Java Get Started

To start writing Java, you need exactly two things:

1. A JDK (Java Development Kit) — gives you javac (compiler) + java (runtime).
2. A text editor or IDE — IntelliJ IDEA, VS Code, Eclipse, or even Notepad.

Verify your install:

bash
javac --version
java --version

You should see something like:

javac 21.0.2
openjdk 21.0.2 2024-01-16

Step 1. Create a file HelloWorld.java:

java Run in compiler
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Hello, Codekilla!");
    }
}

Step 2. Compile (produces bytecode):

bash
javac HelloWorld.java

This creates HelloWorld.class in the same folder.

Step 3. Run (note: no .class extension):

bash
java HelloWorld

Hello, Codekilla!

• javac read HelloWorld.java.
• It translated the source into platform-independent bytecode (HelloWorld.class).
• java launched the JVM, loaded the class, and called main.
• The JIT (Just-In-Time) compiler converted hot bytecode into native machine code on the fly.

Two tools, two jobs.

1. Download IntelliJ IDEA Community (free) from jetbrains.com.
2. Open it, click New Project → Java.
3. Pick your JDK → name the project → finish.
4. Right-click src → New → Java Class → type HelloWorld.
5. Paste the snippet above → click the green ▶ button.

1. Install VS Code.
2. Add the Extension Pack for Java by Microsoft.
3. Open your folder, create HelloWorld.java, hit ▶.

Don't want to install? Use Codekilla's online Java compiler — write code in the browser, click Run, see output.

Real Java projects use Maven or Gradle — they fetch dependencies, compile, test, and package one-command:

bash
mvn package          # Maven
gradle build         # Gradle

You'll meet them later. For now, plain javac/java is fine.

• Running java HelloWorld.class — drop the .class. Use java HelloWorld.
• Filename mismatch — public class HelloWorld requires HelloWorld.java (case-sensitive).
• javac: command not found — JDK not on PATH. Restart terminal or add the JDK's bin/ to PATH.
• Mixing JDK versions — multiple JDKs installed → java --version may differ from javac --version. Use JAVA_HOME to pin one.
• Skipping -Xlint:all — you'll miss warnings the compiler could catch for free.

1. Install + compile — Set up JDK 21 on your machine and compile HelloWorld.java. Confirm java HelloWorld works. Hint: check java --version.
2. Different folder — Compile with javac -d out HelloWorld.java. Run with java -cp out HelloWorld. Hint: out/HelloWorld.class.
3. Read input — Modify the program to read your name with Scanner and greet you. Hint: import java.util.Scanner; new Scanner(System.in).nextLine();.
4. JAR it — Run jar cfe app.jar HelloWorld HelloWorld.class, then java -jar app.jar. Hint: cfe = create-file-entry-point.

💡 Think Like a Programmer: Master your toolchain first. Spending 30 minutes on javac, java, and IntelliJ saves you days of confusing build bugs later.