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(piano music)

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-: So let's solve this problem.

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So we are not able to run this code, right?

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This is not working.

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We want to get hello world on the screen.

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It was working with J Shell

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because JShell says you just a parse a statement,

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I will work.

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But then when you build an application,

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when you run the application,

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how exactly it works behind the scene.

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Once we understand that, we can write this code.

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So in your machine you will be having something

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called JVM.

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Now by default, when you say Java is platform independent is

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because of JVM, because in your machine you will be having

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JVM irrespective of which OS you use.

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It can be Windows, Linux based OS, or Mac

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or whatever famous OS we have.

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It will have JVM installed.

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And of course if you have JVM, you can run Java application

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and if you don't have JVM, you can't run Java application.

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Okay, so we got JVM here, right, now what JVM says,

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hey, I will work.

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But then this one question, when you say JVM will work,

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but JVM works on which environment?

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So underlying we have your OS.

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So this is your OSS

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and then your OS is working on a hardware, right?

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So you'll be having an hardware, it can be a laptop,

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it can be a desktop, it can be a phone, doesn't matter,

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you have an hardware, on top of that we have an OS.

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On top of that we have something called JVM.

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Now, JVM says, okay, my job is to execute a Java code, okay.

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So whatever java code you create, JVM will execute that.

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But then JVM will not accept the code directly.

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See what happens is Java goes in multiple steps in Java

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application, when you as a programmer,

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so let's say you are a programmer here

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and when you write a code,

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you will not directly run write a code which will

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run on JVM, okay?

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And we don't even do that, right?

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We don't even write a code which directly works on the

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hardware and we don't even write a code which works

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directly on OS.

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Java says if you want to make it work, you have

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to run your code on JVM,

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but then JVM will not understand what you are talking about.

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And one more, one more thing we have talked about, right?

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Java is platform independent.

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So what does platform independence means?

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It simply means that you can run your application

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on any machine.

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So when you write a Java code,

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it will run on any machine irrespective

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of the hardware or the OS.

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But the only thing it requires is

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that machine need to have JVM.

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So irrespective of which platform I'm talking about,

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if it has JVM, your application becomes

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platform independent.

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But JVM itself is platform dependent, okay?

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So example, you can't run JVM on iOS.

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So we can't say Java is platform independent when JVM itself

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is dependent.

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So yes, your application is independent, JVM is not.

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So JVM need to be built for a particular OS.

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And let's say if it is windows in my,

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so in my machine it is Mac,

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but let's say if it is Windows,

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we have JVM available on Windows.

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So your JVM says, Hey, I will run your code

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but you have to submit me, not the Java code,

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but something called a byte code.

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So you have to convert your code into a unreadable format

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for a normal human being.

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And this is called a byte code

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because JVM only understands byte code.

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But can you write byte code?

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Of course not.

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You only understand English type of languages, right?

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So whatever we have return the code,

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we have a return system dot out dot print,

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it's English type language, right?

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So what you create is a Java code.

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So this is your Java file or Java code,

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and then you have to convert this Java code.

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So this is Java code, which is more of readable.

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This is not something which you can read.

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So basically you have to convert this into a Java

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code or byte code.

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So for that we have something in between here,

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which is called a compiler.

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Now since this is a Java compiler, we can call it

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as Java C, which is Java compiler.

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So basically you will write this code.

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So this code will be getting compiled into a byte code

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and that byte code goes on the JVM, it'll run.

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Makes sense, right?

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So that means when you write something it will

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go into byte code

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and then byte code will run on JVM.

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Now the only thing is see your project can be a big project,

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right, here I'm writing only one file.

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It can be 10 lines of code.

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It can ,it can be 10 files, it can be a hundred files,

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it can be thousand files.

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Out of all these files, there will be only one file

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with JVM will execute, okay?

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JVM will start with the first file

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and then of course that file will depend upon

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other files and it will execute.

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So JVM says you need to tell me what that file is, okay?

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So if you, even if you have thousand files which you want

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to run in your project, you have to give the first file.

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So in this case, what you will do is you will specify, hey,

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that's my first file.

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So when you run the code, you will specify the first file

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and also that file need

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to have something called a main method, okay?

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So as I, as I mentioned before, right?

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We have print in the same way we have main, so your file,

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the first file need to have a main method.

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So when I say first file, it is not like you have

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to follow a sequence, create this file, then that,

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that file, out of all the files in your project,

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the execution will start from the first file which you

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specify, okay?

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And that file need to have mean, not just mean,

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it is looking for a specific signature.

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Okay, so it has a signature, it means the entire method

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signature, the method name,

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what are the values you are parsing and all this stuff.

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And of course all this term you will, you will get used

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to in some of these sessions.

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Don't worry.

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So first you have to make sure

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that you provide a mean and mean has a particular signature.

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Again, what each word means, that we'll see later.

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So the signature is this, it is public static wide mean.

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So you have to remember, always parse this method.

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I know there are four words,

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but then this is the thing you have to remember

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and mean will accept some arguments, some parameters.

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So you have to parse string A.

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Now again, we'll discuss this, what this string is,

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what is public static void.

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We'll discuss that in some time,

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but mean is looking for the signature.

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So your code will not work without the signature.

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So remember this, so that makes sense.

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What are the steps you have to follow?

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So the first step is

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as a programmer you will be creating a Java file, okay?

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Now once your java file is ready, you will put

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that into compiler, then compiler will compile it.

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Then that code becomes a byte file, a byte code basically.

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Then you will take this byte code, you will run that on JVM.

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Now, once you, once your JVM says, okay,

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I'm running the code, JVM will look

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for this particular syntax.

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It should be there in your code

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and then you will get the output, right?

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Simple stuff, okay.

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Now there's a space in

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between here if you can see, this is a space I have left

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that space intentionally

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because we'll add certain things there.

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So at this point, just remember there's a space here,

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there's a box meaning here.

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We'll add that box in some time.

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Now once we have understood all those things, it's time

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to expand this code.

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Now if you remember, we have

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to write everything in a main, right?

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So this section should be a part of,

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remember what J will ask you for,

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J will ask you for mean.

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So we have to put that into mean.

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So why the syntax for mean, it's very simple, it is public,

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static wide mean.

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We have seen that, of course these words will define

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that words in the upcoming session.

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And once you understand all these terms,

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you are java programmer, right?

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And then this will say string A and that's it.

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This is a syntax you have to follow.

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Now there's one more thing.

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In Java, now Java is object oriented.

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Now what is object oriented means we'll discuss that later.

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But in simple terms,

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object oriented means everything should be an object.

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And to create object we need to class, okay?

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Again, we'll discuss all those

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things in the upcoming session.

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But a class is missing here.

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In fact, if you compile this code, this is what your

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Java say will tell you.

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Java say says, Hey, where is your class?

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Your class is missing.

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Okay, let's do that.

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So let's put that a class here and we'll give a class name.

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At this point we'll keep the class name and file them same.

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And we'll say, open bracket and close,

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and will give an indentation.

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So basically we have to follow this

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thing, always give a tab.

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So every time you open a (indiscernible),

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give a tab, it looks good, okay?

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There's no compulsion, it looks good

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and we love to build stuff which also runs and looks good.

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Okay, this looks good, right?

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Ignore this X and these suggestions,

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these are the suggestions coming from your VS code.

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But you know that part in fact is it

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because happening of this extension,

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let me just remove this.

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So let's not use that

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extension because it will give you a suggestion.

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It's good for development but not for teaching purpose.

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So it adds extra text there.

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Okay, cool.

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So this is how you write your code, okay?

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We got a class, a file name,

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and then public static void mean, in bracket

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you have to mention the parameters.

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Again, we'll discuss that later.

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And the only thing,

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important statement here is system.out.printHelloWorld.

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Okay, now let's see if this works.

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Let's create the screen and compile.

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You can see there's no error.

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But the moment I compiled,

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if you can see on the left hand side,

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we got this extra file.

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We got hello.class.

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Now this is your byte code.

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Remember we have talked about a byte code here.

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This is a file which got created automatically.

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So when you create a Java file, when you send it

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for the compilation, it will create the byte code,

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and the extension for the byte code is dot class file.

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The extension for java code is dot java file.

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And now it is compiled done.

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Right, now it's time to run your code.

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So remember when I mentioned to run, you have to use JVM,

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and for that you have to use a command, which is Java,

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and you have to mention the class name, not a file name

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this time, just a class name.

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The class name is hello.

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And say enter.

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You can see we got hello world.

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So yeah, that's how it works.

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So simple stuff, that's how we can able to complete our code

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and we are able to run this code.

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Now, if you have done until now, give round of applause

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or a pat on your back because you deserve it.

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So this makes sense, right?

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So there's a gap here, right?

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So let's remove this gap.

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So what we have here, so basically, you know when you,

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when you run your code, basically think about your cooking

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something in a kitchen, right?

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So where do we cook?

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Of course in a kitchen, right?

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Why not in a hall or in a bedroom?

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It's because in the kitchen to cook something,

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you need extra things, right?

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Example, if you make a bouillon,

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I don't know if you know the recipe,

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I don't know the recipe,

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but if you make a bouillon,

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you have to add extra ingredients.

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Now those things are kept in the shelf.

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So in the kitchen you'll not just have

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that resources which are there in the platform,

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but also you'll be having extra stuff.

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Now, the same way, if you want to run your Java application,

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you need extra libraries and you also need a runtime.

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The runtime is something which,

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where you can run things, right,

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now you can run something in a JVM.

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So that is part of run.

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But then what if you need some extra libraries?

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You have to, you need, you need a environment as well.

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So we have one more layer here, which is about your OS,

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which is called JRE.

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So when you want to run something, you use JRE to run it.

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Now JRE stands for Java Runtime Environment.

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And what is your JVMJ?

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JVM stands for Java Virtual Machine.

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Okay, so basically we have two things.

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We have a JVM and we have a JRE.

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Now JVM is a part of JRE.

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Now what else you'll be having inside your JVM,

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you will be having some libraries.

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So JVM with libraries is a part of JRE.

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So whenever you run something, it runs inside the JRE

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and JVM is actually responsible to run it.

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Okay, and by default, when you, when you want

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to run your application on any platform, it need

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to have JRE, which will have JVM.

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Now not every machine will have Java C.

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Java C is only for development purpose, right?

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So we can imagine this in this section.

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So basically we have an outer box, which is your JDK,

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which is used by developers.

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Inside, which you'll be having something called JRE.

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And in this you'll be having JVM.

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So as a developer you'll be installing JDK.

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JDK will have JRE.

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JRE will have JVM.

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But to run something on any other machine,

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let's say if you have made the application, you want

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to run this on your friends machine.

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Your friends machine will only have JRE

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and JVM.

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JDK is not required.

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And that's why we say Java is vora, which is write once

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and run anywhere, right?

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So you just have write on your machine,

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you can run it on any other machine, provided you have JRE

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and JVM installed.

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So that's how your Java works behind the scene.

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I hope it makes sense now

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and we'll write some more code in the upcoming videos.