Java Main Method Explained - What Does All That Stuff Mean?

Summary

This video script demystifies the Java 'public static void main(String[] args)' method, explaining its components and purpose. It clarifies that 'public' ensures accessibility, 'static' allows the method to be called without an instance, 'void' signifies no return value, 'main' is the required method name for the Java Runtime Environment (JRE) to execute the program, and 'String[] args' is an array for passing arguments to the program. The script also demonstrates how to use 'args' to accept user input and the importance of handling these arguments to avoid errors like 'ArrayIndexOutOfBoundsException'.

Takeaways

• 😀 The 'public static void main(String[] args)' method is the entry point for a Java program, which must be present for the Java Runtime Environment (JRE) to execute the code.
• 🔓 The 'public' access modifier is necessary because the main method needs to be accessible to the JRE from outside the class.
• 📌 The 'static' keyword means the main method can be called without creating an instance of the class, which is how the JRE invokes it.
• 🛑 The 'void' return type indicates that the main method does not return any value; it simply executes and then the program ends.
• 📝 The name 'main' is not arbitrary; it is the specific name that the JRE looks for to start the program.
• 🔑 The 'String[] args' parameter is an array of strings that can hold command-line arguments passed to the program, allowing for external input.
• 💡 The 'args' parameter provides flexibility, allowing programs to utilize input provided when the program is started.
• 🛠️ IDEs like Eclipse allow for setting arguments for the main method through run configurations, facilitating easy testing with different inputs.
• 🖥️ Outside of an IDE, command-line interfaces can also be used to pass arguments to a Java program by including them after the class name in the 'java' command.
• ⚠️ If the 'args' array is used in the program, it's important to ensure that arguments are passed when running the program to avoid errors like 'ArrayIndexOutOfBoundsException'.
• 🎓 Understanding each component of the 'public static void main(String[] args)' method signature demystifies the Java programming environment and empowers programmers to write more effective code.

Q & A

What is the purpose of the 'public static void main(String[] args)' method in Java?

The 'public static void main(String[] args)' method is the entry point for any Java application. It is the method that the Java Runtime Environment (JRE) calls to start the program execution.

Why is the 'main' method declared as 'public'?

The 'main' method is declared 'public' to ensure it is accessible to the Java Runtime Environment, which needs to call this method to run the program.

What does 'static' mean in the context of the 'main' method?

The 'static' keyword indicates that the method can be called without creating an instance of the class. The JRE calls the main method directly on the class, not on an instance of the class.

Why does the 'main' method have a 'void' return type?

The 'void' return type signifies that the 'main' method does not return any value. When the main method completes execution, the program ends without returning anything.

Can the 'main' method be named something other than 'main'?

No, the 'main' method must be named 'main' because that is the name the Java Runtime Environment looks for when starting the program.

What is the purpose of the 'String[] args' parameter in the 'main' method?

The 'String[] args' parameter is an array of strings that can hold command-line arguments passed to the program when it is executed. It allows the program to receive input from the user or the environment.

How can you pass arguments to a Java program when running it from an IDE like Eclipse?

In Eclipse, you can pass arguments by right-clicking on the class, going to 'Run As', then 'Run Configurations', and entering the arguments in the 'Arguments' tab.

Can you pass arguments to a Java program when running it from the command line?

Yes, you can pass arguments to a Java program from the command line by specifying them after the class name when using the 'java' command to run the program.

What happens if the 'main' method tries to access 'args' without any arguments being passed?

If no arguments are passed and the 'main' method tries to access 'args', it will result in an 'ArrayIndexOutOfBoundsException' because the program is attempting to access an element in an empty array.

What is an example of how the 'args' parameter can be used in a Java program?

An example use of the 'args' parameter is to print a message passed as an argument when starting the program, such as 'System.out.println(args[0]);' to print the first argument.

How can command-line arguments be used to influence the behavior of a Java program?

Command-line arguments can be used to customize the behavior of a Java program by providing input for configuration, setting parameters, or controlling the flow of the program based on the provided values.

Outlines

00:00

📚 Understanding the Java Main Method

This paragraph explains the significance of the 'public static void main(String[] args)' method in Java programming. It clarifies that this method signature is the entry point for the Java Runtime Environment (JRE) to execute a program. The 'public' access modifier ensures that the method can be called from outside the class, while 'static' allows the JRE to call the method without creating an instance of the class. 'Void' indicates that the main method does not return any value, and 'main' is the specific name that the JRE looks for. The 'String[] args' parameter is an array that can hold command-line arguments passed to the program, which can be accessed and utilized within the main method. The explanation includes an example of how to print a message passed as an argument and demonstrates how to pass arguments through an IDE like Eclipse or a command-line interface.

05:00

🛠️ Exploring Java Program Arguments and Execution

The second paragraph delves into the practical use of the 'String[] args' parameter in Java's main method. It shows how to pass arguments to a Java program both within an IDE, such as Eclipse, and from the command line using a terminal or PowerShell. The speaker provides a step-by-step guide on how to modify program arguments in Eclipse's run configurations and demonstrates compiling and running a Java program from the command line, including how to pass arguments directly after the class name. The paragraph also cautions about the potential for an 'ArrayIndexOutOfBoundsException' if the program expects arguments but none are provided, emphasizing the importance of correctly using the 'args' parameter. The summary concludes with an invitation for viewers to engage with the content by liking the video and subscribing for more similar content.

Keywords

💡Java

Java is a high-level, class-based, object-oriented programming language designed for portability and used widely in various computing platforms. In the video, Java is the central theme as the script discusses the specifics of starting programming in Java, particularly the importance of the 'main' method in a Java program.

💡JRE (Java Runtime Environment)

The Java Runtime Environment is a software package that provides the necessary components for running Java applications. In the script, JRE is mentioned as the entity that calls the main method of a Java program to initiate its execution.

💡Main method

The main method is the entry point of any Java application. It is where the program execution begins. The script explains the structure and purpose of the main method signature, which is crucial for the JRE to recognize and execute the program.

💡Method signature

A method signature in Java uniquely identifies a method and includes its name, return type, and parameter list. The script describes the main method's signature, 'public static void main(String[] args)', which is essential for the JRE to call the method.

💡Public

In Java, 'public' is an access modifier that allows a method or class to be accessible from any other class. The script mentions 'public' as a necessary modifier for the main method to be callable by the JRE from outside the class.

💡Static

A 'static' method in Java can be called without creating an instance of the class. The video script explains that the main method is static so that the JRE can call it directly on the class rather than on an instance of the class.

💡Void

'Void' in Java is a data type that represents the absence of any value. The script clarifies that the main method returns 'void', meaning it does not return any value upon completion, which is typical for the entry point of a program.

💡String

In Java, 'String' is a class that represents sequences of characters. The script discusses 'String' in the context of the 'args' parameter of the main method, which is an array of 'String' objects representing command-line arguments passed to the program.

💡Arguments

Arguments in the context of the main method are the command-line inputs passed to the program. The video script provides examples of how to use the 'args' array to access these inputs and incorporate them into the program logic.

💡IDE (Integrated Development Environment)

An Integrated Development Environment is a software application that provides comprehensive facilities for software development. The script mentions using an IDE, specifically Eclipse, to modify program arguments and run Java programs.

💡Array

An array in Java is a data structure that stores a fixed-size sequential collection of elements of the same type. The script explains the 'args' parameter as an array of 'String', which holds the arguments passed to the main method.

💡ArrayIndexOutOfBoundsException

This is a type of exception in Java that occurs when an attempt is made to access an array with an index that is outside the bounds of the array. The script warns about this exception when the program tries to access elements in the 'args' array without any arguments being passed.

Highlights

The Java Runtime Environment (JRE) calls the main method to trigger a program to run.

The main method signature 'public static void main(String[] args)' is designed for the JRE to call the program.

The 'public' keyword makes the method callable by something outside of the class.

A 'private' or 'protected' method would restrict access, making it unusable by the JRE.

The 'static' keyword allows the method to be called without creating an instance of the class.

The JRE calls the main method directly on the class, not on an instance of it.

The 'void' return type indicates that the main method does not return anything.

The main method must be named 'main' for the JRE to recognize and call it.

The 'String[] args' parameter allows passing arguments to the program.

Arguments can be passed through the IDE's run configurations.

Arguments can also be passed when running the program from the command line.

Using arguments can lead to an 'ArrayIndexOutOfBoundsException' if not handled correctly.

Examples given demonstrate how to print a passed argument using 'System.out.println(args[0])'.

The video aims to demystify the Java main method and its components.

The explanation provides clarity on why each part of the main method signature is necessary.

The video encourages viewers to give a thumbs up and subscribe for more content.

Transcripts

when you got started programming in java

i bet somebody just

told you that you had to have this

public static

void main string args and in there is

where you write your code

then i bet you probably thought oh wait

what does that mean wait

wait what's with public static void main

string

what does all that mean and they

probably told you shut up nope don't ask

no shut up stop no just program just do

it

write it put that in there that's what

you got to do

and you probably went whoa okay all

right i'll do it but that question is

probably still going on your head what

does all of that

actually mean so i promise that in just

a few minutes you're gonna know what

each individual piece of that means what

it does and what it's used for

so let's get to it so i think the best

way to

think about what is happening here is

that the jre the java runtime

environment on your computer

has to call your program's main method

to trigger the program to run so for

example i have a class here called main

method explanation that has a main

method

and what the jre is actually going to do

is call this main method

uh something like this so with the main

method

explanation dot

main and it's going to pass in

arguments and all the pieces of this

public static void main string args

uh method signature are designed to make

that call work

so first things first public why is it

public well it's public

because this method has to be callable

by something outside of this class if

this method was something else like

private or protected the jre wouldn't

have the access to call it so the method

has to be public this was private

instead

only this class would be able to call it

and so

it's it's kind of like saying hey i'm a

great program and the jre goes great how

can i run you he's like yeah he can't he

can't

only i can run me and that's pretty

useless program

so next why static so a static

method is a method that can be called on

a class without

needing an instance of that class to run

it against

and that's how the jre is actually going

to call your program it's not going to

create an

object of main method explanation and

then

run a main method on it it's just going

to run the main method directly on the

class

exactly as i have it written here so

here's what it's not going to do

create a new variable of type main

method

explanation called explanation

equals new main method explanation

and then take explanation dot

main it's not going to do that it's

designed to make the call

directly on your class like this so

that's why it has to be static

it's not going to be called on an

instance of your class it's going to be

called on your class

so your main method has to be static now

next is this void now

void is actually the return type of your

main method

just like you have methods that return

an int or a long or a string or whatever

your main method just returns nothing

it's

void it has no need to actually return

anything

so when your program gets called it gets

run and it finishes

it doesn't have to return anything when

this main method is done your program

just ends

now next why is it called main

it just is it's just called main you

can't call it anything else

the jre is going to call your class dot

main and so your main method better be

called main

or won't work so i know at the beginning

i told you

yes people told you and it's just the

way it is in this case

that's just the way it is that's what

the jbm is going to call

that's what you better name your method

now the last part of this is probably

the most

interesting this string array called

args

and what this actually is is an array of

strings that are arguments that can be

passed to your program and if you've

never used it it can actually be pretty

cool

now a lot of programs that you write you

just don't care what is passed in

in this uh array of strings arguments

you don't need to do anything your

program is going to do what it's going

to do

you don't need to give it any other

argument at the beginning you might take

input later or something like that but

you don't need to when you kick off the

program give it input you get input from

the user later or something

but you can do it so i'm going to show

you a quick example of how you can

actually take in an argument and

use it in your program so let's say you

just wanted to print a message from

whoever's starting up your program and

they can

answer that message as the argument you

just want to print it out so you can do

system dot out dot print line

args zero so that's just getting the

zeroth

the first element on this array

and it's going to print it out so how do

you pass in that argument

now i'm going to show you a couple ways

first is in the ide i'm using

i'm using eclipse right here so if

you're programming eclipse for example

if you want to change these arguments

you can

right click anywhere on the class and go

to run as and then click run

configurations

and then you'll see this arguments tab

and that's where you can put in your

arguments

so for example we can put in chimichanga

and apply and then we can hit run and

what that's going to do is

pass in as an array one element array

um with the string chimichanga and

that's going to be at the zeroth

position of those args

as you can see it prints out chimichanga

so you can imagine what kind of things

you can do with this you could

let's say you want to print out

something like x number of times

you can give it that number in the input

arguments there's all kinds of fancy

stuff you could do with that

but also if you're not running in an ide

and you're just using a text editor or

something you can still do this

so here's my folder with main method

explanation.java in it

and i can open a powershell window here

you can use

command window or whatever you've got

this is just basically a command window

and so i can compile my program with

java c

and what's the name of it java c main

method explanation dot java

so it's compiled you can see my class

file here and now i can run that command

with

java space

main method explanation

and then i can just put my arguments

right after that so i can also put

jimmy junga's

and i can see it printed out

chimichangas but something to note if

you are actually

using these arguments in your program

you have to pass them in so for example

here if you don't pass it in you're

going to have problems so we could run

that last

command again just without that

parameter

and you could see oh no i got an array

index out of bounds exception

it was looking for something in an array

that wasn't there and blew up so you can

run into problems if you don't use that

right but

in the right hands it can offer some

pretty cool functionality so i hope you

learned something today hope i've

demystified a little bit of the java

world here today for you and

if i did give me a thumbs up it's super

appreciated

if you'd like to see more videos like

this in the future hit that subscription

button thanks so much everybody see you

next time