Week 1

Environment setup

How Java works

Types and Variables

Arrays

Basic IO

Control Flow

Packages

OOP in Java

Static Members

Practice

Assignment

• Teachers

Back end Track

🎯 Learning Objectives

By the end of this module, you will be able to:

• Explain what the JDK is and what it contains
• Choose the right JDK distribution for your needs
• Install OpenJDK 21 on your operating system
• Verify your installation using the terminal
• Understand what JAVA_HOME is and why it matters
• Set up IntelliJ IDEA and run your first Java program

1. What is the JDK?

The Java Development Kit (JDK) is the software package you install on your machine to write, compile, and run Java programs. It contains three main components:

Think of it this way: the JRE is the engine, the JVM is what runs your code inside that engine, and the dev tools are the toolkit you use to build things.

2. Command-Line Tools You Get with the JDK

When you install the JDK, you get these tools available in your terminal:

You won't use all of these immediately, but it's good to know they exist. For now, javac and java are the two you'll use the most — and your IDE will often run them for you behind the scenes.

3. JDK Distributions

Java is an open standard, which means different organisations can build and distribute their own version of the JDK as long as they comply with the official specification. They all run the same Java code — the difference is who builds and maintains them.

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For this track we use OpenJDK 21 — it's free, widely used, and the version most tools and frameworks support well. Any other free distribution would also work fine.

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4. Installing OpenJDK 21

Follow the instructions for your operating system. If you're not sure which one you have, ask your coach before starting.

Windows

macOS

Linux (Ubuntu / Debian)

5. Verify Your Installation

Once the installation is complete, close and reopen your terminal, then run:

javac -version

You should see output like:

javac 21.0.x

Also verify the runtime:

java -version

Expected output:

openjdk version "21.0.x" ...

<aside> ⚠️

If you see an error like javac: command not found, try closing and reopening your terminal first. If the error persists, your PATH may not include the JDK's bin folder — see the JAVA_HOME section below or ask your coach.

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For more help, visit the official installation guide: https://openjdk.org/install/

6. Understanding JAVA_HOME

Many Java tools — including IntelliJ IDEA, Maven, and Gradle — look for an environment variable called JAVA_HOME. This variable holds the path to the root folder of your JDK installation, so these tools can find the JDK without you having to configure each one manually.

Check if it's already set

# macOS / Linux
echo $JAVA_HOME

# Windows (PowerShell)
echo $env:JAVA_HOME

If it prints a path (e.g. /usr/lib/jvm/java-21-openjdk-amd64), you're good. If it prints nothing, you may need to set it — IntelliJ can usually handle this automatically, but some build tools need it explicitly.

<aside> 💡

In practice, IntelliJ IDEA detects your JDK automatically when you create a project. You only need to set JAVA_HOME manually if a command-line build tool like Maven or Gradle can't find your JDK. Ask your coach if you run into this.

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7. Managing Multiple Java Versions with SDKMAN!

As you work on different projects, you may encounter ones that require different Java versions. SDKMAN! is the standard tool for installing and switching between multiple JDK versions from the terminal — similar to nvm for Node.js.

It works on macOS, Linux, and Windows (via WSL):

# Install SDKMAN!
curl -s "<https://get.sdkman.io>" | bash

# Install a specific JDK
sdk install java 21.0.2-open

# Switch between versions
sdk use java 21.0.2-open

# List available versions
sdk list java

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You don't need SDKMAN! right now, but it's worth knowing it exists. It becomes very useful once you start working on multiple Java projects professionally.

</aside>

More information: https://sdkman.io/

8. Setting Up IntelliJ IDEA

IntelliJ IDEA is the industry-standard IDE for Java development. It handles compiling, running, debugging, and much more — so you can focus on writing code rather than managing tools.

Installation

1. Go to https://www.jetbrains.com/idea/download/
2. Download the Community Edition (free) for your operating system
3. Run the installer and follow the setup wizard

Create Your First Project

1. Open IntelliJ IDEA
2. Click New Project
3. Select Java from the left panel
4. Under JDK, select the OpenJDK 21 you installed — IntelliJ should detect it automatically. If not, click Add JDK and browse to your installation folder
5. Click Next, give your project a name (e.g. hello-java), and click Create

Write and Run Hello World

IntelliJ will create a src folder. Right-click it → New → Java Class → name it Main.

Paste this code:

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

Run it by clicking the green play button next to main, or press Shift + F10.

You should see in the output panel at the bottom:

Hello, World!

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