Install Java (JDK)

Choosing a JDK#

Java has several free, production-grade JDK distributions. For new projects, use a current LTS (Long-Term Support) release:

Popular free distributions: Eclipse Temurin, Amazon Corretto, Oracle OpenJDK. They're all built from the same OpenJDK source and are interchangeable for almost all purposes.

Installing on Windows#

1. Download the Temurin 21 .msi installer from the Adoptium project.
2. Run the installer — make sure "Set JAVA_HOME variable" and "Add to PATH" are checked.
3. Open a new terminal and verify:

java -version
javac -version

Installing on macOS#

Using Homebrew:

brew install --cask temurin
java -version

Installing on Linux (Debian/Ubuntu)#

sudo apt update
sudo apt install openjdk-21-jdk
java -version

Verifying the install#

Both commands should print a matching version:

$ java -version
openjdk version "21.0.3" 2024-04-16 LTS

$ javac -version
javac 21.0.3

If javac is missing but java works, you likely installed a JRE instead of a full JDK — reinstall using the JDK package.

Your first program#

Create a file named exactly Main.java — the file name must match the public class name:

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

Compile and run it from the terminal:

javac Main.java   # produces Main.class (bytecode)
java Main         # runs it on the JVM — note: no ".class" extension

Hello, World!

Using a build tool (recommended beyond "hello world")#

Real projects almost never compile with raw javac — they use Maven or Gradle to manage dependencies, run tests and package the app. A minimal Maven project structure looks like:

my-app/
├── pom.xml
└── src/
    └── main/
        └── java/
            └── Main.java

mvn compile   # compiles the project
mvn package   # builds a runnable .jar

Common mistakes#

• Naming the file differently from the public class it contains (App.java with public class Main will not compile).
• Running java Main.class instead of java Main — the java launcher takes the class name, not a file name.
• Installing only a JRE and wondering why javac isn't found.

Interview questions#

Q: What happens if the file name doesn't match the public class name? The compiler refuses to compile it with an error like class Main is public, should be declared in a file named Main.java.

Q: What's the difference between java Main and java Main.java? java Main runs the already-compiled Main.class bytecode. Modern JDKs (11+) also support java Main.java as a convenience that compiles and runs single-file source in one step, without producing a .class file on disk.