02 - Java Basics/003 First application Hello World App_en.srt

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They will create our first application in Java language.

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Be prepared to have fun.

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Also in this lesson, you will learn what our console applications, how to compile Java code to get

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bytecode and how to execute your first program from console.

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Let's start.

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So what a console applications console application is an application that takes input and displays output

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at command line console with access to three basic data streams, standard input, standard output and

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standard error.

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These applications don't have any graphical user interface.

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They are primarily used nowadays in learning purposes.

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So console application is a lightweight application which might be easily run in console.

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During this course, we'll learn tools which we have in GDP to create powerful applications, but the

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same as learning as a human languages.

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The first thing you learn is alphabet.

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After that, no letters.

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You can create words from words who can create sentences from sentences.

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You can build a story.

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The same is here will start from console applications to learn alphabet and words.

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Here is what we are going to do today as a software engineer will create with you files with Java.

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Extension will call this files source code.

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This is exactly files where programming code written in Java will be stored.

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After that will ask Java Compiler to compile source code to bytecode.

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Java program will do this task.

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Java will produce files with class extension and Java will execute our program.

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What do you need to understand?

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And remember that Shivam doesn't execute Java files.

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Jivin executes class files properly.

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This will help you in the future when you will troubleshoot issues in your real life project.

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So now let's take a look how to create your first program, how to compile it and run theme.

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Please don't be afraid.

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You won't use common line to write your programs on a regular basis.

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In the next, lessons will run special desktop applications designed to create programs.

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But now I'd like to ask you again to open a common line wherever you want on your computer.

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For example, for this lesson I created folder where I will create a program together with you.

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Let's type seemed easier to open a common line probably using right now.

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And you must be kidding me.

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Why do we do this?

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Why don't we just start work with applications which help us to create programs?

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The answer is there are a few points to consider.

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The first one you need to learn how to work with console at least some basics because sometimes not

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always, but sometimes depending on which built tool you will use, probably you would need to work

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with console.

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And sometimes as software engineers you would need to write scripts which will call command line comments.

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Real life example.

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You would like to compile all Java files in your project in one archive, which will be deployed to

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the Web server afterwards.

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Right after compiling, you would like to run specific files, desk classes to be sure that tests are

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not failing in case tests are succeed.

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You finish your build process in case there are failures in the tests.

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You don't proceed with your build.

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That is one of the examples where knowledge of common line comments might come in handy.

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The second point to consider Guice.

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You had just started learning Java.

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You need to understand what compilation process is, what is compilation error and how it is different

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from runtime exception.

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That's why I ask all my students to create program in Notepad and to run it and console at least once

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in their lives in some conference on the first level of interview process of junior software engineers

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where you have quizzes and tests, sometimes there are also questions related to common line.

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We opened the common line to create file with Java extension, why we cannot just do mouse right click

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and create file because you need to enable showing the file extension first in different Windows version.

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It is done in a slightly different way.

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And from default options, you can create file with the extension, which doesn't work for us.

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So let's create file from console because it is easier and it's the same way to create file from the

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terminal on different Windows versions actually.

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Now at least a few ways.

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How to create file from command prompt.

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But let me show one to you.

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Type the following notepad.

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Hello World Dot Java.

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When dialogue is popped up, click yes.

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Now we see file with Java extension created here in case you have Mac just type like this.

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Dutch Hello, World Dot Jalaa.

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The file will be created.

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Let's write our first program, we'll cover right now basic syntax so each Java file can have only one

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public class or interface.

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We'll cover more modifiers during all be topic.

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But for now, just try to understand that in case class is public, client can access it from anywhere,

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including our game, which needs to access this class.

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The name of public class should be the same as the name of a Java file.

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This is true.

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In either case you would have compilation error Gervais's case sensitive.

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So hello world written with Capital H at the beginning is completely different from Hello World with

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H lowercase.

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Java is a case sensitive because it uses a C style syntax case.

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Sensitivity is useful because it lets you infer what the name means based on its case.

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For example, the Java standard for class names is uppercase in the first letter of each word, for

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example, like integer string, etc..

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The standard for variable and method names is lowercase, the first letter of the first word and uppercase

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and all other first letters.

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No print, learn, Czech error, etc.

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And the standard for constants is all uppercase was on this course, some constant like this.

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So after hello, world, let's put curly braces here in Java, curly braces define SCOP.

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Here is the scope of the class, in a few seconds, you'll see the scope of the method each otoscope

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includes in their scope, zazz variable declared in the class scope is visible in the method scope,

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but variables declared in the method scope not visible in Glasgow.

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Now we need to create an entry point for our program by convention.

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This method is publicly accessible, so let's type here public.

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It is static.

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We'll cover more what static is during or P topic.

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It does return nothing.

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That's why return type is void and it has name main by convention.

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This method takes as a parameter array of strings in the next lessons will cover in details what arrays

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are as of now.

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Consider array of strings as a sequence of strings.

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A parameter is a variable in the method definition.

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When the method is called, the arguments are the data you pass into the methods parameters.

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Hope this doesn't bring even more confusion.

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I just want you also to learn the right wording to make your colleagues understand you.

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Public static void main method, which takes array of strings, is our entry point to each Java program.

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JVM knows that it should call exactly this method and begin program execution from this point.

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Let's define scope of this method here in this method will write code which will print hello world text

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to console output will write a system not out how to represent output.

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Stream output stream has such behavior as printing to console print, to learn method prints to console

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and terminate the line.

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Each statement in Java ends with semicolon Prindle and can take strings as arguments that's postering

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Hello World into it.

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In Java, string literals are written inside double quotes and one more new word.

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Literal.

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Literal is a notation for representing a fixed value in source code.

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Here we have string literal.

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Now let's save this file.

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Let's go back to the console and now let's compile our program, Javadoc Hello, world, that Java.

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If you don't see any messages, that means compilation was successful and you can notice that new file

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is appeared in our folder, this file has the same name as our Java file, but has different extension.

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This file contains byte code for our Java virtual machine.

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Now let's execute bytecode on our game.

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Let's step Java.

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Hello, world and press enter.

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In this case, you don't need to specify extension for the file since Java expects only class files

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and you see output here.

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Hello world congratulations.

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You have just written your first console application.

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What are common mistakes?

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You can see compilation error after calling Java and you can see runtime errors.

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Let me reproduce it with you.

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Let's pretend your name your public class incorrectly.

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Not the same as Java filename.

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Let's put capital G at the end and also let's pretend you did a typo in main method name.

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Let's save the file.

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Let's call Jack now.

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We see compilation error, so code is not compiled to bytecode, always read these error messages,

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they can help you to fix issues.

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For example, like here, public class, hello world should be declared in the file with the same name.

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We will just rename our public class.

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Let's compile it one more time, but not forget to save the file before compilation process.

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So now everything is OK and we don't see any compilation errors, and by the way, to call comments,

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which I just used, I use up Arrow.

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Let's run it again.

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And here we have one time exception.

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What does it mean, compile it?

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But error happened during the program execution.

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You try to find humane method, but couldn't hope this information will help you to troubleshoot issues

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in case you will have once.

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Let's recap one more time what we have learned today.

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And now you know what console applications are.

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You understand basic Java syntax.

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You learn how to create hello world application in Java.

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Also, you saw what compilation errors and the runtime errors are.

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You can compile and execute your programs from the console.

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And here's your homework.

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Try to repeat everything what we did during the lesson by yourself.

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Also, I'd like you to write that or shall file, which will compile your source code and we'll start

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your Java application, try to do it by yourself with help of the Internet if needed.

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And we'll check your homework in the next lesson together and read Java Code Convention from the link

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here.

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Pay extra attention to naming convention.

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Java naming convention is a rule to follow as you decide what to name your intent to file, such as

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class baggage, variable, constant method, etc. but it's not forced to follow.

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So it's known as convention, not neutral.

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These conventions, as suggested by several Java communities such as Sun Microsystems and Netscape,

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all the classes, interfaces, packages, methods and fields of Java programming language and given,

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according to the Java naming conventions, if you fail to follow these conventions, it may generate

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confusion.