2.0 is a 

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double." 

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And so the natural question is 

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why? Why do we have both these things? How come like all integers aren't subsumed 

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by the real values, right, you're kind of the mathematician type and you're like, "Yeah, there's like 

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strictly more of these than there are of these. 

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So why are all these like subsumed in here. Why do we have this integer type," okay? 

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And the reason we do boils down to a simple question and the simple 

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question you want to ask yourself 

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is how much versus how many, okay? 

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So if you ask someone, 

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let's say you, 

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ask me, just so I won't embarrass you, "How much do you weigh, Meron?" How much do you weigh, Meron?" 

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I weigh about, you know, 

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155.632 pounds, okay? 

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That makes perfect sense, right. If I can put a decimal point, I can put as 

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many numbers after it, or I could just say 156, right and 

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those are both valid kinds of things. Now 

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you could ask me, 

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"Hey Meron, 

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how much children do you have?" 

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I have 

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2.3 children. 

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Does that make any sense to you? Yeah, it's like we had three until that grisly 

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accident. No, 

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it's just that there's sometimes - I know that's horrible to say. We actually have one, 

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and he's just fine. 

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There's sometimes in the world when you care about counting, and when you care 

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about counting, it doesn't make sense to have fractional values. Those things are integers. They're a 

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how many kind of value. 

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When you're thinking about how much, 

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that's a double value, and you actually want to keep these distinct because if I 

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ask you what's the next number after 1, you say - 2. 

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2 

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because you're thinking integer, and that's perfectly right and if I say 

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what's the next number after 1.0, 

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you say - [Inaudible]. Yeah, 

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you start mumbling and you're like, "Well, I know it's not 2.0. That would have been the obvious answer and that's 

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probably not it. 

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How about 1.0000001, and I'm like, "No you missed a couple of zeros in there. Just keep 

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going. 

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Wait until the end of the quarter and put that 1 and come back and talk to me." 

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In real values, there is no next value, 

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okay, so when you care about counting, it doesn't make sense to use a double. That's 

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when you want to use an integer, so having these things be a distinct type 

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actually make sense 

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and you'll kind of see that as we go along, okay. 

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So the value is basically just what you're actually gonna put into this box over 

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here and it's gonna be some of the values that we actually talked about, 

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so let me show you some examples of how we might use variables, what the 

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syntax for variables actually is in Java. 

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So what we need to do with variables before we use them in Java is we need to 

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declare a variable. It's kind of like declaring your love. If you're gonna 

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have some box somewhere that's got something stored in it, you need to come tell the 

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world, "Hey 

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world, I've got this thing called X. It's cool. 

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Check it out." 

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Here's how you do that in Java. You say 

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INT X, okay? 

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What that does is you specify a type for your variable, what's the box gonna 

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hold. 

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You specify a name for that variable. In this case, the name is X 

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and because all statements end with a semi colon, just like you saw in Karel's 

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world, we put a semi colon. 

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What does this do? 

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It creates a box somewhere in the computer's memory 

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that's name is X that's gonna store an integer in it. 

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Right now, what does it store? [Inaudible]. Nothing. 

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We haven't told it anything to store in particular. 

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And so in Java's world, unless you initialize you always want to think 

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about initializing a variable, giving it some initial value 

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to start off with and there's some rules that we'll talk about as we go along 

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regarding when Java will automatically sort of give you a zero value in there and there's 

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time when it doesn't and sometimes it can get a little bit confusing. So the 

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general rule you want to remember and it's also good software engineering is give variables 

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an initial value when it makes sense, okay? 

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So how might we give it an initial value? Well, on that same line, rather than just putting 

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the semi colon there, we could say X equals 3 

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and what that does is it gives it an initial value of 3 to x. 

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Okay, so you could actually have the 

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declaration of the variable without the initial value, but you always need to 

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give it an initial value before you use it, so a lot of times we just give it the 

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initial value when we actually declare it, okay. 

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If we want it to have something of type double, maybe something called 

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Y, we could have something called Double Y equals 5.2 and what that means is 

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somewhere in memory we have this thing called Y, which has value 5.2 in 

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it, and it's just a real value. 

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So the general form for how you do these declarations 

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is you say type and I'll put little squiggles underneath these just to let you know 

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that this is the general form, 

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the name 

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and then potentially an equal and some 

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initial value, followed by the semi colon, and that's how you do the 

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declaration. And so the question that comes up 

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where do you actually make these declarations and the place you generally 

235
00:11:33,649 --> 00:11:37,220
at least for the time being make the declarations of value, 

236
00:11:37,220 --> 00:11:40,420
for declarations of variables is inside methods. 

237
00:11:40,419 --> 00:11:45,529
So when you create some method like you have public, 

238
00:11:45,529 --> 00:11:46,759
void, 

239
00:11:46,759 --> 00:11:48,450
run 

240
00:11:48,450 --> 00:11:51,569
and you're gonna put everything that's gonna go inside run in there, 

241
00:11:51,568 --> 00:11:54,899
you could, for example, have this line in there and this would create X with a 

242
00:11:54,899 --> 00:11:58,448
value of 3 and Y with a value of 5.2 and you wouldn't have the 

243
00:11:58,448 --> 00:12:02,278
general form there, but for example, you could have those declarations inside 

244
00:12:02,278 --> 00:12:06,879
run and what that means is you have those variables created with some values 

245
00:12:06,879 --> 00:12:09,340
available to you inside this method called run. 

246
00:12:09,340 --> 00:12:12,830
They're only available to you inside that method called run. They're not available 

247
00:12:12,830 --> 00:12:16,800
to you in some other method and we'll talk about how you get values passed around 

248
00:12:16,799 --> 00:12:20,490
between different methods in a few days, but for the time being when you 

249
00:12:20,490 --> 00:12:23,769
have your variables they're only available in that method. Okay, 

250
00:12:23,769 --> 00:12:25,759
so any questions about that - 

251
00:12:25,759 --> 00:12:32,759
variables or values? Uh-huh? If you have them inside the public run method [inaudible], will that method be able to take that variable? 

252
00:12:36,480 --> 00:12:39,970
Well, you won't be defining another method inside run, right; you'll be 

253
00:12:39,970 --> 00:12:42,990
defining another method that's separate from run. It's just a separate method. It won't be 

254
00:12:42,990 --> 00:12:46,409
available in that separate method. It doesn't matter if it's public or private. 

255
00:12:46,409 --> 00:12:50,949
It just not available there. So this public or private thing that you have here doesn't 

256
00:12:50,950 --> 00:12:54,240
affect the visibility of your variables. Your variables are always only visible 

257
00:12:54,240 --> 00:12:57,240
inside the method in which they're declared and set up, okay? 

258
00:12:57,240 --> 00:12:59,448
Now, one other thing that we did when we sort of, 

259
00:12:59,448 --> 00:13:02,668
you know, did this equals thing over here, if we wanted to, we could have actually 

260
00:13:02,668 --> 00:13:04,559
done this in two lines 

261
00:13:04,559 --> 00:13:08,669
rather than setting up X initial value there, we could have said into X and then we 

262
00:13:08,669 --> 00:13:12,849
could have set X equals 3 over here and gotten the same effect. 

263
00:13:12,850 --> 00:13:17,570
And when we say X equals 3, this equals is not equals in mathematics; it's 

264
00:13:17,570 --> 00:13:21,930
actually what's known as an assignment statement in Java or in most programming 

265
00:13:21,929 --> 00:13:22,669
languages. 

266
00:13:22,669 --> 00:13:26,469
And the idea is we're taking some value over here. That's what's on the 

267
00:13:26,470 --> 00:13:30,009
right-hand side of the equals and assigning it to whatever variable is 

268
00:13:30,009 --> 00:13:32,419
listed over on the left-hand side of the equals, 

269
00:13:32,419 --> 00:13:34,368
so we're assigning that value, okay? 

270
00:13:34,369 --> 00:13:37,830
And so the general form for an assignment, 

271
00:13:37,830 --> 00:13:40,770
let's just do that over here, 

272
00:13:40,769 --> 00:13:43,919
is pretty straightforward, you just saw it, 

273
00:13:43,919 --> 00:13:49,379
is variable and this is the name of the variable. So I'll squiggle - equals 

274
00:13:49,379 --> 00:13:53,049
some value or some expression for a value and we'll talk about expressions 

275
00:13:53,049 --> 00:13:56,399
in just a second with a semi colon at the end of it, and that's how you assign a value. 

276
00:13:56,399 --> 00:14:00,279
You can assign values multiple times to a variable, to a variable. It's a variable, the contents change. So you 

277
00:14:00,279 --> 00:14:04,809
say, "Hey, X equals ," and then down here somewhere, "Hey X equals 4," 

278
00:14:04,809 --> 00:14:08,679
and when you get to this line up until you get to that line from here to here, X will 

279
00:14:08,679 --> 00:14:11,878
have had the value 3 and when it gets to this line, it'll stick a 

280
00:14:11,879 --> 00:14:13,058
4 in the box. 

281
00:14:13,058 --> 00:14:16,129
And that's perfectly fine because X is a variable. As long as everything you stick in 

282
00:14:16,129 --> 00:14:20,399
there is of the type of which X is declared, you're okay, okay? 

283
00:14:20,399 --> 00:14:21,879
So what that also means 

284
00:14:21,879 --> 00:14:25,289
is when you're doing Assignment you can do things that are perfectly fine in 

285
00:14:25,289 --> 00:14:28,639
programming, but to mathematicians, they just go nuts; they go crazy like 

286
00:14:28,639 --> 00:14:31,299
veins just burst in your head and you hear these popping noises and 

287
00:14:31,299 --> 00:14:32,719
people dying in the streets, 

288
00:14:32,720 --> 00:14:36,360
which is you can say, "Hey, I have some variable called total. Let me create some 

289
00:14:36,360 --> 00:14:39,789
variable in its total," which I'll give some initial value like 10 and that's a good 

290
00:14:39,789 --> 00:14:40,799
time. 

291
00:14:40,799 --> 00:14:47,189
And then I say, "Hey total equals total plus 1." And if 

292
00:14:47,190 --> 00:14:49,690
you're a mathematician, you look at that and say, 

293
00:14:49,690 --> 00:14:51,600
"No, man. 

294
00:14:51,600 --> 00:14:55,590
Total can't equal total plus 1. That's just not right." And then you got this 

295
00:14:55,590 --> 00:14:58,940
whole philosophical question about, you know, if this is like total is equal to 

296
00:14:58,940 --> 00:15:01,890
infinity and you're just like, "No, no, no, that's not what we're talking about." 

297
00:15:01,889 --> 00:15:05,449
We're talking about assignment. This is into equals, this is an assignment. So what it 

298
00:15:05,450 --> 00:15:07,190
says is evaluate 

299
00:15:07,190 --> 00:15:09,710
the right-hand side, okay? 

300
00:15:09,710 --> 00:15:13,250
Total - what was total's previous value. Go and look it up in the box over here you have some 

301
00:15:13,250 --> 00:15:17,090
box for total. Its value was 10. It looks it up. It says, "Oh, that was 10." I'm gonna 

302
00:15:17,090 --> 00:15:19,139
add 1 to it, that gives me 11. 

303
00:15:19,139 --> 00:15:21,350
What do I do with that 11? 

304
00:15:21,350 --> 00:15:22,870
Stick it back in the box 

305
00:15:22,870 --> 00:15:26,730
named Total. So it just says okay, and it puts an 11 over here and that's perfectly 

306
00:15:26,730 --> 00:15:30,269
fine. So it's perfectly fine to do something like then where you take the old value 

307
00:15:30,269 --> 00:15:35,500
for a variable, you do some manipulation on it, you stick it back in the same variable. Okay? 

308
00:15:35,500 --> 00:15:39,419
So with that said, now that you know about variables, we can go back to a 

309
00:15:39,419 --> 00:15:43,059
couple of the programs that you saw last time and things ill now begin to make a 

310
00:15:43,059 --> 00:15:45,789
little bit more sense, so if you come to the computer, 

311
00:15:45,789 --> 00:15:49,699
remember our little friend, add two integers that you saw last time. Suddenly 

312
00:15:49,700 --> 00:15:53,300
the world opens up a little bit and we say, yeah, add two integers is a console 

313
00:15:53,299 --> 00:15:57,000
program, right. So it's gonna write some text out to a console somewhere 

314
00:15:57,000 --> 00:16:01,549
and we have our run method. We print out a line; we know that the method print 

315
00:16:01,549 --> 00:16:02,909
[inaudible] when we say that, 

316
00:16:02,909 --> 00:16:06,139
whatever we put inside the double quotes gets printed to the screen so it 

317
00:16:06,139 --> 00:16:06,838
prints out 

318
00:16:06,839 --> 00:16:11,040
this program adds two numbers, so we execute that line and after we execute it, it writes it out and 

319
00:16:11,039 --> 00:16:12,699
it comes to the next line. 

320
00:16:12,700 --> 00:16:18,049
Low and behold, what have we done here? We've declared a variable named N1. 

321
00:16:18,048 --> 00:16:21,088
What's the value for that variable that we're gonna assign to it? We're 

322
00:16:21,089 --> 00:16:22,819
going to call some method. 

323
00:16:22,818 --> 00:16:25,929
The method we're gonna call is something called Read INT and this is 

324
00:16:25,929 --> 00:16:28,608
a method that's provided for you in 

325
00:16:28,609 --> 00:16:31,160
console programs. All console programs can do this. 

326
00:16:31,159 --> 00:16:34,269
The way it works is you give it some 

327
00:16:34,269 --> 00:16:37,529
text between two double quotes. It writes that text on the screen just like 

328
00:16:37,529 --> 00:16:41,809
you saw last time and asks for input from the user. Whatever value the user 

329
00:16:41,809 --> 00:16:43,138
types in and hits 

330
00:16:43,139 --> 00:16:48,619
enter, that number is the value that gets what we call return by read INT. It's 

331
00:16:48,619 --> 00:16:51,699
the value that [inaudible]. It's kind of 

332
00:16:51,698 --> 00:16:55,188
giving it back to you. Do what you want. What 

333
00:16:55,188 --> 00:16:58,338
are you gonna do with it? You're gonna assign it to the Box N1, so here's 

334
00:16:58,339 --> 00:17:00,380
the box for N1. 

335
00:17:00,379 --> 00:17:04,200
We execute this statement. It's coming; it's gonna print to enter N1 on the 

336
00:17:04,200 --> 00:17:07,789
screen. It's gonna ask us for a value. Let's say we type in the value 

337
00:17:07,789 --> 00:17:12,379
17 and it sticks that 17 so this expression over here 

338
00:17:12,380 --> 00:17:15,199
evaluates the 17. It sticks 17 in the box for N1, 

339
00:17:15,199 --> 00:17:19,479
goes to the next line, now it declares N2. We have some box. Its initial value 

340
00:17:19,479 --> 00:17:23,949
also is gonna come from the user via this read INT method. So we call read INT. It 

341
00:17:23,949 --> 00:17:27,279
asks us for another value; we type in 25, 

342
00:17:27,279 --> 00:17:29,538
and it sticks that in the box. 

343
00:17:29,538 --> 00:17:32,609
Now, we're gonna declare another; we just declare until the cows come home, right? 

344
00:17:32,609 --> 00:17:36,250
Declaring is fun; it's cheap. It's easy; do it, do it often, right, so we're gonna declare 

345
00:17:36,250 --> 00:17:39,169
another variable total, so we have another box over here for total 

346
00:17:39,169 --> 00:17:42,059
and the value of total that es gonna get assigned there is just whatever value is in 

347
00:17:42,058 --> 00:17:43,440
N1 plus N2. 

348
00:17:43,440 --> 00:17:47,370
So to evaluate this side, it says I need to look up the box N1, so at the 

349
00:17:47,369 --> 00:17:51,199
point where it reaches this line, it evaluates whatever is in N1. 

350
00:17:51,200 --> 00:17:54,909
This isn't some truth that holds for all time. It's just an assignment that it does 

351
00:17:54,909 --> 00:17:56,429
when it reaches that line. 

352
00:17:56,429 --> 00:17:58,530
So it says, find out what's in N1, 

353
00:17:58,529 --> 00:17:59,899
add it to what's in N2. That'll 

354
00:17:59,900 --> 00:18:02,389
give you the value 42; stick that in Total, 

355
00:18:02,388 --> 00:18:05,649
which is what you get. And the last line says, 

356
00:18:05,650 --> 00:18:08,670
I'm gonna print something to the screen where the total is and you see these 

357
00:18:08,670 --> 00:18:10,550
funky plusses and you're like, 

358
00:18:10,549 --> 00:18:14,470
"Whoa, whoa, whoa Meron, I thought plus was light lading things together. What's going 

359
00:18:14,470 --> 00:18:16,358
on? You're like trying to add - 

360
00:18:16,358 --> 00:18:19,369
this thing's got the value of 42. I know that because it's in the box. 

361
00:18:19,369 --> 00:18:21,939
You're trying to add 42 to some text? 

362
00:18:21,940 --> 00:18:23,590
That's not right." Well, 

363
00:18:23,589 --> 00:18:26,970
in most worlds, it would not be right, but it turns out in the Java world it's 

364
00:18:26,970 --> 00:18:28,960
perfectly fine and when it sees something 

365
00:18:28,960 --> 00:18:30,058
that some words 

366
00:18:30,058 --> 00:18:34,859
with a plus sign, this plus sign here is not numerical addition anymore. This is 

367
00:18:34,859 --> 00:18:37,579
essentially you can think of as a concatenation operator. It says, "Take 

368
00:18:37,579 --> 00:18:38,899
whatever text is here 

369
00:18:38,900 --> 00:18:41,059
and add to that text, the 

370
00:18:41,058 --> 00:18:42,729
textural form of this value." 

371
00:18:42,729 --> 00:18:46,088
Well, the textural form of 42 is just the Characters 4 and 2, 

372
00:18:46,088 --> 00:18:47,298
so it sort of 

373
00:18:47,298 --> 00:18:50,868
adds that 42 here and then it adds to the end of that a little period, so it 

374
00:18:50,868 --> 00:18:52,399
concatenates all those things together 

375
00:18:52,400 --> 00:18:55,640
and then prints them out and says the totals 42. 

376
00:18:55,640 --> 00:18:59,380
So any questions about the program? 

377
00:18:59,380 --> 00:19:02,000
If you're feeling good, like you understand what just went on in the program, 

378
00:19:02,000 --> 00:19:03,849
nod your head. 

379
00:19:03,849 --> 00:19:06,089
Rock on. That's what I like to see. All right. 

380
00:19:06,089 --> 00:19:08,819
So with that said, 

381
00:19:08,819 --> 00:19:12,529
we can go back to something else we did last time, right, so now you know how 

382
00:19:12,529 --> 00:19:13,950
all this 

383
00:19:13,950 --> 00:19:16,419
variables and assignments and types 

384
00:19:16,419 --> 00:19:19,360
and you're like, "Hey, Meron, yeah, that's all good and well, but last time you were telling me 

385
00:19:19,359 --> 00:19:21,209
about like 

386
00:19:21,210 --> 00:19:25,390
classes and objects and drawing stuff on the screen like, what's up with all that." 

387
00:19:25,390 --> 00:19:29,040
And this is the time when we actually do sort of the, you know, Reese's 

388
00:19:29,039 --> 00:19:30,288
Peanut Butter Cup. 

389
00:19:30,288 --> 00:19:33,920
Like it's two great tastes that taste great together. We take some 

390
00:19:33,920 --> 00:19:36,900
objects over here and some variables over here and we just slam them together, 

391
00:19:36,900 --> 00:19:40,690
and now you're going to have variables that contain objects. They 

392
00:19:40,690 --> 00:19:41,950
taste great, all right? 

393
00:19:41,950 --> 00:19:45,639
So here are some of the objects that you considered last time, right? We had things 

394
00:19:45,638 --> 00:19:50,219
called, like some classes. You have the G label class and the G -rect class 

395
00:19:50,220 --> 00:19:53,240
and the G-oval class. These are all these graphics classes like a label 

396
00:19:53,240 --> 00:19:57,779
[inaudible] or rectangle or an oval. I'll show you examples of all these things this time. 

397
00:19:57,779 --> 00:19:58,859
And these guys 

398
00:19:58,859 --> 00:20:00,969
were actually all 

399
00:20:00,970 --> 00:20:03,839
classes that 

400
00:20:03,839 --> 00:20:08,819
are part of a hierarchy of other classes and the other classes, they're all 

401
00:20:08,819 --> 00:20:11,619
part of is G 

402
00:20:11,619 --> 00:20:17,019
object? So what G-object is a G-object is a class. Don't let the name fool 

403
00:20:17,019 --> 00:20:20,460
you. Some people see G-object and they think, "Oh, that must be an 

404
00:20:20,460 --> 00:20:21,860
object." No, G-object is a class. 

405
00:20:21,859 --> 00:20:25,949
It just means a graphics object. And guess what, a rectangle or an oval or a line or 

406
00:20:25,950 --> 00:20:28,630
a label or all different graphics objects. 

407
00:20:28,630 --> 00:20:32,030
So what that actually looks like in terms of the hierarchy just like you saw 

408
00:20:32,029 --> 00:20:33,379
last time, 

409
00:20:33,380 --> 00:20:37,650
is the classes are represented, kind of like this in the hierarchy, all of these 

410
00:20:37,650 --> 00:20:42,950
classes are actually G-objects. Okay, they're all sub classes of G-objects. It means they're 

411
00:20:42,950 --> 00:20:43,440
specializations. 

412
00:20:43,440 --> 00:20:46,889
So any behavior that G-object has, all of these things have, 

413
00:20:46,888 --> 00:20:49,829
but they might have their own specialized behavior, as well. 

414
00:20:49,829 --> 00:20:52,569
Now, the interesting thing about all this is that 

415
00:20:52,569 --> 00:20:54,389
when you have classes 

416
00:20:54,390 --> 00:20:57,900
and you create objects, you can store those objects in a variable. You can say, 

417
00:20:57,900 --> 00:21:02,580
"Give me a variable that is a box," and the type that that box holds is an object 

418
00:21:02,579 --> 00:21:06,309
that's the type of some class, so all classes can actually be used as type means, 

419
00:21:06,309 --> 00:21:07,559
okay? 

420
00:21:07,559 --> 00:21:11,569
Now, that's kind of funky. Let me show you an example, okay? 

421
00:21:11,569 --> 00:21:13,639
Here is another program you saw last time. 

422
00:21:13,640 --> 00:21:14,640
Now, 

423
00:21:14,640 --> 00:21:17,870
we can just kind of reveal the mystery about variables. 

424
00:21:17,869 --> 00:21:21,589
We have this program, Hello program that extends the graphics program, right, so it's gonna 

425
00:21:21,589 --> 00:21:22,639
draw us some pictures. 

426
00:21:22,640 --> 00:21:26,509
Well what are we actually doing on this first line? Hey, we're doing a declaration 

427
00:21:26,509 --> 00:21:28,648
and assignments on the first line. 

428
00:21:28,648 --> 00:21:31,548
What's the type of our variable? It's a G-label. 

429
00:21:31,548 --> 00:21:33,920
So we use the class name as the type. 

430
00:21:33,920 --> 00:21:37,680
The name is label, so we have a box called label and what that box is gonna 

431
00:21:37,680 --> 00:21:39,660
hold is any object 

432
00:21:39,660 --> 00:21:41,710
of the class G-label. 

433
00:21:41,710 --> 00:21:44,299
Okay, so that's the type. It's gonna hold any object. 

434
00:21:44,299 --> 00:21:45,589
And the question comes up, 

435
00:21:45,589 --> 00:21:50,278
how do we get an object of type G label. And here's the funky part. The way we get an 

436
00:21:50,278 --> 00:21:52,140
object of type G label is 

437
00:21:52,140 --> 00:21:53,389
we have to ask 

438
00:21:53,388 --> 00:21:55,178
for a new G-label. 

439
00:21:55,179 --> 00:21:57,830
So we give the name of the class here 

440
00:21:57,829 --> 00:22:01,099
and class is depending on the class, you know, see some examples of this 

441
00:22:01,099 --> 00:22:02,228
will take in 

442
00:22:02,229 --> 00:22:07,229
or give in some initial values for what we refer to as parameters, 

443
00:22:07,229 --> 00:22:09,870
and these parameters are all separated by commas. 

444
00:22:09,869 --> 00:22:11,558
But at some initial values, 

445
00:22:11,558 --> 00:22:16,178
that this object uses to sort of set itself up. It initializes the objects. So objects 

446
00:22:16,179 --> 00:22:19,580
rather than just taking one value, can potentially hold multiple values in 

447
00:22:19,579 --> 00:22:20,048
them. 

448
00:22:20,048 --> 00:22:23,629
In this case, the G-label is gonna have Hello World as some text that it's gonna 

449
00:22:23,630 --> 00:22:27,440
display in some location that's 100,75. 

450
00:22:27,440 --> 00:22:31,980
So we say, "Hey G-label, you're the class. I don't have an object of you yet. You're 

451
00:22:31,980 --> 00:22:35,999
the class." When I say, "Give me a new G label, it's sort of like going to the factor. 

452
00:22:35,999 --> 00:22:39,868
You sort of show up at the G-label factory and you say, "Yo, G-label factory. 

453
00:22:39,868 --> 00:22:43,898
Give me a new object," and G-label factory is sort of sitting up there and says, "Well, what kind of new 

454
00:22:43,898 --> 00:22:46,898
object do you want? Yeah, I can give you a new G-label, but how is that initial G label 

455
00:22:46,898 --> 00:22:47,719
going to look," 

456
00:22:47,720 --> 00:22:51,799
and you're providing these parameters that sort of specify what the initial G-label looks like. 

457
00:22:51,799 --> 00:22:56,019
So this line is now created a G-label, okay? 

458
00:22:56,019 --> 00:22:57,569
And it has some initial, 

459
00:22:57,569 --> 00:22:59,549
so when we execute that line, 

460
00:22:59,549 --> 00:23:01,190
it has some initial 

461
00:23:01,190 --> 00:23:04,460
text that it's gonna display and somewhere it's also storing the value is 175 because 

462
00:23:04,460 --> 00:23:06,840
it knows where that's gonna be on the screen. 

463
00:23:06,839 --> 00:23:09,928
Now, what we're gonna do is we're going to tell that object 

464
00:23:09,929 --> 00:23:12,090
to do some additional stuff. 

465
00:23:12,089 --> 00:23:16,869
The way we tell an object to do something is we specify the name of the 

466
00:23:16,869 --> 00:23:20,639
variable for the object, okay. So the object name is label. That's the name of 

467
00:23:20,640 --> 00:23:21,559
the variable. 

468
00:23:21,558 --> 00:23:24,378
And so that's what we say here. We don't say G-label. We say the name of the 

469
00:23:24,378 --> 00:23:28,368
variable label. Then we have a dot; then we have the name of the method that 

470
00:23:28,368 --> 00:23:32,548
we're going to call for that particular object. So it's a little bit different than 

471
00:23:32,548 --> 00:23:34,079
Karel's world, all right. In Karel's world, 

472
00:23:34,079 --> 00:23:36,089
every time you call the method, 

473
00:23:36,089 --> 00:23:38,519
Karel was the one executing that particular method. 

474
00:23:38,519 --> 00:23:42,279
Here when you have multiple objects, you need to specify which object you want 

475
00:23:42,279 --> 00:23:43,779
to execute this method. 

476
00:23:43,779 --> 00:23:46,690
So you say, "Hey, label, yeah, I'm referring to you, buddy. 

477
00:23:46,690 --> 00:23:51,049
Set your fonts to be this particular font, san seraph which you saw last time." It 

478
00:23:51,049 --> 00:23:53,450
just makes it big. It makes it 36-point font. 

479
00:23:53,450 --> 00:23:56,370
And then you say, "Hey label, yeah, you, same guy, 

480
00:23:56,369 --> 00:24:00,179
or gal or it or whatever it may be. Set your color to be red." 

481
00:24:00,180 --> 00:24:02,509
And it says, "Okay, I'll set my color to be red, 

482
00:24:02,509 --> 00:24:03,989
and then finally 

483
00:24:03,989 --> 00:24:08,440
if I want to take this label and stick it up onto my graphics canvas which is 

484
00:24:08,440 --> 00:24:11,099
actually the program I'm looking at. Right, I've done all this stuff and my program's still 

485
00:24:11,098 --> 00:24:14,269
bringing out a blank screen. It's like here you go, nothing going on here. I 

486
00:24:14,269 --> 00:24:16,179
say add and the name of 

487
00:24:16,179 --> 00:24:19,940
the object that I want to add, right because now I'm holding this little object called label. This 

488
00:24:19,940 --> 00:24:22,759
is my little felt Hello World, and I want to say, "Hey 

489
00:24:22,759 --> 00:24:25,899
take the felt hello world that's so cute and just stick it in the world," 

490
00:24:25,898 --> 00:24:28,609
so we add it to the 

491
00:24:28,609 --> 00:24:30,429
graphics program, basically 

492
00:24:30,430 --> 00:24:33,779
and it shows up in the screen where it knows it's supposed to show up at a 

493
00:24:33,779 --> 00:24:36,109


494
00:24:36,109 --> 00:24:36,668
100,75. 

495
00:24:36,669 --> 00:24:40,240
So the general form for this, the people will refer to when you see something like 

496
00:24:40,240 --> 00:24:44,269
this, you have name of the variable and the method name and the 

497
00:24:44,269 --> 00:24:47,278
Java-esc names that we give to these, just so we hear other 

498
00:24:47,278 --> 00:24:48,029
programmers 

499
00:24:48,029 --> 00:24:50,259
refer to them, you know what they're talking about, 

500
00:24:50,259 --> 00:24:53,259
is they say the object is the receiver. 

501
00:24:53,259 --> 00:24:57,019
So the object here is the one who's receiving or being told to do 

502
00:24:57,019 --> 00:24:57,819
something, 

503
00:24:57,819 --> 00:25:00,769
okay? So the name of the variable is the receiver 

504
00:25:00,769 --> 00:25:04,250
and the method that's being called is called the message and so there's this whole 

505
00:25:04,250 --> 00:25:08,569
notion of sending messages, kind of like you could think of programming as I am, right, 

506
00:25:08,569 --> 00:25:12,049
and in your little I am you have like your friend, Bob, and your friend, Sally and your 

507
00:25:12,049 --> 00:25:14,178
friend, Dave, and your friend, Label, 

508
00:25:14,179 --> 00:25:18,140
right. And Label is just sitting there in I am and Label is not very talkative and once 

509
00:25:18,140 --> 00:25:21,640
in awhile you can send a message to I am which is a form of message 

510
00:25:21,640 --> 00:25:24,519
that label understands and then Label will do something. 

511
00:25:24,519 --> 00:25:27,660
So this is called the message that we send to the receiver, 

512
00:25:27,660 --> 00:25:31,860
is kind of the nomenclature. But a lot of times, you'll just hear people say, I made a 

513
00:25:31,859 --> 00:25:35,449
method call on the object and it means exactly the same thing. Again, we like to give 

514
00:25:35,450 --> 00:25:37,730
funky names to simple concepts. Okay, 

515
00:25:37,730 --> 00:25:44,730
so any questions about that, uh-huh? If you add the label first, then you start [inaudible]. 

516
00:25:45,140 --> 00:25:49,000
Yeah, you can still, after you sort of put the little object up there that says, 

517
00:25:49,000 --> 00:25:50,589
"Oh, the object's in the world, and 

518
00:25:50,589 --> 00:25:54,169
say, "Object change your color to red," and that's where it's a lot cooler than the felt 

519
00:25:54,170 --> 00:25:57,130
animals you had when you were a kid, because it'll just change to red right there on 

520
00:25:57,130 --> 00:25:57,919
the screen 

521
00:25:57,919 --> 00:26:00,759
because once it's been added, it's up on your canvas, 

522
00:26:00,759 --> 00:26:04,190
and now any messages that you send to it will actually change its appearance on 

523
00:26:04,190 --> 00:26:06,250
the canvas. Okay. 

524
00:26:06,250 --> 00:26:09,778
So with that said, let's kind of go through and understand a little bit more 

525
00:26:09,778 --> 00:26:11,679
about what are some of these 

526
00:26:11,679 --> 00:26:12,190


527
00:26:12,190 --> 00:26:16,570
messages or methods that we can send to particular kinds of objects. 

528
00:26:16,569 --> 00:26:20,859
So to do that, first let's talk about the graphics world that we live in. 

529
00:26:20,859 --> 00:26:23,219
There's a couple of things you want to think about the graphics world. How do 

530
00:26:23,220 --> 00:26:26,589
we specify all these things like locations and how is it laid out. Well 

531
00:26:26,589 --> 00:26:30,490
the origins, the upper left, so the upper left corner of your graphics window is 

532
00:26:30,490 --> 00:26:32,460
location 00. 

533
00:26:32,460 --> 00:26:36,620
Every thing in your graphics window is measured in pixels. Pixels are the little 

534
00:26:36,619 --> 00:26:39,250
dots on your screen, so if you get out a magnifying glass and you go up to your monitor 

535
00:26:39,250 --> 00:26:43,099
and you look real close and you get like eye strain and then you want to sue, 

536
00:26:43,099 --> 00:26:46,679
don't worry. It's just fine. At least you saw a pixel and it brought you al little closer 

537
00:26:46,680 --> 00:26:47,750
to Nirvana. 

538
00:26:47,750 --> 00:26:51,679
So a pixel is just a little squares essentially on your screen 

539
00:26:51,679 --> 00:26:53,490
that could actually get displayed. 

540
00:26:53,490 --> 00:26:56,819
All the numbers that we refer to are in terms of pixels. 

541
00:26:56,819 --> 00:27:01,269
So when we think about X and Y coordinates, X is how far you go across, 

542
00:27:01,269 --> 00:27:04,460
sort of across the screen and Y is how far you go down. 

543
00:27:04,460 --> 00:27:08,990
The X coordinates just increase to the right from 00 and the Y coordinates, 

544
00:27:08,990 --> 00:27:12,569
different than what you're used to in mathematics, Y coordinates go down. So 

545
00:27:12,569 --> 00:27:16,099
normally, we think of this as a negative Y direction. Not so in Java's world. This is 

546
00:27:16,099 --> 00:27:20,488
the positive Y direction going down. Hey that's just the way life is. It's been 

547
00:27:20,489 --> 00:27:24,329
like that since I was a wee tike, or actually, since Java came around. Okay, 

548
00:27:24,329 --> 00:27:28,429
and so you might wonder, "Hey, if I have some label, like Hello World, and I 

549
00:27:28,430 --> 00:27:30,298
specify some location, 

550
00:27:30,298 --> 00:27:33,969
where is that really on this Hello World thing and so 

551
00:27:33,969 --> 00:27:37,269
the G-label coordinates, the coordinates that 100,75 that we gave it were the X and Y 

552
00:27:37,269 --> 00:27:41,259
location of the base line of the first character. So that first pixel right 

553
00:27:41,259 --> 00:27:45,919
there on the lower left hand corner of the H, okay? 

554
00:27:45,919 --> 00:27:52,919
And so that's 175. That's how things were laid out. Uh-huh Is it possibly to specify [inaudible] using ratios? 

555
00:27:56,048 --> 00:27:59,039
Well, everything's in terms of pixels. There's some stuff you'll see later on in the 

556
00:27:59,039 --> 00:28:02,809
class where we actually specify some things like in polar coordinates if you're familiar 

557
00:28:02,809 --> 00:28:05,278
with polar coordinates, you'll get to that later on. 

558
00:28:05,278 --> 00:28:08,038
But you want to think of these numbers as just being absolute pixels and you 

559
00:28:08,038 --> 00:28:10,990
can do some math to figure out where those pixels should actually be. 

560
00:28:10,990 --> 00:28:14,308
But they're still just pixels, 

561
00:28:14,308 --> 00:28:16,269
just way off. All right, 

562
00:28:16,269 --> 00:28:17,869
so 

563
00:28:17,869 --> 00:28:21,449
remember all of these classes are organized in some hierarchy and everything 

564
00:28:21,450 --> 00:28:25,500
that we talked about, right, G-label, G rect, G-oval and G-line are all G-objects, 

565
00:28:25,500 --> 00:28:29,359
which means any behavior that a G-object has, all of these puppies have. 

566
00:28:29,359 --> 00:28:32,879
So let's start off by thinking about what are the behaviors or methods that are 

567
00:28:32,880 --> 00:28:37,049
available on G-objects. So one is set color; you just saw an example of that, 

568
00:28:37,048 --> 00:28:40,168
right, you specified the object, you say set color and you give it some color and I'll show you 

569
00:28:40,169 --> 00:28:41,538
where the colors come from 

570
00:28:41,538 --> 00:28:46,308
and it sets the color of the specified object to the color you just specified 

571
00:28:46,308 --> 00:28:48,629
as this thing called a parameter. 

572
00:28:48,630 --> 00:28:52,320
So the thing that's inside the 

573
00:28:52,319 --> 00:28:53,389
parentheses 

574
00:28:53,390 --> 00:28:58,070
in our formal speak, we refer to as parameter. So methods have parameters. 

575
00:28:58,069 --> 00:29:01,569
They're the values that you provide inside the parentheses 

576
00:29:01,569 --> 00:29:04,179
when you invoke a particular method, okay? 

577
00:29:04,180 --> 00:29:06,920
So there's a single parameter there, color. 

578
00:29:06,920 --> 00:29:10,769
There's also something there called set location, which takes an X and Y coordinates. 

579
00:29:10,769 --> 00:29:12,279
Again X and Y are in pixels 

580
00:29:12,279 --> 00:29:16,690
and it sets the location of that object to that XY, so this method has two 

581
00:29:16,690 --> 00:29:18,288
parameters, X and Y. 

582
00:29:18,288 --> 00:29:22,980
And you might say, "But Meron, when I created the G-label before, didn't I 

583
00:29:22,980 --> 00:29:27,130
specify the X and Y location when I told the factory to give me one of 

584
00:29:27,130 --> 00:29:29,049
these G-label objects?" Yeah, you did, 

585
00:29:29,048 --> 00:29:30,418
but you didn't have to, 

586
00:29:30,419 --> 00:29:33,989
and it turns out there's times you don't want to; you want to just say, "Hey give me a 

587
00:29:33,989 --> 00:29:37,639
label that's got the words Hello World in it and later on I'll figure out where 

588
00:29:37,638 --> 00:29:41,089
it's actually gonna go on the screen, so I'll specify what the X and Y are," but 

589
00:29:41,089 --> 00:29:43,368
until you specify the X and Y, it doesn't know, 

590
00:29:43,368 --> 00:29:46,368
but you can specify after you've created the object, is the 

591
00:29:46,368 --> 00:29:47,859
important thing. And 

592
00:29:47,859 --> 00:29:50,269
move, and here's the funky thing. You can actually tell an object, like you have 

593
00:29:50,269 --> 00:29:50,979
your little, 

594
00:29:50,979 --> 00:29:53,769
you know, Hello World somewhere on the screen and it's kind of like the 

595
00:29:53,769 --> 00:29:57,519
furniture idea of objects. You're like, "Yeah, I don't like how it looks there. Move it a little to 

596
00:29:57,519 --> 00:29:58,679
the right. 

597
00:29:58,679 --> 00:30:00,929
And so there's a move method 

598
00:30:00,929 --> 00:30:04,280
and its coordinates if you're sort of a Calculus person or DX and DY, and if you're 

599
00:30:04,279 --> 00:30:07,779
not a Calculus person, don't worry, you don't need to be a Calculus. This is the closest we 

600
00:30:07,779 --> 00:30:10,740
get to Calculus in this class. Rock on. DX 

601
00:30:10,740 --> 00:30:14,970
and DY just means how many pixels in the X direction and the Y 

602
00:30:14,970 --> 00:30:15,950
direction should you 

603
00:30:15,950 --> 00:30:18,569
move this object from its previous location 

604
00:30:18,569 --> 00:30:21,740
and these can be positive or negative, because you can actually have something 

605
00:30:21,740 --> 00:30:24,420
negative say in the Y direction and it'll move it up on the screen. 

606
00:30:24,420 --> 00:30:27,680
So it's just the offset or how much you want to, so 

607
00:30:27,680 --> 00:30:30,580
think of it as the difference in X and the difference in Y, how much do 

608
00:30:30,579 --> 00:30:37,329
you want to change where it moved. Uh-huh, question? Do you use a set location XY and then [inaudible]? 

609
00:30:37,329 --> 00:30:44,329
Can you do set location and so you - [Inaudible] 

610
00:30:44,659 --> 00:30:48,409
later define X and Y. No, if you want to say set location XY, you're 

611
00:30:48,409 --> 00:30:51,730
referring to two variables X and Y, so there's variables X and Y and each have already been 

612
00:30:51,730 --> 00:30:56,299
declared and have values. [Inaudible]. 

613
00:30:56,299 --> 00:31:00,419
Yeah, so anything that's a parameter here, you can actually use a variable rather 

614
00:31:00,420 --> 00:31:04,259
than an actual value, like an actual number, and we'll see some examples of that as we go 

615
00:31:04,259 --> 00:31:05,619
along. 

616
00:31:05,619 --> 00:31:06,829
[Inaudible]. 

617
00:31:06,829 --> 00:31:10,429
So where do all these colors come from? It turns out these colors come from a 

618
00:31:10,430 --> 00:31:13,750
standard library that someone in the Java world wrote that's called 

619
00:31:13,750 --> 00:31:18,179
the Java AWT package, so if you want to use these colors at the top of the 

620
00:31:18,179 --> 00:31:22,820
program, and I'll show you and example, that should say, importJava.AWT.star 

621
00:31:22,819 --> 00:31:26,599
and these are all the colors, so they're all the name color, dot and then what the 

622
00:31:26,599 --> 00:31:27,678
color actually is. 

623
00:31:27,679 --> 00:31:31,880
So you just saw an example where we used colored dot red to make Hello World 

624
00:31:31,880 --> 00:31:34,559
red, but there's all these different colors and they're all in your book, so you don't need 

625
00:31:34,558 --> 00:31:36,240
to scribble them down hurriedly, but 

626
00:31:36,240 --> 00:31:39,200
you know, different shades or black or grey or white and 

627
00:31:39,200 --> 00:31:42,919
you know, magenta and sienna if you're sort of a color photography sort of person, but 

628
00:31:42,919 --> 00:31:46,649
there's a whole set of colors and you can just go to town on them. Okay. 

629
00:31:46,648 --> 00:31:51,308
So all G objects, all of these things respond to these methods because 

630
00:31:51,308 --> 00:31:55,569
these guys are all objects, so any of those three methods will work on any of these 

631
00:31:55,569 --> 00:31:58,329
objects, of objects of any of these types. But 

632
00:31:58,329 --> 00:32:01,569
there are some additional behaviors, there are some additional methods 

633
00:32:01,569 --> 00:32:05,829
that some of the specialized classes, the sub classes actually implement. So 

634
00:32:05,829 --> 00:32:06,359
G-label 

635
00:32:06,359 --> 00:32:10,119
has some additional things that it does beyond just what a G-Object in 

636
00:32:10,119 --> 00:32:10,998
general would do. 

637
00:32:10,999 --> 00:32:12,950
And the big one, 

638
00:32:12,950 --> 00:32:16,029
well first of all, it's got what we refer to as the constructor. You can think of 

639
00:32:16,029 --> 00:32:19,019
the constructor and I'm gonna do a little violence to the term, 

640
00:32:19,019 --> 00:32:22,269
but you can think of the constructor as a factory, okay? 

641
00:32:22,269 --> 00:32:26,509
What the constructor says is, "Hey, I want a new one of these guys," so I use the word 

642
00:32:26,509 --> 00:32:29,869
new and then I specify the constructor, okay? 

643
00:32:29,869 --> 00:32:35,138
The constructor is the name of the class of which you're going to get a new object 

644
00:32:35,138 --> 00:32:38,898
and then some initial parameters. So the initial perimeters for a G-label for 

645
00:32:38,898 --> 00:32:39,338
example, 

646
00:32:39,338 --> 00:32:43,028
could be what you just saw, the text that you wanted to display and its initial X 

647
00:32:43,028 --> 00:32:45,329
and Y coordinate, okay. 

648
00:32:45,329 --> 00:32:46,779
There's some other ones. 

649
00:32:46,779 --> 00:32:50,049
For example, you can change the fonts, right, it makes sense for a label. It 

650
00:32:50,049 --> 00:32:53,369
doesn't make sense for a square or rectangle, to say, "Hey, square, you're font's 

651
00:32:53,369 --> 00:32:54,959
gonna be Helvetica." And 

652
00:32:54,960 --> 00:32:56,420
the square is like, 

653
00:32:56,420 --> 00:32:59,539
"I'm four lines, man, I don't have a font. What are you talking about?" 

654
00:32:59,539 --> 00:33:03,609
That's why the notion of setting a font isn't something that all G-objects have. 

655
00:33:03,609 --> 00:33:07,729
It's only something that G-labels have which means the G-rect and G-oval and G-line 

656
00:33:07,730 --> 00:33:09,140
don't have a set font. 

657
00:33:09,140 --> 00:33:12,320
This is just something that you can do with a G-label and what it does it says 

658
00:33:12,319 --> 00:33:13,710
specify the font 

659
00:33:13,710 --> 00:33:16,289
that you want that label to show up in on the screen, 

660
00:33:16,289 --> 00:33:20,049
and the general form for specifying the font - this is all in your book and it shows you examples of the 

661
00:33:20,049 --> 00:33:22,139
fonts, but the general form is 

662
00:33:22,140 --> 00:33:24,880
inside double quotes, because you're gonna give it as a piece of text, you 

663
00:33:24,880 --> 00:33:29,190
specify the family of the font. That would be something like times or Helvetica. 

664
00:33:29,190 --> 00:33:33,389
The style of the font, which is, you know, plain or bold or italic or bold or 

665
00:33:33,388 --> 00:33:36,699
italic, so yeah, now in fact you can do all those funky things your word processor does and 

666
00:33:36,700 --> 00:33:40,380
then the size of your font, how big or small you actually want it to be 

667
00:33:40,380 --> 00:33:43,580
and you specify that as some texts and that would be 

668
00:33:43,579 --> 00:33:47,869
what you send over to this sent font 

669
00:33:47,869 --> 00:33:49,588
method to set the text. Okay, 

670
00:33:49,588 --> 00:33:55,799
so any questions about that? 

671
00:33:55,799 --> 00:34:02,799
Uh-huh? [Inaudible]. No, you get them all through graphics program. Rock on. All right, so 

672
00:34:04,000 --> 00:34:11,000
how do we draw some geometrical objects? Have another question? [Inaudible]. 

673
00:34:13,659 --> 00:34:17,338
If you're doing a graphics program without any colors, you don't need 

674
00:34:17,338 --> 00:34:23,679
Java.awt. But if we're doing one with color, then we need Java.awt in addition? Yeah, 

675
00:34:23,679 --> 00:34:26,979
in addition to a graphics program, and Iell give you an example of that in just a second. I 

676
00:34:26,978 --> 00:34:29,068
have to show you a program that has it. 

677
00:34:29,068 --> 00:34:32,369
So drawing geometrical objects - it turns out some of these other guys have funky 

678
00:34:32,369 --> 00:34:35,579
things that you can do specifically with them. So first of all, there's the 

679
00:34:35,579 --> 00:34:38,199
constructors, the factories from which you can build new things. So 

680
00:34:38,199 --> 00:34:39,729
how do I make a new rectangle? 

681
00:34:39,728 --> 00:34:45,348
I say, "Hey, I want a new G-rect." I specify the X and Y location. It's the upper 

682
00:34:45,349 --> 00:34:49,028
left-hand corner of that rectangle and then I specify the width and height of 

683
00:34:49,028 --> 00:34:51,389
that rectangle in pixels. Okay. 

684
00:34:51,389 --> 00:34:55,399
So that's the basic idea is upper left-hand corner and then 

685
00:34:55,398 --> 00:34:59,639
width and height of the rectangle. 

686
00:34:59,639 --> 00:35:02,958
Similarly, for oval - ovals are kind of funky because you look at this and you're like, 

687
00:35:02,958 --> 00:35:07,078
"Hey I have an X and Y and I have a height. I didn't like -width and height like 

688
00:35:07,079 --> 00:35:10,789
ovals. I thought an oval was defined by like two [inaudible] and that whole thing 

689
00:35:10,789 --> 00:35:14,219
where you have a string. Did you ever do that - you stick the two like nails in the board and you 

690
00:35:14,219 --> 00:35:16,400
put the string around it and you draw 

691
00:35:16,400 --> 00:35:17,409
anyone? 

692
00:35:17,409 --> 00:35:19,989
There's like two people - yeah, like sorry. 

693
00:35:19,989 --> 00:35:23,489
Do it; go get a piece of wood, stick two nails in it, put some string around it and go get a 

694
00:35:23,489 --> 00:35:25,659
pencil, something you can draw ovals 

695
00:35:25,659 --> 00:35:29,248
and you can draw a 1 and if you're like 6 years old, you'll draw like 1,000, 

696
00:35:29,248 --> 00:35:34,068
and if you're 18, and draw one, you're like, "Yeah, that was just stupid." 

697
00:35:34,068 --> 00:35:37,708
But the basic idea behind an oval is the reason why we specify this way is 

698
00:35:37,708 --> 00:35:42,828
you specify sort of an imaginary rectangle for the oval and the oval shows 

699
00:35:42,829 --> 00:35:45,010
up as an oval that just happens to touch 

700
00:35:45,010 --> 00:35:49,040
the sides of the rectangle, which is how we sort of specify how wide and broad it is. 

701
00:35:49,039 --> 00:35:52,349
So think imaginary rectangle and sort of the four ends of the oval would sort of 

702
00:35:52,349 --> 00:35:54,538
be touching it, and I'll show you that 

703
00:35:54,539 --> 00:35:55,339
momentarily. 

704
00:35:55,338 --> 00:35:58,978
G line is very straightforward. G-line, the way we specify line just like 

705
00:35:58,978 --> 00:35:59,748
you know, 

706
00:35:59,748 --> 00:36:03,039
the days of yore with Euclid, line is defined by two points, 

707
00:36:03,039 --> 00:36:06,360
right, that's an infinite line, but we can think of where it ends at the two points. So 

708
00:36:06,360 --> 00:36:10,829
we specify an initial XY location and an ending XY location and it will just 

709
00:36:10,829 --> 00:36:13,130
draw a line between those two points, 

710
00:36:13,130 --> 00:36:20,130
pretty straightforward. Uh-huh? [Inaudible] is the set location defined as the lower left corner of your object, and 

711
00:36:25,179 --> 00:36:30,910
the G-rectangle [inaudible] left corner? Well, for set location is the lower left corner of for textural objects, 

712
00:36:30,909 --> 00:36:36,068
and then it becomes different for rectangles and ovals and stuff like that, yeah. 

713
00:36:36,068 --> 00:36:39,009
So it's a slight variation because that's just because we're trying to 

714
00:36:39,009 --> 00:36:43,498
deal with, you know, texts and rectangles symmetrically and it's tough to do, okay. 

715
00:36:43,498 --> 00:36:47,389
There's also some additional methods that are just shared by G rect and G-oval. 

716
00:36:47,389 --> 00:36:49,489
They don't actually apply to lines 

717
00:36:49,489 --> 00:36:52,420
or to labels for that sense, 

718
00:36:52,420 --> 00:36:55,710
rectangles and ovals can be filled, so they can either be an outline. You can either have 

719
00:36:55,710 --> 00:36:58,720
a rectangle that looks just like this which is an outline 

720
00:36:58,719 --> 00:37:02,628
or it can be filled in which means it's just a big solid 

721
00:37:02,628 --> 00:37:04,318
chunk of stuff. So 

722
00:37:04,318 --> 00:37:08,088
you said if it's filled, you say this is either true or false with the actual 

723
00:37:08,088 --> 00:37:12,219
word true or false. So if you've set it to true, then when you draw it on there, 

724
00:37:12,219 --> 00:37:15,898
you say, "Hey, put this thing up on my canvas." By adding it to the canvas, it 

725
00:37:15,898 --> 00:37:18,228
will show up filled in; it will show up solid. 

726
00:37:18,228 --> 00:37:21,308
And if this is set to false, it just shows up as the outline of the object, 

727
00:37:21,309 --> 00:37:24,749
whether or not that's an oval or a rectangle. 

728
00:37:24,748 --> 00:37:28,719
And almost done here. There's also a notion of a set fill color, 

729
00:37:28,719 --> 00:37:32,209
and you might be wondering, you're like, "But Meron, you told me that set color. 

730
00:37:32,210 --> 00:37:33,639
Here's where things get a little funky, 

731
00:37:33,639 --> 00:37:35,298
you told me that set color 

732
00:37:35,298 --> 00:37:38,298
is something that was defined for a G-object, 

733
00:37:38,298 --> 00:37:42,130
so if set color is defined over here, doesn't that mean that a 

734
00:37:42,130 --> 00:37:45,579
G-rect and a G oval already can set that color?" 

735
00:37:45,579 --> 00:37:46,890
Yes, that's true. 

736
00:37:46,889 --> 00:37:51,019
"So what's the set fill color all about?" And this is a slight variation. When you 

737
00:37:51,019 --> 00:37:52,489
set color for something, 

738
00:37:52,489 --> 00:37:54,828
you set the color for the whole thing 

739
00:37:54,829 --> 00:37:57,519
if that's solid or if it's an outline, you're setting the color for the whole 

740
00:37:57,518 --> 00:38:02,808
thing. When you set fill color, you are just setting the color for this internal 

741
00:38:02,809 --> 00:38:06,548
portion. So imagine like this was your rectangle, 

742
00:38:06,548 --> 00:38:09,619
what color used to fill it in can actually be different than what color 

743
00:38:09,619 --> 00:38:12,879
the line is. So if you want the line, say, to be black and you want the fill 

744
00:38:12,878 --> 00:38:15,210
to be red, you can actually do that 

745
00:38:15,210 --> 00:38:18,599
by setting the fill color to be red and then the outline is still whatever 

746
00:38:18,599 --> 00:38:20,829
color the actual outline was 

747
00:38:20,829 --> 00:38:25,420
if you want to do that. Okay, so if the color - if the interior is fill, so the set 

748
00:38:25,420 --> 00:38:28,829
fill has to be true, then you can set the fill color to be different. That's a 

749
00:38:28,829 --> 00:38:31,898
minor thing, but it's kind of fun if you're drawing stuff, okay? 

750
00:38:31,898 --> 00:38:35,568
So any questions about that? Oh, 

751
00:38:35,568 --> 00:38:42,568
wait - we have a question over there, uh-huh. [Inaudible]. 

752
00:38:43,619 --> 00:38:46,239
Oh, in the place of this word fill? Yeah. 

753
00:38:46,239 --> 00:38:49,920
It's either the word true or the word false. 

754
00:38:49,920 --> 00:38:53,700
So if you say true, set filled is true which means it will be a 

755
00:38:53,699 --> 00:39:00,699
solid thing; it will be filled in. Uh-huh? If your when you're using the [inaudible], you specify a [inaudible], what is the why would you use the set location [inaudible]. 

756
00:39:08,009 --> 00:39:09,619
Maybe you wanna do some animation. 

757
00:39:09,619 --> 00:39:13,940
Say like you have like your bunny in the world and he's gonna want to move along. [Inaudible]. 

758
00:39:13,940 --> 00:39:16,880
Well, it's set somewhere to begin with and then later on, guess what you're 

759
00:39:16,880 --> 00:39:20,960
actually gonna be doing a program that involves animation. Rock on. 

760
00:39:20,960 --> 00:39:23,349
And you will move things around. 

761
00:39:23,349 --> 00:39:27,509
So that's where it comes from. All righty. So 

762
00:39:27,509 --> 00:39:30,920
one last thing that might be useful to you, sometimes you want to say, center 

763
00:39:30,920 --> 00:39:33,278
something in the graphics window or draw something in the bottom, how do you know 

764
00:39:33,278 --> 00:39:35,170
how big the graphics 

765
00:39:35,170 --> 00:39:36,639
window actually is. 

766
00:39:36,639 --> 00:39:39,299
There's two methods from the graphics 

767
00:39:39,300 --> 00:39:40,229
program class, 

768
00:39:40,228 --> 00:39:43,918
so these methods you might notice they don't have any objects, that is their 

769
00:39:43,918 --> 00:39:47,609
receiver, and the reason for that is that both of these methods, 

770
00:39:47,610 --> 00:39:51,099
their receiver is the graphics program, so if you don't specify 

771
00:39:51,099 --> 00:39:53,009
the receiver for a method, 

772
00:39:53,009 --> 00:39:57,369
the thing that is actually receiving the method is the encapsulating class. Okay, 

773
00:39:57,369 --> 00:40:01,099
just like in Karel, when you had Karel and you said move and you didn't specify an object for 

774
00:40:01,099 --> 00:40:02,068
the move, 

775
00:40:02,068 --> 00:40:04,418
the move is going into Karel, so get width and get height. 

776
00:40:04,418 --> 00:40:08,088
We'll return to you the width and the height of the graphics window in pixels. So 

777
00:40:08,088 --> 00:40:11,369
these actually give you back a numeric value that you can, for example, assign to a 

778
00:40:11,369 --> 00:40:18,369
variable and then do manipulations on it. Uh 

779
00:40:22,608 --> 00:40:26,199
huh. If you don't fill in [inaudible] something like set color to change the entire background of the graphics [inaudible]? Well, you can make the huge rectangle the size of the graphics 

780
00:40:26,199 --> 00:40:29,180
window and set it to be filled and set it to be a color and then you can 

781
00:40:29,179 --> 00:40:35,788
change the whole background that way. Uh-huh? Can you then change the size of the graphics window? 

782
00:40:35,789 --> 00:40:39,219
There is some stuff for later on, but for right now, just think of a fixed size 

783
00:40:39,219 --> 00:40:40,599
graphics window. Yeah, 

784
00:40:40,599 --> 00:40:43,559
so let me push on just a little bit if we can 

785
00:40:43,559 --> 00:40:46,650
because what I want to actually show you now is a graphics window that 

786
00:40:46,650 --> 00:40:50,189
makes, or a graphics program that makes use or a bunch of this stuff 

787
00:40:50,188 --> 00:40:53,828
so over here, here's a little program called fun graphics, okay. 

788
00:40:53,829 --> 00:40:57,869
And so we have a comment up at the top and here are all the imports you need. Notice I 

789
00:40:57,869 --> 00:40:58,980
[inaudible] for you. 

790
00:40:58,980 --> 00:41:02,559
We need acm.graphicstar because we're gonna write a graphics program. 

791
00:41:02,559 --> 00:41:06,619
We need acm.program.star because all programs that you write in this class are gonna 

792
00:41:06,619 --> 00:41:07,339
need that. 

793
00:41:07,340 --> 00:41:12,260
And because we're gonna use colors we additionally need Java.awt.star. 

794
00:41:12,260 --> 00:41:15,080
If we weren't gonna use colors, we wouldn't need that, but it's needed for the 

795
00:41:15,079 --> 00:41:22,079
colors. Uh-huh. 

796
00:41:23,239 --> 00:41:26,068
[Inaudible]. It did; it might have just been hidden 

797
00:41:26,068 --> 00:41:29,338
because sometimes it just appears hidden until you expand it out, but it actually 

798
00:41:29,338 --> 00:41:30,328
had it in there. 

799
00:41:30,329 --> 00:41:33,548
So here's what we're gonna do. Let me just run this program, show you what 

800
00:41:33,548 --> 00:41:36,818
it displays on the screen and then show you how we actually get that display, so it's just 

801
00:41:36,818 --> 00:41:41,150
like, "Hey Meron, you're just drawing garbage on my screen," no, there's really like 

802
00:41:41,150 --> 00:41:43,829
a method to this madness. Not really, 

803
00:41:43,829 --> 00:41:47,360
there are multiple methods to this madness [inaudible]. I 

804
00:41:47,360 --> 00:41:49,030


805
00:41:49,030 --> 00:41:50,139
know, 

806
00:41:50,139 --> 00:41:51,338
sometimes 

807
00:41:51,338 --> 00:41:54,409
computer scientists should not be - that was actually not a canned joke. 

808
00:41:54,409 --> 00:41:58,679
That just came tome - how horrible is that? 

809
00:41:58,679 --> 00:42:00,579
So here is what's going on. 

810
00:42:00,579 --> 00:42:03,038
We drew a bunch of stuff on that screen, how did it get there. 

811
00:42:03,039 --> 00:42:06,679
Well, hey Hello World; we probably knew how that got there. This should all be familiar to 

812
00:42:06,679 --> 00:42:08,838
you now. We got a new G-label. 

813
00:42:08,838 --> 00:42:12,759
We had some variable called label that we assigned it in. We set its font. We set its 

814
00:42:12,759 --> 00:42:15,900
color to red. Because we set its color to red, that's why we need 

815
00:42:15,900 --> 00:42:18,980
www.Java.awt because we're using colors. 

816
00:42:18,980 --> 00:42:22,969
We set its color to red and we said, "Hey, I got Hello World, throw it up on the 

817
00:42:22,969 --> 00:42:26,389
screen," so we add label. You give it the name of the variable and 

818
00:42:26,389 --> 00:42:27,539
there it appears. 

819
00:42:27,539 --> 00:42:29,409
Over here we say, "Hey G-rect. 

820
00:42:29,409 --> 00:42:33,779
I want a new rectangle that's upper left-hand coordinate is 10,10 and 

821
00:42:33,780 --> 00:42:38,360
it's size is gonna be 50,50, which means it is a square. It's not filled 

822
00:42:38,360 --> 00:42:41,960
in. I haven't specified its color right," so if you don't say set fill, it's not automatically 

823
00:42:41,960 --> 00:42:44,858
filled in. If you don't specify the color to automatically black 

824
00:42:44,858 --> 00:42:45,848
and then I add it 

825
00:42:45,849 --> 00:42:50,949
and then you get that little box all the way up in the upper left-hand corner. Then, I say, "Hey, 

826
00:42:50,949 --> 00:42:54,628
you know what, I want some other G-rect because it's cooler." It's gonna have 

827
00:42:54,628 --> 00:42:58,469
colors and it's gonna be bigger and better and it's gonna be nationwide, 

828
00:42:58,469 --> 00:43:01,349
and so what it's gonna do is I'm gonna have rect 2 which is 

829
00:43:01,349 --> 00:43:05,369
another variable of Type G-rect. It's perfectly fine for me to have multiple 

830
00:43:05,369 --> 00:43:08,058
objects of the same type which they knew G-rect 

831
00:43:08,059 --> 00:43:12,829
that's upper left-hand coordinates is 300,75 and whose size is 

832
00:43:12,829 --> 00:43:16,539
honking - it's 200 by 100, so it's big long thing, 

833
00:43:16,539 --> 00:43:19,939
and it's gonna be filled in and I'm gonna set its fill color to be red, so the 

834
00:43:19,938 --> 00:43:21,308
whole thing is red, 

835
00:43:21,309 --> 00:43:22,819
throw it up there 

836
00:43:22,818 --> 00:43:25,358
and so what I get is that big, rectangle. 

837
00:43:25,358 --> 00:43:28,969
Just ignore the big oval one in front of it for now; I get this big red 

838
00:43:28,969 --> 00:43:29,869
rectangle up there. 

839
00:43:29,869 --> 00:43:32,960
And I say, "Well, that's kind of cool, but I want to see what this oval thing is all 

840
00:43:32,960 --> 00:43:33,809
about. 

841
00:43:33,809 --> 00:43:36,259
So Hey, Meron, 

842
00:43:36,259 --> 00:43:37,690
make me an oval." 

843
00:43:37,690 --> 00:43:41,920
And I'm like, "All right, well, G-oval will make you an oval," and the dimensions of the oval, 

844
00:43:41,920 --> 00:43:45,990
its upper left-hand coordinate and its size are exactly the same as the 

845
00:43:45,989 --> 00:43:49,199
rectangle, and the reason for doing that is to show you the oval in relation 

846
00:43:49,199 --> 00:43:51,219
to the rectangle that you specified. 

847
00:43:51,219 --> 00:43:53,058
It's set filled is true 

848
00:43:53,059 --> 00:43:57,380
and it's set fill color, not its set color, but its set fill color 

849
00:43:57,380 --> 00:44:00,869
is set to be green, which means the internal dividend will be green, the outline 

850
00:44:00,869 --> 00:44:04,359
of the oval will still be black, because we did not set its color and then we 

851
00:44:04,360 --> 00:44:04,939
added 

852
00:44:04,938 --> 00:44:08,739
and there you get that green oval and you can notice its four ends touch the same 

853
00:44:08,739 --> 00:44:10,130
four ends as this rectangle 

854
00:44:10,130 --> 00:44:14,349
and if you look real closely there's actually a green line that demarcates the 

855
00:44:14,349 --> 00:44:17,660
oval and then the middle fill is actually green. It might be a little hard to see, but 

856
00:44:17,659 --> 00:44:18,788
that's the case. 

857
00:44:18,789 --> 00:44:22,230
Then we also have a line, so I will just call this my funky line, 

858
00:44:22,230 --> 00:44:26,400
and my funky line is a line that starts at 100,150 and it extends to 

859
00:44:26,400 --> 00:44:29,099
the location 200,175 

860
00:44:29,099 --> 00:44:31,208
and there is my funky line. 

861
00:44:31,208 --> 00:44:33,328
Yeah, it's a funky line. All right, 

862
00:44:33,329 --> 00:44:35,548
and then I add another thing and you're like, 

863
00:44:35,548 --> 00:44:37,248
"Dude where's my line." 

864
00:44:37,248 --> 00:44:41,548
And so Dude where is my line is a line that should cut across the entire graphics window 

865
00:44:41,548 --> 00:44:45,088
because it starts at 00, well not the entire graphics window. It starts at 00 

866
00:44:45,088 --> 00:44:48,288
in the upper left-hand corner and goes to 100,100, and this is a common error which is 

867
00:44:48,289 --> 00:44:49,999
why I how it to you. You look up there and you're like, 

868
00:44:49,998 --> 00:44:54,308
"But Meron, there's only one line," and that's your funky line or my funky line as the case 

869
00:44:54,309 --> 00:44:55,180
may be. 

870
00:44:55,179 --> 00:44:57,989
Where is Dude where is my line? 

871
00:44:57,989 --> 00:45:01,378
And it's not up there. Why is it not up there? [Inaudible]. 

872
00:45:01,378 --> 00:45:05,099
Yeah. You did not add it to the graphics content, okay. 

873
00:45:05,099 --> 00:45:08,279
Common error - people will go ahead and create something and add all the colors and set all 

874
00:45:08,280 --> 00:45:12,200
the sides and be like, "Ah, I'm rocking," and then they'll run their program and nothing shows up and they 

875
00:45:12,199 --> 00:45:16,538
start tearing their hair out and beating their section leader and it's a bad time, and they're sitting there 

876
00:45:16,539 --> 00:45:17,930
contemplating life in jail 

877
00:45:17,929 --> 00:45:19,298
and then they realize, 

878
00:45:19,298 --> 00:45:22,659
"I just should have added it to the canvas, all right." 

879
00:45:22,659 --> 00:45:26,769
But you can't give that as an excuse to the judge. All right. So any questions about 

880
00:45:26,768 --> 00:45:28,358
that? 

881
00:45:28,358 --> 00:45:31,858
All right. Let me very quickly - I need to push on real quickly. So I want to tell you 

882
00:45:31,858 --> 00:45:35,068
a little bit about expressions before we 

883
00:45:35,068 --> 00:45:37,958
finish up and we'll do more expressions, but 

884
00:45:37,958 --> 00:45:42,608
you need to know some basic stuff on expressions. 

885
00:45:42,608 --> 00:45:47,548
So all an expression is is you already saw one. And expression is something like INT 

886
00:45:47,548 --> 00:45:52,009
total, equals N1 plus N2. This is an expression. 

887
00:45:52,009 --> 00:45:55,829
It's just some mathematical expression that tells us how things kind of relate 

888
00:45:55,829 --> 00:45:56,729
to each other. 

889
00:45:56,728 --> 00:46:01,038
And basically all an expression is it's a bunch of terms and these terms can be either 

890
00:46:01,039 --> 00:46:02,440
individual variables 

891
00:46:02,440 --> 00:46:07,409
or they can be constant values, like they ca be like the Number 2 or 3 

892
00:46:07,409 --> 00:46:11,499
or they could be some method called like Read INT that actually gets a 

893
00:46:11,498 --> 00:46:14,178
value from the user, for example, 

894
00:46:14,179 --> 00:46:17,898
and there's some operators between them, like the addition operator, and so when 

895
00:46:17,898 --> 00:46:21,548
you take some terms and you put operators between them to combine the 

896
00:46:21,548 --> 00:46:22,119
terms 

897
00:46:22,119 --> 00:46:25,450
that's called an expression, all right. Fairly straightforward stuff - you've probably seen these. 

898
00:46:25,449 --> 00:46:27,639
There's a bunch of operators you should know about. 

899
00:46:27,639 --> 00:46:31,539
The operators are things like plus, minus, 

900
00:46:31,539 --> 00:46:36,778
times, which is actually a star or asterisk, which is how we refer to times. It's not as X - 

901
00:46:36,778 --> 00:46:40,838
division and then strangely enough, this thing's that looks like percent and it's 

902
00:46:40,838 --> 00:46:44,778
not the percentage operator, although you might think that. This is what's known as 

903
00:46:44,778 --> 00:46:50,268
the remainder operator, okay. So these puppies generally work like you 

904
00:46:50,268 --> 00:46:54,838
would think, add two numbers, subtract two numbers. There's also a uniary minus. 

905
00:46:54,838 --> 00:46:58,009
The uniary minus is the funky mathematical way of saying negative sign. 

906
00:46:58,009 --> 00:47:00,009
So you could actually say like 

907
00:47:00,009 --> 00:47:05,449
equals negative N1 plus N2. That's like a negative minus. That's perfectly fine. 

908
00:47:05,449 --> 00:47:09,579
Okay, or you can use that same thing, you know, as subtraction if 

909
00:47:09,579 --> 00:47:11,959
it's between two terms. 

910
00:47:11,958 --> 00:47:12,899
Multiplication, division, 

911
00:47:12,900 --> 00:47:17,259
the thing about division that is funky is it depends on how division is used, 

912
00:47:17,259 --> 00:47:20,009
okay and I'll tell you a little bit more about that next time, but I want to talk 

913
00:47:20,009 --> 00:47:21,769
about remainder right now. 

914
00:47:21,768 --> 00:47:24,649
And remember back in second grade, when you did like 

915
00:47:24,650 --> 00:47:26,340
short division and you said, 

916
00:47:26,340 --> 00:47:30,759
"Oh, I had 5 and 2 - how many times does 2 go into 5 - 

917
00:47:30,759 --> 00:47:35,838
2 and you were like remainder 1. Anyone remember that? 

918
00:47:35,838 --> 00:47:39,048
Wasn't that such good time? I loved remainder - like 

919
00:47:39,048 --> 00:47:42,438
that's the online like math. I'm like - and when the remainder was 0, I 

920
00:47:42,438 --> 00:47:46,139
got pissed. 

921
00:47:46,139 --> 00:47:49,748
But that's what remainder was all about. It's just what's left after you do the 

922
00:47:49,748 --> 00:47:53,438
division. So that's the way you should think about percentage. It's the remainder 

923
00:47:53,438 --> 00:47:55,408
operator, and you're like, "So how does that work?" 

924
00:47:55,409 --> 00:48:00,019
So what that means is if I say 7 remainder 2, 

925
00:48:00,018 --> 00:48:05,438
the value of this is 1 because it's after I divide 7 by 2 and I see 

926
00:48:05,438 --> 00:48:08,858
how many whole times 2 goes into 7, what's the remainder, 

927
00:48:08,858 --> 00:48:09,748
it's 1, okay? 

928
00:48:09,748 --> 00:48:13,788
You could say something funky, like, hey what's 7 

929
00:48:13,789 --> 00:48:14,770
remainder 20? You're 

930
00:48:14,769 --> 00:48:20,380
like well, 20 doesn't go into 7, Meron. So that would be 0 remainder - 

931
00:48:20,380 --> 00:48:22,088
rock on. 

932
00:48:22,088 --> 00:48:24,058
So that's the way it works. 

933
00:48:24,059 --> 00:48:27,309
It's not the modulus operator. If you're a mathematician, or if you've worked with 

934
00:48:27,309 --> 00:48:31,119
other languages and you've seen clusters and you're like, "Oh, that's the modular operator." 

935
00:48:31,119 --> 00:48:35,189
It's not in Java; it's remainder an dif you try to apply it to negative numbers, 

936
00:48:35,188 --> 00:48:38,818
unexpected things, what's not unexpected; it's just not what you would have expected in the 

937
00:48:38,818 --> 00:48:41,378
mathematical sense. So as far as you see it in the book and everything we do in 

938
00:48:41,378 --> 00:48:44,929
this class, just apply it to positive integers and you can only apply it to 

939
00:48:44,929 --> 00:48:48,568
integers so there's no remainder with valued doubles, 

940
00:48:48,568 --> 00:48:51,518
there's no remainder. And so when we come back next time we'll talk a little bit 

941
00:48:51,518 --> 00:48:53,679
about some of these expressions. 

942
00:48:53,679 --> 00:48:56,328
Any questions before we break? All right, and 

943
00:48:56,329 --> 00:48:58,999
I'll see you - oh, there's a question. Why don't you 

944
00:48:58,998 --> 00:49:00,358
just come up and ask it. Thank you.