Unnamed Classes and Instance Main Methods in Java

Last updated: January 8, 2024

Written by: Alexandru Borza

Reviewed by: Saajan Nagendra

Core Java

>= Java 21

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1. Introduction

Java 21 is here, and among the new features, we can see how Java is becoming increasingly accessible for beginners with the unnamed classes and instance main methods. The introductions of these are crucial steps forward in making Java a more beginner-friendly programming language.

In this tutorial, we’ll explore these new features and understand how they make the learning curve smoother for students.

2. Writing a Basic Java Program

Traditionally, for beginners, writing their first Java program was a little more complicated than in other programming languages. A basic Java program required the declaration of a public class. This class encloses a public static void main(String[] args) method, serving as the entry point of the program.

All of this is just to write a “Hello world” in the console:

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

Java 21 greatly simplifies the way we can write a simple program:

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

We’ll go into more detail about how we achieved this syntax simplification using the new features.

3. Instance Main Methods

The introduction of instance main() methods allows developers to utilize a more dynamic approach to initializing their applications.

3.1. Understanding Instance Main Methods

This has changed the way Java programs declare their entry points. In fact, Java earlier required the presence of a static main() method with a String[] parameter in the public class, as we saw in the previous section.

This new protocol is more lenient. It enables the use of main() methods with varied access levels: public, protected, or default (package).

Also, it doesn’t require the methods to be static or to have a String[] parameter:

class HelloWorld {
    void main() {
        System.out.println("Hello, World!");
    }
}

3.2. Choosing a Launch Protocol

The refined launch protocol chooses automatically a starting point for our program, taking into account both availability and access level.

Instance main() methods should always have a non-private access level. Moreover, the launch protocol follows a specific order to decide which method to use:

a static void main(String[] args) method declared in the launched class
a static void main() method declared in the launched class
a void main(String[] args) instance method declared in the launched class or inherited from a superclass
a void main() instance method

When a class declares an instance main() method and inherits a standard static main() method, the system invokes the instance main() method. In such cases, the JVM issues a warning at runtime.

For example, let’s suppose we have a superclass HelloWorldSuper, that implements a long-established main() method:

public class HelloWorldSuper {
    public static void main(String[] args) {
        System.out.println("Hello from the superclass");
    }
}

This superclass is extended by a HelloWorldChild class:

public class HelloWorldChild extends HelloWorldSuper {
    void main() {
        System.out.println("Hello, World!");
    }
}

Let’s compile the superclass and run the child using the — source 21 and –enable-preview flags:

javac --source 21 --enable-preview HelloWorldSuper.java
java --source 21 --enable-preview HelloWorldChild

We’ll get the following output in the console:

WARNING: "void HelloWorldChild.main()" chosen over "public static void HelloWorldSuper.main(java.lang.String[])"
Hello, World!

We can see how the JVM warns us that we have two possible entry points in our program.

4. Unnamed Classes

Unnamed classes are a significant feature designed to simplify the learning curve for beginners. It allows methods, fields, and classes to exist without explicit class declarations.

Typically, in Java, every class exists within a package and every package within a module. Unnamed classes, however, exist in the unnamed package and unnamed module. They are final and can only extend the Object class without implementing any interface.

Given all this, we can declare the main() method without declaring the class explicitly in the code:

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

Using these two new features, we managed to turn the program into a very simple one that can be much easier to understand by any person who starts programming in Java.

An unnamed class is almost exactly like an explicitly declared class. Other methods or variables are interpreted as members of the unnamed class, so we can add them to our class:

private String getMessage() {
    return "Hello, World!";
}
void main() {
    System.out.println(getMessage());
}

Despite their simplicity and flexibility, unnamed classes have inherent limitations.

Direct constructions or references by name are impossible, and they don’t define any API accessible from other classes. This inaccessibility also causes the Javadoc tool to falter when generating API documentation for such classes. However, future Java releases may adjust and enhance these behaviors.

5. Conclusion

In this article, we learned that Java 21, with the introduction of unnamed classes and instance main() methods, has made significant progress in enhancing user experience, especially for those at the beginning of their programming journey.

By simplifying the structural aspects of programming, these features allow novices to focus on logical thinking and problem-solving more quickly.

The code backing this article is available on GitHub. Once you're logged in as a Baeldung Pro Member, start learning and coding on the project.

Of course, Azure Container Apps has really solid support for our ecosystem, from a number of build options, managed Java components, native metrics, dynamic logger, and quite a bit more.

To learn more about Java features on Azure Container Apps, visit the documentation page.

You can also ask questions and leave feedback on the Azure Container Apps GitHub page.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.

Orkes is the leading workflow orchestration platform built to enable teams to transform the way they develop, connect, and deploy applications, microservices, AI agents, and more.

Try a 14-Day Free Trial of Orkes Conductor today.