Installing Java

Scope: getting a JDK on your machine, confirming it works, and knowing which Java runs when you type java. Package names below were checked against Debian/Ubuntu docs in October 2026; Homebrew and Windows steps follow the vendors' own instructions.

Pick a JDK

You want an LTS release (see What Is Java?). Two free, reputable sources cover everyone:

Source	Good for	Package / how
Your Linux distro	Debian/Ubuntu/Mint	sudo apt install openjdk-21-jdk (or openjdk-25-jdk on Debian 13+)
Temurin (Eclipse Adoptium)	Windows, macOS, any Linux	Installer or archive from adoptium.net

Avoid Oracle's branded JDK download unless you enjoy reading license agreements — OpenJDK builds are the same language, free for any use.

Which version? 21 if you want maximum compatibility with existing tutorials and employers; 25 if you want the current LTS. Everything in this guide works on 17 and up.

Linux (Debian/Ubuntu/Mint)

sudo apt update
sudo apt install openjdk-21-jdk

Check:

java -version
javac -version

Expected (patch numbers move over time; the 21 is what matters):

openjdk version "21.0.7" 2025-04-15
OpenJDK Runtime Environment (build 21.0.7+6-Ubuntu-0ubuntu124.04)
...
javac 21.0.7

Both commands must work. java runs programs; javac compiles them. If javac is missing you installed a JRE package by mistake — install the -jdk one.

macOS

brew install --cask temurin21

Check: same java -version / javac -version as above.

Without Homebrew, download the macOS installer from adoptium.net and run it.

Windows

Download the Windows MSI installer from adoptium.net, run it, and accept the option to set JAVA_HOME and update PATH (the installer offers both — say yes).

Check in PowerShell:

java -version
javac -version

If you have several Javas

Developers accumulate JDKs. java -version shows the one your PATH finds first. To see all of them:

# Linux: lists every installed java
update-alternatives --list java

# macOS: lists every installed JDK
/usr/libexec/java_home -V

On Windows, check JAVA_HOME in Settings → System → About → Advanced system settings → Environment Variables. Point it at the JDK you want (e.g. C:\Program Files\Eclipse Adoptium\jdk-21.0.7.6-hotspot) and make sure %JAVA_HOME%\bin is on PATH.

A handy extra: jshell

The JDK ships a read-eval-print loop called jshell (Java 9+). It runs snippets instantly without files — perfect for trying things from this guide:

jshell

|  Welcome to JShell -- Version 21.0.7
|  For an introduction type: /help intro

jshell> 3 * 7
$1 ==> 21

jshell> /exit

Type /exit to leave. You'll see jshell> prompts in later chapters wherever a quick experiment fits.

What this doesn't cover

Maven and Gradle — the build tools real Java projects use instead of raw javac. You don't need them for this guide; every program here compiles with plain javac. When your project outgrows single files, learn Maven first (simpler mental model), Gradle second.