Java for Beginners in One Hour | Derek Wan | Skillshare

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Java for Beginners in One Hour

teacher avatar Derek Wan, Software Engineer and Teacher

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Taught by industry leaders & working professionals
Topics include illustration, design, photography, and more

Watch this class and thousands more

Get unlimited access to every class
Taught by industry leaders & working professionals
Topics include illustration, design, photography, and more

Lessons in This Class

10 Lessons (1h 8m)
    • 1. Introduction

      3:35
    • 2. Program Anatomy

      4:39
    • 3. Methods

      13:05
    • 4. Replit Demo

      6:35
    • 5. Control

      12:25
    • 6. Objects Part 1

      7:54
    • 7. Objects Part 2

      3:37
    • 8. Comments

      2:25
    • 9. Arrays and ArrayLists

      6:56
    • 10. Sudoku

      6:32
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About This Class

This class covers essential topics for every programmer and teaches you everything you need to know to build a fully functioning, interactive Sudoku game. This is not a class that takes shortcuts; instead, I've distilled the essentials of Java to an hour so you can quickly build a foundation on which you can expand your programming skillset. 

What You'll Learn

  • Access Modifiers
  • File Structure
  • Packages
  • Methods
  • Replit
  • Boolean Expressions
  • Conditional Logic Control
  • Loops
  • Objects
  • Instance vs Static Paradigms
  • Comments
  • Arrays
  • ArrayLists

No prior experience or knowledge is required. Perhaps even more importantly, no specialized hardware or setup is required either, since all code will be run through a sandboxed environment in your browser. You can focus all your energy on just Java, instead of haggling with your computer's quirks. All you need for this course is Internet, your device, and an hour of time. 

Slides can be found herehttps://drive.google.com/drive/folders/1W_K68_lFghJ2PsFXYkt035Ikt4X-lxW2?usp=sharing

Part 2 installment coming soon.

Meet Your Teacher

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Derek Wan

Software Engineer and Teacher

Teacher

I was formerly an instructor for the largest computer science course at the University of California, Berkeley, where I taught undergraduates for several years. I am now a full-time software engineer working on experimentation platforms at a large tech company based in Cupertino, California. However, I'm still passionate about tech education and decided to start making Skillshare courses to get more people excited about coding! I have more courses in the works, but check out what I have so far by scrolling down.

Website | Github | Research | Thesis

 

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Transcripts

1. Introduction: Hi there. My name is Derek and welcome to my course on Java. You probably have your reasons for being here. Maybe you've been job Clinton recently only to scroll down to the qualifications section and realized that they require you to know Java and other programming languages as a basic prerequisite. Maybe you've been looking at compensation bands and realize that software engineers are paid very generously. Or maybe you just want to learn something interesting to kick off the new year. Regardless of why you ended up here, java is a great skill to learn. But what is Java? Java is a performance and popular programming language, meaning that you'll be able to write fast programs. You'll be in high demand for your skills. Java's also easy to pick up and transferable, meaning that once you learn Java, it will make learning other programming languages even easier. Now that all sounds great, but there are tons of other programming classes out there. Why should they choose this one? Well, we've got lots in store for you. Firstly, we have all the source code in this course ready to go for you in an online environment, meaning that all you gotta do is press the green Run button. No need to spend hours banging your head against your computer trying to figure out what's wrong with your local setup. With one click of a button, you're ready to see the output of your programs. We also have a very visual resume ready project for YouTube, meaning that at the end of this course, you'll have something to show off to your friends and put on your resume. We have concrete example code samples in every lesson so that you don't get confused by abstract concepts are concepts are introduced with an easy to follow and visual walk-through. Finally, I will be actively maintained in this course. So feel free to post questions if things are not clear. Before I let you loose into the course content, I should probably first introduce myself. I am a professional software engineer and for my entire career, I have worked at some of the most famous and valuable tech companies in the world. I have real-world experience delivering concrete technical products, some of which you may have encountered. Before I was a software engineer. I taught undergraduate Computer Science at the University of California, Berkeley, where we regularly solve thousands of students this semester. Course was named one of the top five in the nation by Bloomberg. And I'm currently one of the most popular instructors under 61, a departmental YouTube channel where we've amassed about 900,000 views as of 2022. During my time at Berkeley, I also want multiple teaching awards and was given the distinguished GSA Award in 2020, which was given to the top 13 instructors and the entire computer science department out of several hundred. Finally, I hold a graduate degree in computer science also from Brooklyn. As you can see, I love all things tech and I love teaching. I'm super excited to have you guys along for the ride with me. One last thing before we start the course, I ask that you please create an account on repl it.com and verify your e-mail once you have done so. Now, I know I know. Maybe you just made a Skillshare account and you're probably not in the mood to make another account. Well, as I alluded to earlier, we have an online environment setup for you to play around with and to build your project. And believe me, the alternative is much, much worse. The alternative to the online environment would be setting up a local dev environment on your computer. There are 1 million ways that could go wrong and it could take hours to troubleshoot. So I ask that you spare a couple of minutes right now to please make an account right here. That's it for me in this video, I'll see you in the next one. If you'd like to access the slides. The link will be posted in the course description. 3. Methods: Welcome friends. In this video, we're going to talk about methods. First of all, what is a method? A method is pretty much anything that performs an action within a class. And the definition for inaction is pretty broad. It could be anything. It could be simple arithmetic. There could be running a machine learning algorithm. It could be rendering a frame. Classes contain methods, and methods contain all the logic within a program. Now, methods must be declared before we can start writing logic in them. There are four components to this declaration. First, we have the access modifier. We have four types, public, private, private, and protected. Access modifier on a mirth method works exactly the same way it works for classes. If the axis is not explicitly stated, the method defaults to package private, just like a class would. A public method is one that can be referenced from any class. Panda package private method is one that can only be accessed if it is referenced from a method of a class within the same package. Also recall that previously promised, I will explain private and protected once we started talking about methods, well, it turns out we need to know even more about Java before we can talk about protected. So we'll leave that one out for now. However, we can talk about private code that is marked private can only be accessed within the same class. So e.g. if I declared method X to be private, that means that only other methods within the same class would be allowed to reference method X. Methods of other classes regardless of their package, wouldn't be allowed to reference a private method. Next, we have static. We can choose to omit the word static in our Declaration, in which case the method will default to non-static. Now, don't worry about what static actually means just yet. Unfortunately, you'll need more context on Java before you'll be able to understand it. So for now, just be aware that the static keyword exists and it comes after the access modifier. Next we have the return type which must always be present. The return type is, as the name suggests, the type of the value that is returned by a method. In this case, nothing is returned by this method. So we say its return type is void. Now, you may be a bit confused about what it means for a method to return something. The return value of a method is the value output. E.g. imagine we have a method called add and all it does is it adds two integers together. So e.g. add of 1.2 gives us three. In programming, we will call this three the returned value of the function add. Return value is synonymous with the output value. And in this camp and in our ad example, the type of the return value would be an integer, since three is an integer. Finally, we have the method name. Names generally can be whatever you want them to be. The only restriction, I believe is that the first character of the method name must be a letter. Now, let's talk about types in Java. Like we said in the previous slide, every method needs to return type. So it's important that we understand what some of those types may be. First, we have a string, which is a doubly quoted human readable piece of text. Recall in the previous slide, we had Hello World, right here, right here. This is a doubly quota human-readable text. It is an example of a string. Next, we have an integer, an int, which is Java speak for integer. We also have floats, which is Java's way of saying a number with a decimal point, like 1.3 e.g. we have booleans which are values that can only be true or false. We also have array types, which looked like int bracket, bracket or Boolean bracket bracket. An array of items is basically just an ordered collection of elements of the type. So e.g. if we specify int bracket bracket, then that means that that value is going to be an ordered collection of integers. And if we have Boolean bracket, bracket, then that is going to be a collection of true or false values. Java has a lot more types than justice, but these are some of the most common types will see. And they'll serve as a good foundation for the remainder of this course. Next, we need to talk about method arguments. A method argument is just a formal way of saying the inputs or parameters of a method. The arguments of the method are specified in parentheses after the name of the method. And the Java type of the arguments must be explicitly specified. E.g. here we have a string array type and we're given the name of the argument args. This means that when we call this method, we must give it an input of type string array. Otherwise, Java will refuse to compile the code and will not run. There are a couple of subtle points you should be aware of. The first is that this method main over here is very special. Typically a Java application, we will have several files and we would need to specify an entry point. Otherwise, Java would have no way of knowing which piece of code just start running first. So in this case, we would specify that we want the helloworld class to be our entry point. At which point java would then look for a main method that has exactly this signature, public static, void main with one argument that has type string array. It will run this method if it exists as the initial entry point into the program. Otherwise, Java will throw an error saying that there's no main method defined. Second, if you want to have a method taken multiple arguments, we will need to explicitly type each of those arguments and separate them with a comma. In this ad example, we have two arguments, x and y, and we specify the types Int for both of them. And this function add simply returns the result of adding these two numbers return x plus y. As a side note, note that when we specify a return type and the method signature, e.g. over here, we must include a line inside this method that explicitly says return in Maine, e.g. we have a void, which means we return nothing. Note that there is no line in this method that says return. Third, it's totally fine to have zero arguments to. You would simply put an empty set of parentheses after the method name. Now, you may be thinking that this requirement explicitly type everything in Java is really burdensome and only exists to slow you down. But if that were true, Java wouldn't be one of the most popular programming languages and the software industry today. The reason we have these requirements is that these allow Java to perform something we call compile-time checking. Before you run a Java program, your computer must first convert the code into something called byte code. We call this process compilation. When there's compilation is complete and you run the program, you're actually running the bytecode that is produced. Now the advantage of having a compile step before running the code is that the Java compiler is able to analyze your code and perform some basic sanity checks. One of those sanity checks is that all the specified types in your logic are consistent. E.g. as we mentioned earlier at here, expects two integers. Imagine if I tried passing the number two, a string, hello world as arguments into ad. Well that would make no sense. How do you add the number two to a piece of text Hello World. So Java's advantage is that it will run these sanity checks at compile time and notify you of any errors before the code is run. In other languages without compile-time checking, there may be an issue in your code that you wouldn't know about until the code is run. And that could be very devastating if your code is deployed in say, a bank or maybe on Google, which is used by 1 billion people every day. Now, let's talk about the method body. First. The method body is simply all the code within these brackets. There's not really any restriction on what you can do in the body and the method body can be as long as you want. Regardless of whatever logic you decide to perform. The main thing you need to ensure is that the type of the return value needs to match the specified return in the method declaration. Here, we're returning some, which is a variable we declared and initialized with the value x plus y. Since x and y are both ints, the addition of two is also an int. Therefore, returning sum is compliant with our requirement to return an int in this method. The next point I want to emphasize is that the return line is terminal. Java compilation will fail if you try to put any code after the return line in a method, which makes sense since the return value is the output value of the method. Why perform additional logic after you've already output a final value? We also need to talk a bit about variables that you create and use within a method. Just like method arguments, variables you defined within the method must be explicitly typed when you create them. And you can name them anything you want as long as the first character of the name is a letter. After declaring the variable sum as an int, e.g. we can no longer change its type within the same method. So I cannot do something like int sum equals x plus y. And then in the next line do string sum equals hello world. Although you can't change the type of sum, you can change its value. And when you do so, you do not need to explicitly type it. So after the mine int sum equals x plus y, I can do something like sum equals one without the, without the explicit type declaration. And by the way, these three statements are only true within the same method. I could have an entirely different method called Subtract. And subtract, I could declare sum to be a string if I want it to. That variable would have to stay a string within that method. Method variables in one method are independent of method variables in another method. Now that we know how to define methods, Let's look at how we use them. We call this a method call. When we referenced the method name and passing arguments in parenthesis, as you can probably guess, out of 12 will cause the code return x plus y to be executed. And that returned value is now going to be bound to X in Maine. By the way, this thing we've been using all along, system dot out, dot print LN is also a method. It consists of a reference to a method name and has parentheses in which we pass an argument for our travel to print out to the screen. In this case, the value of x, which is three. The reason we need all these dots is going to make more sense in the future video. Pop quiz. How does Java know to print out the value three, in this case, instead of the literal character x that we passed into print, go ahead and pause the video. I will reveal the answer in 321. The reason java knows to print three is that this x is not, WE quoted at the X had double-quotes around it. Java would treat x as a literal character x, since it does not have double quotes around it, java understands that x is a variable name with a value attached to it. So it will first evaluate what x is and then print out the result of that evaluation. Here are the links to the code samples using this video. Since we've covered quite a bit of content in this video, I ask that you please visit these links. Click, Fork, ripple, and tinker with the code. It doesn't have to be anything fancy. Just experiment with adding variables to method bodies or maybe change the return type and the method declaration. And by the way, remember to add semi-colons. Java lines must end in a semicolon unless you're opening up a new code block with squiggly brackets. 4. Replit Demo: In this video, I want to give you a quick overview of the ripple environment. And what I mean when I say I encourage you to tinker with the code. First, make sure you have made an account on repl it.com. I know it's kind of annoying to set up another account, but the alternative to this is to set up a local programming environment on your computer. The problem, that is, I don't know what computer you have. I don't know what OS you're running and I don't know what environment variables are, aliases you have setup on your computer. When I used to teach at UC Berkeley because sometimes take me more than an hour just to get a local environment setup for a small group of maybe ten students. And given that skill share classes basically have unlimited capacity, the overhead associated with setup alone would take prohibitively long time. This is a class about Java, not about computers and repl, it has already set up Java for you. I think this is an extremely valuable resource that you can use, plus it's free for our use cases. Now, if we visit these links in the slides, Here's what it looks like when we visit the link in the slide. Once you've made a REPL account, go ahead and click this for grappled button over here. Now we have a copy of the project in your account. This is the code from the video on programming anatomy. The sidebar over here is where you can explore the file structure of the project. We can click into the folder to expand its contents and we click, Insert a file names to display their contents here. In this example, the main method here just prints Hello World. So let's sanity check. Go ahead and click the green button. You can ignore the cryptic texts over here. This is just REPL its system under the hood, issuing build, and run commands. Now let's try forking another ripple and make changes to it. Here's the code from the methods video. Let's go ahead and try doing the same thing. So here's the link. Again. Let's click Fork ripple, and fork it to our account. Now let's see what we can do. First, just sanity check with the green button and ensure that this, these values in the main method are printed out. Okay, so we can now see that we printed out the number three. And if you take a look at the code, this should, this should make sense. We have out of 12 and we're printing out the result of adding 1.2, which is three. Now let's see if we're able to add two method calls together. So this would be the first example of tinkering with the code. So let's say we did add one to plus one. Since calling one will return a number one. Sounds reasonable that perhaps one can be added to the result of adding 1.2. Let's go ahead and run this. Now, you see that we get an error. It says we have 21 says not a statement, do other ones as expected, a semicolon. So as you may have noticed, all Java lines end with a semicolon, other than the ones that's open, a new code blocks such as method declarations. So perhaps it would be prudent for us to add a semicolon at the end here. Now let's go ahead and run this code again. Okay, So we still have this, not a statement error, but at least we got rid of the semicolon error. So not a statement is a pretty big error. So let's go ahead and Google that. Let's go ahead and copy and paste this. Go to Google and go Java. Not a statement. We click on the first result Stack. Overflow is an extremely helpful resource for most programmers. And we can scroll down to the answer right here. And notice what they say. They say. Java restricts the types of expressions that are allowed and so-called expression statements. It disallows semantically meaningless statements like zero or a plus b. Hey, guess what? Isn't this exactly what we were doing earlier. We're doing add one to plus one semicolon, which is basically the same for as a plus b semi-colon. If we scroll down some more, you'll see that they have a quote from the Java language specification. Certain kinds of expressions may be used as statements by following them with semi-colons. And then we take a look at the list right here. Well, we have an assignment that is listed as one example of a valid expression. So let's go back to our code and turn this into an assignment expression that would look like this. Y equals add one to plus one. Let's go ahead and run this one more time. And now the code compiles and runs successfully. We don't see the result of adding 12.1 because we didn't print it out. So let's go ahead and add that line and run again. Okay, and now we got a four, which makes sense because we've added one to two and then add a one more one to that, which is four. Now let's try to finding a method, another method that does not have the static keyword and see what happens. So perhaps this call this public to return to. And again, note that I have omitted the static keyword. Now let's try seeing what happens if we do 12 plus one plus two. Let's go ahead and run this code. Interesting. We have another Java error this time. Non-static method cannot be referenced from a static context. So they even have this even point directly to the non-static method that is causing the air. So this is one of Java error that is actually slightly more helpful. They tell you that non-static methods cannot be referenced from static methods. And two is our non-static method. Now we haven't actually talked about what static means. So don't worry about it. We're going to talk about that in a future video and this error message will make more sense. Then the key takeaway is the importance of tinkering with code. Even though we broke the code multiple times, we learn things about Java that will help us later down the line. It's best that you make these mistakes now on many projects associated with these videos so that you don't get confused later when working on larger projects. 5. Control: Welcome friends. In this video, we're going to talk about control. In order to perform more complex logic flow in Java, we need to first understand some common operators. First, we have some basic arithmetic operators that are pretty self-explanatory. These are the symbols of plus, minus and times, and they work exactly as you might expect. They add, subtract, and multiply respectively. The slash symbol is Java's division operator. But one thing you should note is that division on integers in Java, a floor divide operation, meaning that it rounds down to the nearest integer, e.g. 3/2, and normal parlance is 1.5. However, in Java three, Florida by two is one. Since we round down from 1.5% symbol is called modulo or mod for short. It returns the remainder of a division, e.g. 14 mod four is two. Why? Because four goes into 143 times and we're left with the remainder of two. If you're unfamiliar with what a remainder is, I recommend you look it up, then come back to this video and replay the last couple of seconds to confirm that 14 mod four indeed equals two. The equals sign here is how we assign values to variables which we saw earlier, like int x equals one. We also have some Boolean operators, as the name suggests, these are operators that returns true or false. First, we have some pretty self-explanatory operators greater than, less than, greater than or equal to, and less than or equal to. This exclamation mark flips true to false and false to true. We'll see you in the next slide how the precise syntax of this works. Now we have double equals. This means return true if equal, return false if not equal. Remember this, since it's kinda tricky, double equals sign is used to compare equality. Single equal sign is used to assign values. Double ampersand means return true if both conditions are true, otherwise return false. And this double straight-line means to return true if either condition is true, otherwise return false. This operator might not be familiar to most people. So here's where it is. On a Logitech keyboard. It's usually next to the bracket keys. Let's look at some example usage. First, one plus two is obviously 3.1 minus two is obviously negative one. Nothing tricky there. Now, for quotient where we have 3/2, recall that Java division on integers is for division 3/2 is one. Since we're rounding down from 1.5, product is equal to six. Nothing tricky there. Three times two is six, and Java doesn't do anything special with that. The remainder is equal to one because two goes into five twice and we have a one leftover after getting, after multiplying two-by-two, this boolean true, true underscore zero is equal to true because we added this exclamation mark right before a false. So the exclamation mark flips the false expressions to true expressions and true expressions to false expressions. So therefore, this is true, true underscore one is also true because three is indeed greater than negative one. True underscore two is also true because we are negating zero equals equals one. Well, zero does not equal one. So this expression in parentheses, it's going to return false. Then the inversion of that is going to be true. So true underscore two becomes true. False underscore zero is equal to false. Because remember, double ampersand only returns true if both expressions are true. True underscore zero is true. However, the inversion of true underscore one is going to be false. So therefore, it is not true that both are true, and so therefore the result is false. Finally, true underscore three is going to be equal to true because we call this double straight-line thing, it returns true if either of the conditions is true. In this case, false underscore zero is false. However, true underscore zero is true. And so therefore, because one of the conditions was true, the entire expression evaluates to true. Now that we've covered basic operators, Let's look at how we can control the flow of logic within a program. First up, we have the if-else pattern. We place a Boolean expression within parentheses immediately following the IF operator. If this expression here evaluates to true, then we will perform all logic within the curly braces that immediately follow the Boolean expression. In this case, return x plus y. This expression evaluates to false. Then we would skip all the code enclosing the curly braces right here, and instead execute the code within the curly braces of the else case right here, in this case returns zero. Recall from an earlier video that I mentioned that return is terminal and we shouldn't have any lines of code after a return line. So why is it that here we have multiple return lines right here? Think about it and I'll reveal the answer in 321. The reason is that only one of these Code Blocks is ever run each time the method is called. These code blocks are mutually exclusive 100% of the time. The subtle point about return mines being terminal is that they are terminal only for the given block of code in which they exist. And this makes sense if you think about it. This method example one has been declared to return an integer. Imagine if we only wrote this return x plus y line, but we did not have this return zero line. Then further imagine that we pass in an x such that this expression evaluates to false. Then we would skip over this return line and come to the else block where we do not have a return zero anymore, then this method will return nothing. And then this method would be breaking this promise that it will return an integer. So this would lead to a Java compilation failure. Here's another if-else pattern, but this time utilizing some operators that we talked about earlier. Once again, we enclose the entire boolean expression in parenthesis. If x mod two is equal to one, or y mm R2 is equal to zero. Then we execute this entire code block returned X plus Y. Otherwise, if both conditions are false, we return zero. Here's another example, this time introducing a new operator. Else if the name of the operator is pretty intuitive, and it does what you might expect if this Boolean expression were to evaluate to false. It would skip over the return x plus y line and then try to evaluate the Boolean expression contained in the else-if line. If this expression x mod three is equal to one, evaluates to true, then we return x. Otherwise, if this expression were false, we would skip over return x and come to the else case, in which case we return y. Now, let's talk about the wild pattern. Sometimes you may find it helpful to run a certain code block multiple times. E.g. say you want to print out numbers from one-to-one thousand. One way to do so would be to write out system.out.print LAN 1,000 times. But another way to do so would be to write a single system dot out dot print LN, and enclose it within a wild code block. So here's an example. The way this works is we first evaluate the expression here. And while n is greater than zero, if this is true, then we run all the code within this code block. At the end of the code block, we returned to the start and evaluate this expression again. Is n greater than zero? If so, we run the code block again. And then we repeat, returning back to this Boolean expression over and over again until it is false. This syntax here might be a little unfamiliar to you. Let's break it down starting with this line, n equals n minus one. Recall, the equals operator means to assign a value to a variable n. But before we can assign a value to n, we need to first understand what is the value of this expression, n minus one. So basically what this does is it first subtracts one from n, and then once it does so, assign this value back to n. So effectively, we're just decreasing the value of n by one. So if n comes in as a five, we evaluate five minus 15, minus one is four, and then we assign the value four back to n. So n is now for these other two lines and minus, minus and minus equals one, they're actually both shorthand for the same thing, n equals n minus one. So this entire code block is effectively decrementing n by three every single time it is run. Similar syntax exist if you want to increase the value of n, we could simply do n plus plus and plus equals one and n equals n plus one. I'd also like to impart a warning. Java performs many sanity checks at compile time. But one thing that does not check is whether your loop will terminate. E.g. imagine that these three lines were all incrementing. We're increasing the value of n. So n plus plus and plus equals one and n equals n plus one. Now imagine we also pass in a value of n that is already larger than zero, then this code block will increase n even more. And this expression right here, and greater than zero will never be false. And so therefore, this block of code is going to run forever. If this happens, your code won't error and the compiler also won't error. It'll just keep running forever until you recognize the issue and stop the code. So be mindful when you write loops and make sure your condition will terminate eventually. Here is our final control pattern for this video. Here's an example. We have four followed by three expressions enclosed within parenthesis and delimited with semi-colons. The first expression is in initialization step. The variable i does not exist yet in this method, so we both declare and assign it to a value zero. Next, we have the Boolean expression, similar to the Boolean expression in the while loop. This is a Boolean expression that is going to be evaluated before the code block is run. If it is true, we run the code block. If it is false, we break out of the code block and run any code that comes after this for-loop. Finally, we changed the state. This I plus plus is run after the code block is run. So since we're starting from zero and running until I is no longer less than n and I is increasing by one on every single iteration. We're effectively just printing numbers from zero to n minus one. Here are a couple of more examples of for-loops. Note here that we can initialize i, an entirely separate line of code. If no initialization is needed for the for-loop. Let me just put a semicolon and leave the first initialization expression empty. Here we have another example. I want you to take particular note of the initialization here. Recall from our previous lesson that once a variable has been declared, future references to that variable do not need to type to be specified. That is the reason that here we only say I equals three, but not int I equals three. The same rules apply to both the initialization expression and standalone expressions outside the for loop parentheses. Now, you may be wondering to yourself, What's the difference between a for loop and a while loop other than the fact that there's syntax is different? Well, the answer is they're actually effectively the same thing. Really, the only major difference is their syntax. So don't think too hard about what the differences between these two might be. Here's the link to the code used in this video. As always, please click the link, click for grapple and experimental at the code. One thing I'd like for you to try this time is if you run into any Java errors that you might find kinda cryptic and encourage you to copy and paste that error message into Google. The root cause of error messages is usually pretty similar. And often you'll find really helpful results from programming forums that Google direct C2. 6. Objects Part 1: Welcome friends. Today we're going to talk about objects. Now, I realized that, that sounds really dumb, that we're going to talk about objects. But I promise you that you're gonna learn something valuable by the end of this lesson. Now, before we get into the content of this video, let me try to convince you why you should care about objects at all. So far, we've only been dealing with the functional aspect of Java, such as methods and control logic. However, Java is actually what we call an object oriented language. This means that we view our programs as objects with behaviors rather than as a collection of functions. This may be a bit difficult to grasp. One analogy you might think of is how you view a video game. Let's say Mario, e.g. in case you're not familiar with Mario. Here's a screenshot from one of the Mario games. This guy here is Mario. And the premise of the game is yet to get past a bunch of obstacles to reach a princess. And these funny-looking mushroom things over here, they usually move and that they touch you, you lose a life. Now, a Mario game is obviously a piece of code, right? There's probably some piece of code that checks the distance from the mushroom to Mario. And at the distance is less than some threshold. Then we subtract one from Mario's lives. But that's not how we think of this game. We don't think of these mushrooms as methods that are constantly checking for distance to Mario. And we don't think a Mario as a bunch of methods that are constantly calculating his vertical and horizontal velocity. We just think of Mario as a single entity that exhibits jumping and running behavior. And we think of these mushrooms as entities that are actively trying to stop us from saving the princess. That paradigm shift is the same one we have toward Java as a functional language versus Java as an object oriented one. Even if we're not coding a video game, developers find it useful to think of their code as discrete objects exhibiting behavior instead of as a collection of abstract functions. As you work on larger and larger Java projects, it becomes significantly easier to understand the program as a whole. If you have a solid object oriented design, because that's the way humans perceive the world as a collection of objects. Here's an example of a class that exhibits object oriented programming, as well as a main method that utilizes this class as an object. First, I'd like to point out that despite some unfamiliar syntax, there is actually a lot here that we've already covered. Here. We have packages, we have public classes. We have a main method that serves as the entry point to the program. And we have method declarations here. And we have an if else block here with a Boolean expression. So the amount of stuff we need to know to understand object oriented programming is not too bad at all. Let's talk about this code block here. This is something we call a constructor. One way you can tell that this is a constructor is that there is no return type and the method declaration and the name of this code block matches the name of the class. But other than the fact that the declaration of this code block is different from that of a method. Basically acts like a method. And this code block is executed upon instantiation. Instantiation is when we create a new instance of an object. It might be a bit difficult to understand when instances in terms of this Dog class. Let's instead recall our previous examples. When we create an integer variables. Imagine we have two integer variables, x and y. Maybe x is equal to one and y is equal to two. Since both x and y are integers, you can think of them as two instances of the integer type. Only they have different values. Similarly, you can imagine we might have two variables, x and y, both of which have typed Dog and have different values for their name and age. An instance of an object is exactly what it sounds like. It's a version of an object. We'll talk about how to instantiate an object in the next video. For now, all you need to know is that when instantiation does occur, this code block will be the first thing that is executed. And we'll also talk about what this actually means. This.name equals name and this dot age equals age. These right here are examples of what we call instance variables. To be more specific, they are declaring instance variables, but not initializing them to any particular value. We could give these variables and initial value upon declaration, such as private string name equals Bob, right here in this line. However, usually we delegate the variable initialization to the constructor, since the constructor is going to be the first thing that is called anyway. And perhaps it's not a smart idea to name all of our dogs, bob. That would be extremely confusing for all of our puppy friends. As you can see, just like any other variable in Java, these instance variables must have an explicitly declared type. They also need access modifiers, just like classes and methods. There are two important differences between method variables, instance variables. The first difference is that instance variables persist across method calls. A method variable only exists for as long as the method is still running. Once a method returns all its variable values are cleared from memory. For objects, these instance variable values will persist as long as the object instance exists. This will make more sense when we see an example of object instantiation in the next video. The second difference is their access rules. Method variables are only accessible within the innermost code block in which they are created. E.g. if we had a while loop inside the method getName. And inside that while loop, we created a variable. Variable would only be accessible within the code block of the while loop. However, instance variables are accessible from all the non-static methods within the same class. We're going to talk about what static actually means in the next video. But for now, all you need to know is that a non-static method is one that does not have static before they're declared return type and the method declaration. As you can see here, we can return the value of name and age from these methods here. These instance variables are accessible to these methods since these methods are non-static, the prefix this dot tells Java that we're referring to an instance variable called name. Now, you might think that this is a ridiculous requirement that we had to add even more language rules to refer to instance variables. But there's a good reason for this. Can you tell what it is? Pause the video and I'll reveal the answer in 321. The reason is that we might have naming conflicts between instance variables and methods variables. One example of this is actually right here in the constructor. We have the variable name declared in the arguments of the constructor. If I were to omit the, this prefix in this line, this.name equals name, then we would basically just have name equals name, which does nothing because name is already equal to name. So the, this prefix is important for disambiguating between instance variables and methods variables. If there is no this prefix and there is a method variable with the same name as the instance variable, then Java will assume you're referring to the method variable. Here's the code used in this video. As always, please click the link, click Fork ripple, and experiment. If you encounter any cryptic error messages, please try pasting your error message into Google and see what other people have to say. Otherwise, feel free to post questions in this course. 7. Objects Part 2: Welcome friends. In this video, we're going to wrap up our discussion of objects. We mentioned object instantiation in the previous video. Let's see how it works. First, recall that the dog constructor looks like this. It has two arguments, name and age. In our main class, we create two instances of the dark class like so. First, as per the Java tradition, we explicitly specify the type of the variable we are creating, which is the dark type. The next part over here, it looks almost like a method call, except that we have this keyword, new. The new keyword paired with the constructor call tells Java to first allocate memory for a copy of the dog object. And once that memory has been allocated, it calls the constructor. Running whatever code is in the constructor definition. After running these two lines of code, Bobby and Sam now have their name and age data saved inside the first dog and second dog instances. Since we save that data into their instance variables which are persisted. Now, let's talk about static. Here's the dog class from the last video, but with a few add-ons. We've added the static keyword between the access modifier and the type in the method and variable declaration. Recall that variables and methods without the static keyword or non-static, also known as instance variables and instance methods. As the name suggests, instance variables and instance methods are specific to instances of an object. However, static variables and static methods are the opposite. They cannot be bound to a specific instance of an object. This dark example might help you understand. If you want to know the number of legs on a dog, do you need a specific dog to answer that question? Of course not. You know that dogs in general have four legs. The number of legs on a dog is not specific to a particular instance of a dog. On the other hand, if you want to know the name or the age of the dog, we need a very specific dog to answer that question. So instance methods and variables are specific to particular instances of the class. Static methods and variables apply to all instances of the class. Let's take a look at how we query static methods versus instance methods. Since non legs is a public and static attribute, we can ask access it directly with dog dot non legs. Note that dog is not a specific instance of the dog class. The dog class itself has no name or age. We can also access get numb tails directly to the dot class without an instance. However, for get age, we need to access it through a specific instance. Second dog, since only specific instances of dogs have age data associated with them. Also, note that we can still access static methods and variables from an instance of an object. And again, if you think about it, it's pretty intuitive. I can't tell you the age of the dog without a specific dog, but I can't tell you how many legs and tails dogs have in general, if I give you a specific Dr. reference. Here's the code used in this video. As always, please click the link for the ripple and tinker with the code to see what works and what doesn't. 8. Comments: Welcome friends. In this video, we're going to talk about comments. Sometimes Java code can be very confusing, especially if you have a code block that calls other methods and those methods called even more methods. This causes problems later down the line as you'll find that sometimes you have trouble understanding your code just after a few days. And if you ever plan on collaborating with other people, you'll need to make sure that your collaborators are able to understand your code to. In order to assist with code readability, Java has a way for developers to add human-readable notes to their code. Here's one example. Inline comments are prefaced with double slash and they're usually added either after the code in the same line or on the line immediately above the code, the comment is describing the texts that falls. The double slash is a human readable piece of text that is not executable by Java. It exists solely to assist the human reader, the program with understanding with the adjacent code is doing. Here's another example. This is what we call a Java doc comment. There are tools out there that will generate documentation for your code by scanning files for comments that follow this format. We're not going to use a Java doc tool in this course, but it's good for you to know how to write neat and pretty method descriptions. Java, doc, comments, or multiline. They begin with a slash star star, and star slash. Lines in-between should have a single asterisk at the beginning. Typically, java dot comments have a high-level description of the method like we have here, followed by Program and at return tags. If we had multiple arguments, we would have multiple app per m tags. Each one would firstName the perimeter it is describing, followed by a description of what the argument is. The return tag is present for a non-void methods and it contains a description of what is returned. It's not necessary to adhere strictly to this format if you're not generating documentation with the Java doc tool. But even if you aren't, it's generally good practice to follow this format whenever you can. Here's the code used in this video. Please visit the link, click, Fork, ripple, and tinker with the code. 9. Arrays and ArrayLists: Welcome friends. Congratulations on making it this far in the course. In this video, we're going to talk about arrays and ArrayLists. A couple of videos ago. When we first covered Java types, we mentioned the array type, but we didn't really go into too much detail about what it is. Today. We're going to revisit arrays in much greater detail. Recall that we first saw a string array in the method signature of the main method. Here's one way we can create an array. If you already know what values we want to use to initialize, we can populate it directly like so by specifying the type of the array, in this case an integer array, and by including the values and curly braces like so. If you don't know what values you want to use just yet, we can first initialize an array of zeros with the new keyword and by specifying the length of the array in brackets. Then when we're ready to populate the array, we can set the elements directly like so. A couple of things to note here. First, an array's length is fixed and cannot be changed once it's created. So both array one and array two are going to be length three and they will always be. So. Second, array indices begin at zero. In many languages including Java, almost all indexing across data structures, we'll start from zero as well. Now, if you want to print out all the elements in the array one by one, we create a variable I to refer to the index of the array. And we increment this value I by one until we reach the length of the array, which we can look up by using this attribute length. We can access an individual element in the array with these brackets and index from 01 or two, which referred to the first, second and third elements respectively in the array. So those are arrays. Next up we have ArrayLists, which are a more flexible version of arrays. Similar to arrays. Arraylists are also ordered collections of items. However, one important conceptual distinction is that ArrayLists are re-sizable and ArrayLists also have different syntax. Let's explore these differences. The first is that ArrayList are initialized like so. The thing in the angle brackets specifies the type of element in the ordered collection. And we only need to explicitly provide the type on the left side of the initialization. This line would instantiate an ArrayList with length zero. Also noticed that we use capital integer here instead of lowercase int. This is something we call a box to type in Java. It's not super relevant for this course, so we don't need to spend too much time on it. We call the int type a primitive type, and the integer type with a capital I, a reference type. Ints and integers are generally interconvertible and map one-to-one to each other. E.g. an int value of one would map to an integer value of one. All you really need to know is that the reference type integer with a capital I is needed for some data structures such as ArrayLists. Since ArrayLists can only accept reference types as its elements. This is just the way java was designed. So don't dwell on it too much. Just keep it in mind. If you ever tried to use an array list the next time you go grocery shopping. Next, Here's how you add elements to the list. So one is at index 012 is at index one, negative seven is at index 2.19 is at index three. We can also remove elements from the list, which will delete the element at the specified index and decrease the list size by one. In this case, we're going to be removing index two, which in our example is going to be the value of negative seven. So after running this line, we will have the value one at index zero, the value 12 at index one, and devalue 19 at index two. Since all values after index two are going to be shifted by one to account for the deletion. Another difference is the way we get the length of the array. While I erase had a public attribute length. Arraylists have a public method called size. We also have list dot get here instead of brackets to access elements. We're introducing a lot of methods and attributes specific to erase an ArrayList in this video, which might be a bit overwhelming. If you feel this way. Don't worry. Computer science is not a discipline-based on memorization. If you ever forget something small, or if you're trying to find some extra functionality to these data structures, just Google it. Google results for basic programming questions are usually very relevant since many people before you likely had the same questions to ask when they started programming. Finally, I'd like to show you a shorthand way of iterating through the list. This code block does exactly the same thing as this code block above the int element tells Java that we're treating each item in the list as a variable called element with type int and colon list tells Java that this is the list we want to iterate over. As a side note, we could have also written the type as integer with a capital a here. Remember, int and integer are generally interconvertible. We only needed to use integer with a capital I when we declare the type of the ArrayList initially appear. Anyhow on the first iteration of this code block, element is going to be bound to one. On the second iteration, it's going to be bound to 12. You get the idea. Java just handles the incrementing of the index variable for you with this simplified syntax. Now, you may be asking yourself, ArrayList can do everything that arrays can. And ArrayLists even have extra functionalities since they can be resized. So why didn't the Java developers just removed or erased from the language? Pause the video and I'll reveal the answer in 321. The reason is that there is a performance trade-offs involved when using an ArrayList instead of an array. Arraylist with reference types are generally a bit slower than arrays that use primitive types. For most use cases, though, the performance trade-off isn't really noticeable enough to make a difference. But it's something to keep in mind if you ever happen to be working on something that is sensitive to latency, like if you're programming and ultra high precision laser, or you're trying to build an ultra-fast stock trading. But here's the code using this video. Please visit the link, click Fork, ripple, and tinker with the code. 10. Sudoku: Hello friends. Congratulations on finishing all the instructional videos in this course. Today, we get to use what we've learned to build an interactive visual game that you can share it on my resume and with your friends. I've linked a fairly comprehensive document in the projects and resources tab. Go ahead and click this link. And it should take you to this document. I'll let you read the majority of this document on your own time, but I'm going to go through it briefly right now, just so you at least have a general idea of what's going on. I'd like to preface this by saying that this project is not meant to be super-duper, easy, or finished within 10 min. I know some of the other courses on this platform are dead simple, like write a class that has three methods in it. But I don't really think you're going to learn that much if that's all you're doing after going through these videos. So this project may require more thinking than other projects if you've done other Java courses on this platform, but don't be discouraged. It was designed that way. So you can get experienced with a real coding assignment that actually encourages you to understand object oriented programming. Be sure to read this entire document. It is likely that you'll have many questions about the project that are already answered here. Before you begin. Make sure that you have created an account on repl it.com, and also makes sure that you have verified your email. Otherwise you won't be able to fork and make changes to the files. Once you've done so, visit this link and fork the raffle. This project that we've linked here is called a skeleton. What this means is that some of the code has been done for you. Many methods have been left blank and have a comment to do blocks of code that have the todo comment means that you had to fill in the code for that section. In this project, you're going to be implementing Sudoku in case you're not familiar. I've written a description of this game in the overview section here. Here, I've written a brief description of each of the files in this project. The order in which these are listed is the order in which you should complete the project. You should do self first, then row, then block, then board. Main has already been done for you. Here, I've written out what I just told you explicitly. After you complete each class, you should run all the unit tests and ensure that the unit tests related to the class you just finished our passing. First. After you complete cell, you should make sure that all the units has related to sell pass. After you complete row, you should make sure that all the unit tests associated with cell and row pass, so on so forth. The next section here tells you how to actually run the test. In the left panel of the tools section on repl it, you'll be able to access tools. Tools. And after you click on the Tools tab, you should be able to see unit tests. Once you click on unit tests, you should be able to see this window pop up, which has a Run Test button. Even though we haven't started coding yet. Let's just run the unit test and see what happens. Let's come back to our Sudoku skeleton. Let's, again, we go to the Tools tab. We click on unit tests and we click Run tests. After we click Run tests, we can go over to the console tab. I already have it open here, but in case you don't, you can find it in the tools section. So in tools, you go to console. And now we can see the output. In this case we had zero out of 20 tests pass see errors above. So this is good. This means that all of our tests are actually meaningful. Think about it. If any of our tests passed when no code has even been written, that would be a very bad sign. That would mean that the test isn't really testing anything at all. So after you complete the project, all of these tests should pass. The next section here, I've written some debugging tips in case you have some trouble. However, since you are likely new to programming, it's totally okay if you're unable to solve the issue alone. If you run into any issues, you have two options. Option one is to take a quick peek at the solution, which I posted at the very bottom of this document. Option two is to post a question on the Skillshare course under the, under the discussion tab. And I'll get back to you as soon as I can. Finally, after you've passed all the unit tests, you can click the big green button. Playing the game should look like this. Here's my solution. So you click the green button. After it's done compiling, it runs the program and it prints out this Sudoku board that has some values filled in. Now we can play with this game. So let's say e.g. that we would like to put the number, let's say two in dislocation. So this is going to be row number one, since this row of periods, row zero. So this is row one, and this is column one, because this one right here is column zero. Let's say set 11 to be two. And you can see that two has now been populated in dislocation. If we want to undo that, we can do clear 11 because 11 is the coordinate that we're clearing and we'll get that to be empty again. Now, let's try to put in a nine here, which shouldn't work because we already have a nine in this block. We set 11 to B9. It's kinda tell you block zero has multiple names and it will not let you put a nine there. If you get frustrated and you don't want to finish a Sudoku puzzle anymore, you can just type solve. And now it will solve the entire board for you. And it will exit the program telling you that you need to run the run button again, if you want a new puzzle, There's a possibility that the unit tests I wrote or not comprehensive. I'm fairly certain that if you pass a unit test, you should be able to run the game successfully. However, if you pass all the unit tests and you're still unable to run the game properly. I'd recommend you just cross compare your solution with the one I posted. When you find the discrepancy, please post a note in the Skillshare course so I can fix the skeleton or unit test if they need to be fixed. Once again, remember to read this document in its entirety and refer back to it frequently. And that's all I had to say for this video. Have fun on your first real coding assignment.