How to install Java and compile a single Java file

Install Java in the Ubuntu Operating system

Run the below command to install Java 21 in the Ubuntu operating system

sudo apt install openjdk-21-jdk

Install Java in the Centos Operating system

Run the below command to install Java 21 in the Centos operating system

sudo yum install java-21-openjdk

To check the Java version

Run the below command to check the installed Java version

java --version

Output:

ubuntu@vignesh-jenkins2:~$ java --version
openjdk 21.0.4 2024-07-16
OpenJDK Runtime Environment (build 21.0.4+7-Ubuntu-1ubuntu224.04)
OpenJDK 64-Bit Server VM (build 21.0.4+7-Ubuntu-1ubuntu224.04, mixed mode, sharing)

Compiling and running a Java file

1. Create a sample Java file HelloWorld.java

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

Output:

ubuntu@vignesh:~$ ll
-rw-rw-r-- 1 ubuntu ubuntu  118 Aug 12 10:18 HelloWorld.java

2. Compile the Java file HelloWorld.java

Run the javac command followed by file name to compile the Java file. E.g. javac HelloWorld.java

javac HelloWorld.java

Once the compilation finishes, it will create a HelloWorld.class file

3. Run the ls -l command to check the created HelloWorld.class file

ls -l

Output:

ubuntu@vignesh:~$ ll
-rw-rw-r-- 1 ubuntu ubuntu 427 Aug 12 10:24 HelloWorld.class
-rw-rw-r-- 1 ubuntu ubuntu 118 Aug 12 10:18 HelloWorld.java

4. Run the Java program

Type the java command following the Filename without extension to run the Java program E.g. java HelloWorld

java HelloWorld

Output:

ubuntu@vignesh:~$ java HelloWorld 
Hello, World!

Why do we need to compile the Java code?

The Java code is only understood by humans, computers/machines cannot understand this code. The computer can understand only 0’s and 1’s

So the Java compiler will convert the Java code to machine understandable code

What is a compiler?

• A compiler is a program that inputs a high-level language and outputs a low-level language (such as assembly or machine code).
• It is a computer program that converts programming language code into machine code (human-readable code to a binary 0 and 1 bit language for a computer processor to understand).
• The computer then executes the machine code to complete the task.

Key points of Compiler:

• Compilers check all types of errors, limits, and ranges.
• It takes longer to run and also requires more memory.