Java — Get Started

Install the JDK

Before writing Java code you need the JDK — the Java Development Kit. It contains the compiler, standard libraries, and the runtime.

1. Go to adoptium.net
2. Download the installer for your operating system
3. Run it and accept the defaults

The installer sets up JAVA_HOME and adds java and javac to your PATH automatically.

Verify the installation

Open a terminal and type:

java -version
javac -version

You should see version numbers for both. If javac is not found, the installer didn't add it to your PATH — fix that before continuing.

Your first program

Create a file called Main.java:

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

Save it, open a terminal in the same folder, and run:

javac Main.java
java Main

Output:

Hello, world!

Two commands did two different things:

Command	What it does
javac Main.java	Compiles your source file into bytecode (Main.class)
java Main	Runs the compiled bytecode on the Java Virtual Machine

This two-step process is what "write once, run anywhere" means — the .class file works on any machine that has a JVM.

Every Java program needs a class

Unlike Python or JavaScript, Java forces you to wrap code inside a class. The class name must match the file name:

// File: Hello.java
public class Hello {
    public static void main(String[] args) {
        System.out.println("This works.");
    }
}

If the class name doesn't match the file name, the compiler rejects it. This strictness feels heavy at first, but it catches typos and naming problems before your code ever runs.

The main method explained

public static void main(String[] args)

Each word has a purpose:

• public — anyone can call this method
• static — you don't need an object to run it; Java can start it directly
• void — this method doesn't return a value
• main — the special name Java looks for as the entry point
• String[] args — receives command-line arguments (you can ignore it for now)

You don't need to memorize every keyword today. Just know that public static void main(String[] args) is the standard launch pad, and every beginner program starts with it.

Packages and project structure

Real Java projects have many files. The folder structure matches package names — a dot-separated namespace:

com/
  example/
    app/
      Main.java

At the top of Main.java you'd declare:

package com.example.app;

Packages keep code organized and prevent naming collisions. When your project grows beyond a single file, start organizing classes into packages.

IDE vs terminal

You can compile from the terminal, but most Java developers use an IDE (Integrated Development Environment):

• IntelliJ IDEA Community — free, the industry standard
• VS Code — lighter, install the "Extension Pack for Java"
• Eclipse —老牌 free option, still maintained

An IDE gives you autocomplete, error highlighting, debugging, and project management — tools that save hours once your code goes beyond one file.

For learning, either approach works. The terminal keeps you close to what's actually happening; the IDE keeps you productive.

Mini Practice

1. Install the JDK and verify both java and javac versions
2. Create Main.java and print your name
3. Add a second println that prints the current year
4. Break the code — remove public from the class line and read the error
5. Rename the file without renaming the class — see what the compiler says

Errors are documentation. Read them carefully — Java's compiler messages tell you exactly what's wrong and often where to fix it.

Next: syntax rules that govern every Java file →