Java is not bundled with your OS by default. To write and run Java programs, you must install the JDK (Java Development Kit).

This section shows how to install the JDK manually using .zip (for Windows) or .tar.gz (for Linux), and how to verify the installation.

1.Download the JDK

• Visit https://jdk.java.net or your preferred vendor.
• Download the .zip archive suitable for Windows.

2.Extract the JDK

• Extract the archive to a location such as:

C:\Java\jdk-<version>

3.Set Environment Variables

• Open PowerShell and set the following:

setx JAVA_HOME "C:\Java\jdk-<version>"
setx PATH "%PATH%;%JAVA_HOME%\bin"

4.Verify Installation

• Open a new Command Prompt window and run:

java -version
javac -version

• You should see the installed version displayed.

1.Download the JDK

• Visit https://jdk.java.net
• Download the .tar.gz archive for Linux.

2.Extract the JDK

tar -xvzf jdk-21_linux-x64_bin.tar.gz
sudo mv jdk-21 /opt/java/

3.Set Environment Variables

• Edit ~/.bashrc (or ~/.zshrc if using Zsh):

export JAVA_HOME=/opt/java/jdk-21
export PATH=$JAVA_HOME/bin:$PATH

• Apply the changes:

source ~/.bashrc

4.Verify Installation

java -version
javac -version

Now your JDK is installed and configured. You’re ready to write, compile, and run Java programs.

Let’s create and run a simple Java program to verify the setup and understand the basic development workflow.

Open any text editor and write the following code:

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

Save the file as:

HelloWorld.java

Note: The file name must match the class name (HelloWorld).

You can directly run the .java file using the Java interpreter:

java HelloWorld.java

This should print:

Hello, Java!

Let’s look at what happens when you compile the Java source file.

javac HelloWorld.java
ls

This produces a file called HelloWorld.class, which contains bytecode.

You can inspect this bytecode using:

javap HelloWorld

The full JDK is around 300 MB. But your program might only need the core Java module java.base.

You can use jlink to create a lightweight runtime:

jlink --add-modules java.base --output my_jre

Check the size of my_jre — it should be significantly smaller.

You can now run your program using the custom JRE:

./my_jre/bin/java HelloWorld

This demonstrates how Java’s modularity allows you to create minimal runtimes tailored to your application.

Let me know if you'd like this broken into two Markdown files — one for **installation**, another for **your first program** — or we can continue with the next topic.