Как написать на джаве hello world

В этом уроке вы напишите свою первую программу на Java! Для этого вам потребуется компилятор Java из набора JDK (набор инструментов разработки). Если вы ещё не установили JDK, вы можете это сделать, прочитав наш предыдущий урок.

Что вам понадобится

Для первой программы на Java потребуется следующее:

• Установленный Java Development Kit
• Любой редактор кода. Для начала сгодится и блокнот

Исходный код

Исходный код программы – это код на каком-либо языке программирования. В нашем случае это язык программирования Java. Программисты пишут исходный код, который понятен им и компилятору. Затем компилятор преобразует исходный код программы в байт код, который понятен виртуальной машине Java (Java Virtual Machine, JVM). JVM это среда выполнения программ, скомпилированных в виде байт кода.

Рекомендую создать отдельную директорию для ваших исходников. Далее в тексте будет использоваться директория C:Javafirst

Исходный код Java программ содержится в файлах с расширением .java. В этих файлах хранятся классы Java. Давайте напишем свой первый класс:

public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

Здесь мы объявили класс HelloWorld, указали в нём точку входа (метод main) и скомандовали вывести в консоль строку «Hello, World!».

Сохраните этот исходник в файле с названием HelloWorld.java. Будьте внимательны: название файла должно совпадать с названием класса, а расширение должно быть .java.

Теперь откройте консоль (нажмите комбинацию Win+R и укажите cmd):

В открывшейся консоли перейдите в каталог с исходниками:

cd c:Javafirst

Теперь нам требуется скомпилировать исходный код в байт код.

Компилирование

Давайте укажем компилятору Java, что нам требуется скомпилировать исходный код в файле HelloWorld.java:

javac HelloWorld.java

Компилятор javac должен молча отработать и скомпилировать наш исходный код.

Успешным результатом компилирования будет создание нового файла HelloWorld.class в папке с исходником:

Выполнение программы

Давайте же наконец запустим нашу первую программу!

Выполните в консоли следующую команду:

java HelloWorld

Здесь мы запускаем на выполнение наш класс HelloWorld. Обратите внимание, что при запуске программы на выполнение не требуется указывать расширения файлов, так как указывается именно название класса (не файла).

Если вы увидели надпись «Hello, World!», поздравляю, значит, у вас всё получилось и программа корректно выполнилась!

Резюме

В этом уроке вы научились компилировать первую программу с помощью компилятора javac и запускать её на выполнение с помощью виртуальной машины java. В следующих уроках вы узнаете, из чего состоят классы Java.

Об этой статье

Была ли эта статья полезной?

Java is one of the most popular and widely used programming languages and platforms. Java is fast, reliable, and secure. Java is used in every nook and corner from desktop to web applications, scientific supercomputers to gaming consoles, cell phones to the Internet.

Java is easy to learn, and its syntax is simple and easy to understand. It is based on C++ (so easier for programmers who know C++).

The process of Java programming can be simplified in three steps: 

• Create the program by typing it into a text editor and saving it to a file – HelloWorld.java.
• Compile it by typing “javac HelloWorld.java” in the terminal window.
• Execute (or run) it by typing “java HelloWorld” in the terminal window.

The below-given program is the most simple program of Java printing “Hello World” to the screen. Let us try to understand every bit of code step by step.

Time Complexity: O(1)

Space Complexity: O(1)

The “Hello World!” program consists of three primary components: the HelloWorld class definition, the main method, and source code comments. The following explanation will provide you with a basic understanding of the code: 

1. Class definition

This line uses the keyword class to declare that a new class is being defined. 

class HelloWorld {
    //
    //Statements
}

2. HelloWorld 

It is an identifier that is the name of the class. The entire class definition, including all of its members, will be between the opening curly brace “{” and the closing curly brace “}“.

3. main method: 

In the Java programming language, every application must contain a main method. The main function(method) is the entry point of your Java application, and it’s mandatory in a Java program. whose signature in Java is: 

public static void main(String[] args)

• public: So that JVM can execute the method from anywhere.
• static: The main method is to be called without an object. The modifiers public and static can be written in either order.
• void: The main method doesn’t return anything.
• main(): Name configured in the JVM. The main method must be inside the class definition. The compiler executes the codes starting always from the main function.
• String[]: The main method accepts a single argument, i.e., an array of elements of type String.

Like in C/C++, the main method is the entry point for your application and will subsequently invoke all the other methods required by your program.

The next line of code is shown here. Notice that it occurs inside the main() method. 

System.out.println("Hello, World");

This line outputs the string “Hello, World” followed by a new line on the screen. Output is accomplished by the built-in println( ) method. The System is a predefined class that provides access to the system, and out is the variable of type output stream connected to the console.

Comments

They can either be multiline or single-line comments. 

// This is a simple Java program. 
// Call this file "HelloWorld.java". 

This is a single-line comment. This type of comment must begin with // as in C/C++. For multiline comments, they must begin from /* and end with */. 

Important Points 

• The name of the class defined by the program is HelloWorld, which is the same as the name of the file(HelloWorld.java). This is not a coincidence. In Java, all codes must reside inside a class, and there is at most one public class which contains the main() method.
• By convention, the name of the main class(a class that contains the main method) should match the name of the file that holds the program.
• Every Java program must have a class definition that matches the filename (class name and file name should be same).

Compiling the program 

• After successfully setting up the environment, we can open a terminal in both Windows/Unix and go to the directory where the file – HelloWorld.java is present.
• Now, to compile the HelloWorld program, execute the compiler – javac, to specify the name of the source file on the command line, as shown:

javac HelloWorld.java 

• The compiler creates a HelloWorld.class (in the current working directory) that contains the bytecode version of the program. Now, to execute our program, JVM(Java Virtual Machine) needs to be called using java, specifying the name of the class file on the command line, as shown:

java HelloWorld

• This will print “Hello World” to the terminal screen.
   

In Windows

In Linux

This article is contributed by Gaurav Miglani. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

Hello World

tutorial
java

• Creating a .java File
• Compiling
• Running
• Java Classes
• The main() Function
• Command Line Arguments
• The Java API
• Import Statements
• Homework
• Next: Creating Classes

Now that you installed Java, it’s time to write some code!

This tutorial assumes that you’ve already been through the Processing tutorials and understand what code is, and what objects and classes are.

In Processing, we used the Processing editor to write and run our sketches. The Processing editor makes that easy, but it hid a lot of the details from us. To better understand what’s going on behind the scenes, we’re going to create our files using a basic text editor, and we’re going to compile and run our files using the command prompt. Eventually we’ll use a fancier editor, but for now let’s stick with doing things the “manual” way.

Creating a .java File

Open up your basic text editor. I use jEdit, but you can use Notepad or whatever came with your computer. You don’t want to use a fancy word processor like Microsoft Word though, because that kind of editor adds formatting and stuff that we don’t want in our code. Just use a basic text editor!

Type this into your editor:

public class HelloWorld{
	public static void main(String[] args){
		System.out.println("Happy Coding!");
	}
}

And then save that to a file named HelloWorld.java. (Note that you can save files with any extension, and they’re still just basic text files. But we need to use the .java extension so the next step works.)

Compiling

Even though we’re writing code, the computer can’t actually understand the code yet. So we have to translate it into a language that the computer does understand. This is called compiling the code. The Java Development Kit (the JDK) contains a bunch of programming tools, and one of them is a compiler that converts .java files (which are just basic text files that contain Java code) into files that the computer can run.

So now that you have a .java file, you need to compile it.

• Open the command prompt and cd to the directory that contains the .java file.
• Type javac HelloWorld.java and press enter.

That should create a new file named HelloWorld.class in that same directory.

This is a Java class file, and it contains Java bytecode, which is instructions that the computer (specifically, the Java runtime environment) can read and execute. You can try to open the .class file in your basic text editor, but it will look like gibberish- that’s because it’s designed for the computer to understand, not humans!

Running

Now that you have a .class file, you can tell Java to run that file. From the command prompt, type java HelloWorld and press enter. Notice that you don’t include the .class part!

That command runs our program, which for now just prints a message to the console. Here’s what the whole process looks like from start to finish:

Java Classes

Remember that you can create a class to organize variables and functions into a single unit.

In Processing, you can also have “sketch-level” code: variables and functions that aren’t inside a class. But Java doesn’t have sketch-level code, so everything must be inside a class!

That’s why our file starts with a class definition:

The public keyword just means that other classes can uses this class. We’ll talk more about this later.

The main() Function

Remember that Processing automatically calls the setup() and draw() functions for you, so any code you put in them is run for you. Instead of setup() and draw(), Java uses a function named main().

The main() function is public (which means other classes can call it) and static (we’ll talk more about what that later), and it doesn’t return anything (it has a void return type). It also takes a String[] array as an argument. So its declaration looks like this:

public static void main(String[] args){

Finally, inside the main() function we place any code we want to run when the program is executed. For now we’re just printing out a message to the console (the command prompt), so we just call the System.out.println() function:

System.out.println("Happy Coding!");

Putting it all together, it looks like this:

public class HelloWorld{
	public static void main(String[] args){
		System.out.println("Happy Coding!");
	}
}

Command Line Arguments

(I’m assuming that you remember arrays from the Processing arrays tutorial. If you don’t remember, you might want to read through that again.)

Notice that the main() function takes a String[] array argument. Let’s write a program that prints out the values of that array:

public class ArgsPrinter{
	public static void main(String[] args){
		System.out.println("args.length: " + args.length);
		for(int i = 0; i < args.length; i++){
			System.out.println("args[" + i + "]: " + args[i]);
		}
	}
}

Type this program into your basic text editor, save it as a file named ArgsPrinter.java, then open a command prompt to that directory. Type javac ArgsPrinter.java and press enter to compile that code into a class file, and then type java ArgsPrinter and press enter to run this code.

If you run this code, you’ll notice that it just prints out one line:

So the args argument is just an empty array. That doesn’t seem very useful!

Command line arguments are a way for a user to provide input to a command line program. They’re usually just a set of words listed after the command. You’ve actually already used them! When you type javac HelloWorld.java, you’re running the javac command and passing in HelloWorld.java as an argument.

So let’s run the program again, and this time let’s pass in command line arguments by adding them after the class name. Type this into your command prompt and press enter:

java ArgsPrinter one two three

This runs our ArgsPrinter program, and gives it command line arguments one, two, and three. If we hit enter, we see this:

C:UsersKevinDesktopHelloJava>java ArgsPrinter one two three
args.length: 3
args[0]: one
args[1]: two
args[2]: three

So, the args variable is an array that holds any command line arguments the user gives to the program. This is the most basic way we can get input from a user.

The Java API

The Java API is a webpage that lists every class, function, and variable that you can use in Java. It’s a lot like Processing’s reference page, but the Java API is much, much larger.

This should be your first stop whenever you have a question about how something in Java works. This might seem overwhelming at first, but you don’t have to know every class or memorize anything. Instead, you can search for a specific class and get more info about it when you need it.

For example, let’s say you want to use the ArrayList class. Find the ArrayList class in the list on the left, then click it to get more info about it. That page will give you a general description and tell you about the constructors and functions you can call.

Import Statements

Let’s say you created a program that uses the Scanner class, which lets you get user input from the command prompt. You can read more about the Scanner class in the Java API.

For now, type this code into your basic text editor and save it to a file named Greeter.java:

public class Greeter{	
	public static void main(String[] args){
		
		System.out.println("What's your name? (Please type your name and press enter.");
		
		Scanner scanner = new Scanner(System.in);
		String name = scanner.nextLine();
		
		System.out.println("Hello " + name + "!");
	}
}

Now try to compile your program by opening a command prompt to the directory that contains this file, then typing javac Greeter.java and pressing enter. You’ll get an error:

C:UsersKevinDesktopHelloJava>javac Greeter.java
Greeter.java:6: error: cannot find symbol
                Scanner scanner = new Scanner(System.in);
                ^

We’ll talk more about how to read this error later, but for now just know that this error says it can’t find the Scanner class. It’s in the Java API, so what’s going on?

In the Java API webpage, notice that the Scanner class is inside the java.util package:

This means that we have to import the class before we can use it. Import statements go at the top of our code:

import java.util.Scanner;

public class Greeter{	
	public static void main(String[] args){
		
		System.out.println("What's your name? (Please type your name and press enter.");
		
		Scanner scanner = new Scanner(System.in);
		String name = scanner.nextLine();
		
		System.out.println("Hello " + name + "!");
	}
}

Now we can compile and run this code, and it works! Hooray!

This program prompts the user for their name, which they can enter by typing into the command prompt and pressing enter. Then the code uses the Scanner class to read what the user typed. You can use the Scanner class to interact with the user, even without a fancy user interface!

Note that we didn’t have to import the System class, because it’s in the java.lang package (check for yourself by looking it up in the Java API!. Anything in the java.lang package does not need to be imported.

Homework

• Explain in your own words what the javac and java commands do.
• Write a program that takes a single command line argument: the user’s name. Print out a message saying hello to that name. What if the user provides their first and last name? Show a friendly error message if the user does not supply any command line arguments.
• Write a program that uses the Scanner class to ask for numbers from the user. After every number, print out the total of all of the entered numbers, as well as the min, max, and average.
• Write a program that requests a width, height, and character, and outputs a box of that character. For example, if the user enters a width of 3, height of 4, and character of %, you’d print:

• Also request whether the box should be filled. So if a user enters a width of 5, height of 4, character of &, and that the box should not be filled, you’d print:

Next: Creating Classes