Core Java

Java 21: Unnamed Class & Instance Main

Java 21 introduces unnamed classes and instance main methods, enhancing simplicity and flexibility. Unnamed classes allow for quick, on-the-fly class definitions without a formal name, streamlining test code, or one-off implementations. Instance main methods enable executing code within an instance’s context directly, facilitating more intuitive object-oriented programming practices. Let us delve into understanding Java 21 unnamed classes and instance main methods.

1. Exploring Unnamed Classes in Java 21

Java 21 brings a fascinating feature to the Java programming language: Unnamed Classes. This innovative addition allows developers to create classes on the fly without needing to give them explicit names. Unnamed classes are particularly useful for creating quick implementations of interfaces or abstract classes for single-use purposes, such as event handling or temporary data processing.

1.1 Why Use Unnamed Classes?

Unnamed classes simplify the coding process by reducing the boilerplate code required for small, one-off implementations. They make the codebase cleaner and more readable, especially when the implementation is straightforward or only relevant within a narrow scope. Additionally, unnamed classes enhance the encapsulation by keeping temporary implementations closely tied to where they are used.

1.2 Example of Unnamed Classes

Consider a scenario where we want to run a simple thread without creating a named implementation of the Runnable interface:

public class UnnamedClassDemo {
    public static void main(String[] args) {
        new Object() {
            void startThread() {
                new Thread(new Runnable() {
                    public void run() {
                        System.out.println("Thread running inside an unnamed class.");

In this example, we create an unnamed class with a method startThread that initializes a new Thread object using an unnamed implementation of the Runnable interface. This approach allows us to keep our code concise and focused on functionality rather than class structure.

1.3 Core Rules for Unnamed Classes

  • Contextual Declaration: Unnamed classes must be declared and instantiated within a specific context, typically where they are needed. They cannot be declared separately and then instantiated.
  • Single Instance: When you declare an unnamed class, it is instantiated immediately. You cannot create multiple instances of an unnamed class as you would with a named class.
  • Extending Classes or Implementing Interfaces: An unnamed class must either extend a class or implement an interface, but it cannot do both simultaneously.
  • Method Overrides: Unnamed classes are often used to provide immediate implementations of abstract methods from a superclass or interface methods.
  • No Constructor Definition: Since unnamed classes do not have names, you cannot define a constructor as you would in a named class. Instead, instance initialization blocks or object initializers are used if initialization is necessary.
  • Scope Limited to Enclosing Block: The scope of an unnamed class is limited to the block in which it is declared, making it inaccessible outside of this context.
  • Arguments Passing: You can pass arguments to the superclass constructor or to the implemented interface methods directly at the time of unnamed class creation.

1.4 Best Practices for Using Unnamed Classes

While unnamed classes offer a streamlined way to implement quick solutions, their use should be limited to situations where a full class declaration is unnecessary or overly cumbersome. They are best used in scenarios requiring minimal implementation efforts, such as creating listeners for GUI events or small runnable objects. Understanding when and where to use unnamed classes is key to maintaining clean, readable, and maintainable code.

2. Understanding Instance Main Methods in Java 21

Java 21 introduces a new feature that allows the main method to be an instance method rather than a static one. This significant change enables developers to write more object-oriented code by allowing direct access to instance variables and methods from the main method, facilitating a smoother transition to object-oriented principles right from the program’s entry point.

2.1 The Significance of Instance Main Methods

Traditionally, the main method in Java applications is static, meaning it can be called without creating an instance of the class. While this approach works well for small programs, it often leads to a less object-oriented design in larger applications. Instance’s main methods address this issue by promoting a design where objects and their interactions are at the forefront, enabling a more natural and intuitive programming model.

2.2 Example of Instance Main Methods

Let’s explore how instance main methods can be used in Java 21:

public class InstanceMainDemo {
    private String message = "Hello from the instance main method!";

    public void main(String[] args) {

    public void additionalMethod() {
        System.out.println("This is an additional method called from the main method.");

    public static void main(String[] args) {
        new InstanceMainDemo().main(args);

In this example, the class InstanceMainDemo includes an instance main method and an additional instance method called additionalMethod. The static main method creates an instance of InstanceMainDemo and calls the instance main method on it, demonstrating how instance methods can be seamlessly integrated into the application’s entry point.

2.3 Advantages of Instance Main Methods in Java

  • Enhanced Object-Oriented Programming: Instance main methods align more closely with object-oriented programming principles by allowing direct access to an object’s state and behaviors. This approach encourages developers to think in terms of objects from the outset, promoting a design that is more naturally object-oriented.
  • Simplified Code Structure: By eliminating the need for static context in the application’s entry point, instance main methods simplify the code structure. Developers can initiate and manipulate objects directly, leading to clearer, more intuitive code that is easier to read and maintain.
  • Improved Encapsulation: Instance main methods improve encapsulation by allowing main methods to access private fields and methods directly. This encapsulation supports better data protection and hides implementation details from the user, adhering to a core tenet of object-oriented design.
  • Facilitated Testing and Modularity: The use of instance main methods facilitates testing and enhances modularity. Developers can more easily instantiate classes and test their methods without relying on static contexts, allowing for more flexible and comprehensive testing scenarios.

3. Conclusion

In conclusion, the introduction of instance main methods and unnamed classes in Java significantly enhances the language’s flexibility and adherence to object-oriented programming principles. Instance main methods shift the paradigm towards a more object-centric approach, enabling developers to directly access and manipulate object states and behaviors from the program’s entry point. This shift not only simplifies the code structure but also promotes better encapsulation and modularity, which are crucial for building robust and maintainable applications. On the other hand, unnamed classes offer a streamlined way to implement interfaces or extend classes on the fly, reducing boilerplate code and improving code readability for temporary or single-use implementations. Together, these features offer Java developers powerful tools to write cleaner, more efficient code while adhering to the principles of object-oriented design.


An experience full-stack engineer well versed with Core Java, Spring/Springboot, MVC, Security, AOP, Frontend (Angular & React), and cloud technologies (such as AWS, GCP, Jenkins, Docker, K8).
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