Linux run java main

«Hello World!» for Solaris OS, Linux, and Mac OS X

It’s time to write your first application! These detailed instructions are for users of Solaris OS, Linux, and Mac OS X. Instructions for other platforms are in «Hello World!» for Microsoft Windows and «Hello World!» for the NetBeans IDE.

A Checklist

To write your first program, you’ll need:

1. The Java SE Development Kit 8 (JDK 8) You can download the version for Solaris OS, Linux, or Mac OS X. (Make sure you download the JDK, not the JRE.) Consult the installation instructions.
2. A text editor In this example, we’ll use Pico, an editor available for many UNIX-based platforms. You can easily adapt these instructions if you use a different text editor, such as vi or emacs .

These two items are all you’ll need to write your first application.

Creating Your First Application

Your first application, HelloWorldApp , will simply display the greeting «Hello world!». To create this program, you will:

• Create a source file A source file contains code, written in the Java programming language, that you and other programmers can understand. You can use any text editor to create and edit source files.
• Compile the source file into a .class file The Java programming language compiler ( javac ) takes your source file and translates its text into instructions that the Java virtual machine can understand. The instructions contained within this .class file are known as bytecodes.
• Run the program The Java application launcher tool ( java ) uses the Java virtual machine to run your application.

Create a Source File

To create a source file, you have two options:

• You can save the file HelloWorldApp.java on your computer and avoid a lot of typing. Then, you can go straight to Compile the Source File.
• Or, you can use the following (longer) instructions.

First, open a shell, or «terminal,» window.

When you first bring up the prompt, your current directory will usually be your home directory. You can change your current directory to your home directory at any time by typing cd at the prompt and then pressing Return.

The source files you create should be kept in a separate directory. You can create a directory by using the command mkdir . For example, to create the directory examples/java in the /tmp directory, use the following commands:

cd /tmp mkdir examples cd examples mkdir java

To change your current directory to this new directory, you then enter:

Now you can start creating your source file.

Start the Pico editor by typing pico at the prompt and pressing Return. If the system responds with the message pico: command not found , then Pico is most likely unavailable. Consult your system administrator for more information, or use another editor.

When you start Pico, it’ll display a new, blank buffer. This is the area in which you will type your code.

Type the following code into the new buffer:

/** * The HelloWorldApp class implements an application that * simply prints "Hello World!" to standard output. */ class HelloWorldApp < public static void main(String[] args) < System.out.println("Hello World!"); // Display the string. >>

Be Careful When You Type

Note: Type all code, commands, and file names exactly as shown. Both the compiler ( javac ) and launcher ( java ) are case-sensitive, so you must capitalize consistently.

HelloWorldApp is not the same as helloworldapp .

Save the code in a file with the name HelloWorldApp.java . In the Pico editor, you do this by typing Ctrl-O and then, at the bottom where you see the prompt File Name to write: , entering the directory in which you wish to create the file, followed by HelloWorldApp.java . For example, if you wish to save HelloWorldApp.java in the directory /tmp/examples/java , then you type /tmp/examples/java/HelloWorldApp.java and press Return.

You can type Ctrl-X to exit Pico.

Compile the Source File into a .class File

Bring up another shell window. To compile your source file, change your current directory to the directory where your file is located. For example, if your source directory is /tmp/examples/java , type the following command at the prompt and press Return:

If you enter pwd at the prompt, you should see the current directory, which in this example has been changed to /tmp/examples/java .

If you enter ls at the prompt, you should see your file.

Results of the ls command, showing the .java source file.

Now are ready to compile the source file. At the prompt, type the following command and press Return.

The compiler has generated a bytecode file, HelloWorldApp.class . At the prompt, type ls to see the new file that was generated: the following figure .

Results of the ls command, showing the generated .class file.

Now that you have a .class file, you can run your program.

If you encounter problems with the instructions in this step, consult the Common Problems (and Their Solutions).

Run the Program

In the same directory, enter at the prompt:

The next figure shows what you should now see.

The output prints «Hello World!» to the screen.

Congratulations! Your program works!

If you encounter problems with the instructions in this step, consult the Common Problems (and Their Solutions).

Previous page: «Hello World!» for Microsoft Windows
Next page: A Closer Look at the «Hello World!» Application

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How to Run Java Programs in Ubuntu

So, you have started learning Java programming? That’s good.

And you want to run the java programs on your Linux system? Even better.

Let me show how to run Java in terminal in Ubuntu and other Linux distributions.

Running Java programs in Ubuntu

Let’s go in proper steps here.

Step 1: Install Java compiler

To run a Java program, you need to compile the program first. You need Java compiler for this purpose.

The Java compiler is part of JDK (Java Development Kit). You need to install JDK in order to compile and run Java programs.

First, check if you already have Java Compiler installed on your system:

If you see an error like “Command ‘javac’ not found, but can be installed with”, this means you need to install Java Development Kit.

The simplest way to install JDK on Ubuntu is to go with the default offering from Ubuntu:

sudo apt install default-jdk

You’ll be asked to enter your account’s password. When you type the password, nothing is seen on the screen. That is normal. Just enter your password blindly. When asked, press the enter key or Y key.

The above command should work for other Debian and Ubuntu based distributions like Linux Mint, elementary OS etc. For other distributions, use your distribution’s package manager. The package name could also be different.

Once installed, verify that javac is available now.

Step 2: Compile Java program in Linux

You need to have a Java program file for this reason. Let’s say you create a new Java program file named HelloWorld.java and it has the following content:

You can use Nano editor in terminal or Gedit graphical text editor for writing your Java programs.

If there is no error, the above command produces no output.

When you compile the Java program, it generates a .class file with the class name you used in your program. You have to run this class file.

Step 3: Run the Java class file

You do not need to specify the class extension here. Just the name of the class. And this time, you use the command java, not javac.

This will print Hello World on the screen for my program.

And that’s how you run a Java program in the Linux terminal.

This was the simplest of the example. The sample program had just one class. The Java compiler creates a class file for each class in your program. Things get complicated for bigger programs and projects.

This is why I advise installing Eclipse on Ubuntu for proper Java programming. It is easier to program in an IDE.

I hope you find this tutorial helpful. Questions or suggestions? The comment section is all yours.

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Your First Java Program in the Ubuntu Terminal

If you are new to Java Programming in the Ubuntu operating system, this simple tutorial will guide you along in writing and compiling your first Java Hello World program. The applications that you need for this purpose include the Java Runtime Environment and the Java Development Kit. This article covers the installation of these two through the Ubuntu command line. You can then write your first program in a text editor such as gedit, nano or even some graphical text writing tool. You will then compile your program in order to create a class that you can then execute in order to run your Java program.

We have run the commands and procedures mentioned in this article on an Ubuntu 18.04 LTS system.

Java Installation

In order to compile and run a basic Java program, you need to have the following two software installed on your Ubuntu system:

Let us first open the Ubuntu command line, the Terminal, in order to install these two. You can open the Terminal either through the system Dash or the Ctrl+alt+T shortcut.

Since we will be installing Java through the apt utility, first let us update our apt repositories through the following command:

The next thing is to run the following command:

This will ensure either Java Runtime Environment is already installed on your system or not. If yes, it will also let you know which version of Java you have on your system.

In my case, the output shows that I do not have Java installed on my system yet.

In order to install the Java Runtime Environment on your system, run the following command as root:

The system will prompt a Y/n option for you to continue installation. Please enter Y to continue, after which JRE will be installed on your system.

You can then check the installed version of Java as follows:

After installing the JRE, let us check if we have the Java Development Kit installed on our system or not. This can be done by checking the version of your Java Compiler, javac, as follows:

The above output shows that I need to install the Java compiler or the JDK on my system.

You can install it through the following command as root:

$ sudo apt install default-jdk

The system will prompt a Y/n option for you to continue installation. Please enter Y to continue, after which JDK will be installed on your system.

You can then check the installed version of the Java Compiler as follows:

The basic installations you need to have before running a Java program are now complete.

Your First Java Program

Before starting to write Java programs, it is best practice to make a dedicated directory for all your Java related work. I am creating such a directory through the following command in my home folder:

Then move to that directory as follows:

The next step is to write your first Java program. You can write it in any of your favorite text editors. Here, I am using the gedit editor to write the program. Run the following command in order to open a new java file in the gedit editor:

Then copy the following code in your file:

This program is simply intended to print “Hello World” on your screen.

Save the file and close it.

Then is the time to compile your first program through the Java compiler as follows:

The ls command will show that the compiler will create a class based on the class in your Java code:

In order to run the compiled program, run the following program:

In this article, you have learned to install both the Java Runtime Environment and the Java Development Kit used to compile and run your java programs. We also wrote a simple Hello World program in Java and ran to see if you and your system are ready to move to the more complex world of Java Programming.

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