Learning Note: Java 21 Feature – Unnamed Classes and Instance Main Methods (Preview)

In my ongoing study of Java 21, guided by the “Java 21 – Exploring the Latest Innovations for 2024” course, I’ve reached the section on one of the most beginner-friendly enhancements: unnamed classes and instance main methods. This feature is currently in preview, but it’s a significant step toward making Java more accessible, especially for newcomers.

Motivation: Reducing Verbosity for Beginners

Traditionally, writing a simple Java program (like “Hello, World”) requires a lot of boilerplate code: defining a class, writing public static void main(String[] args), and then adding the print statement. For beginners, this can be overwhelming and unnecessarily complex.

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

Java 21 aims to minimize this verbosity. The new feature allows you to write much simpler entry-point code, making the language more approachable for students and those new to Java.

What’s New: Minimal Entry Point

With this preview feature, you can now write a Java program like this:

void main() {
    System.out.println("Hello, Java 21!");
}

• No need for a class declaration.
• No need for public static.
• The focus is on the logic, not the structure.

Setting Up and Running an Unnamed Class in VSCode

If you’re using Visual Studio Code (VSCode), the process is straightforward:

1. Create a New Java Project:

   • Open the Command Palette (Ctrl+Shift+P or Cmd+Shift+P).
   • Type and select “Create Java Project.”
   • Choose “No build tools” for simplicity.
   • Select a folder and provide a project name.

2. Write Your Minimal Java Program:

   • In the src folder, create a file (e.g., App.java).
   • Write your code using the new minimal style.

3. Compile and Run with Preview Features:

   • Open a terminal and navigate to your src directory.
   • Compile with preview enabled:

     javac --release 21 --enable-preview App.java

   • Run with preview enabled:

     java --enable-preview App

   • You should see your output (e.g., “Hello, Java 21!”).

Note: If you try to run this code without enabling preview features, it will not compile or run, as this feature is not yet standard.

Key Takeaways

• Purpose: Make Java simpler for beginners by reducing required boilerplate for entry-point programs.
• How: Allows omitting the class declaration and public static for main methods.
• Status: Feature is under preview in Java 21, so you must enable preview features during compilation and execution.
• Benefit: Easier for new learners and quick prototyping.

This feature, although small, reflects Java’s ongoing effort to modernize and lower the entry barrier for new programmers. I find it a welcome change, especially as I continue to explore more of Java 21’s innovations in my learning journey.