Compact Source Files and Instance Main Method in Java 25

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The Java programming language has always been known for its reliability, structure, and scalability. It is widely used for building large and complex enterprise applications that can be maintained over many years by large teams. However, this same structure that makes Java powerful can also make it intimidating for beginners.

In Java 25, the introduction of compact source files and instance main methods aims to simplify the learning experience. These features make it easier for students and new developers to write their first Java programs without being overwhelmed by the full complexity of the language.

This article explains what these new features are, why they were introduced, and how they make Java programming more beginner-friendly—without changing the essence of the language.

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Why Java Needed Simplification

Java was designed for programming-in-the-large—a style that focuses on building large, modular applications with features like encapsulation, reuse, and namespace management. These concepts are essential for professional software development, but they are unnecessary and confusing for beginners who are just learning how to think logically and write basic code.

When someone starts learning to program, their focus should be on programming-in-the-small—understanding how to work with variables, control flow, loops, and methods. They do not need to worry about classes, access modifiers, or modules. Unfortunately, traditional Java forces these constructs onto beginners from the very first “Hello, World!” example.

For example, to simply print a message, you had to write:

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

For someone who has never written code before, this raises several confusing questions: What is a class? Why is public needed? What does static mean? What is String[] args?

None of these concepts are relevant at the beginner stage. Recognizing this, Java 25 introduces two important features to make early learning simpler and more intuitive: compact source files and instance main methods.

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What Are Compact Source Files?

A compact source file allows you to write Java code without explicitly declaring a class. You can directly write methods and variables at the top level of your source file. When the Java compiler encounters such a file, it automatically creates an implicit class that contains these methods and fields.

This means you can now write:

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

and it will run exactly as expected.

How It Works Internally

Behind the scenes, the Java compiler performs the following steps:

1. It creates an implicit final class that contains the unenclosed methods and fields from your file.
2. This implicit class automatically extends java.lang.Object but does not implement any interfaces.
3. It has a default, no-argument constructor and no additional constructors.
4. It must contain a launchable main method, either a static or instance version, for the program to run.
5. The class resides in the unnamed package and the unnamed module, which means you don’t need to declare them manually.

The idea is to allow students to start writing Java programs without the burden of understanding class structures too early.

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Why Compact Source Files Matter

Compact source files remove the boilerplate that clutters simple programs. They allow students to focus on writing logic rather than memorizing syntax. Once they become comfortable with basic programming concepts, they can naturally progress to learning classes, objects, and packages.

This approach also benefits experienced developers who occasionally need to write quick scripts or small utilities in Java. Instead of creating an entire project structure, they can write a self-contained file and execute it directly from the command line.

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Writing Compact Source Files

Here are a few examples to demonstrate what compact source files look like in practice.

Example 1: Simple Program

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

Example 2: Using a Method

String greeting() {
    return "Welcome to Java 25!";
}

void main() {
    System.out.println(greeting());
}

Example 3: Using a Field

String message = "Java 25 is easier than ever.";

void main() {
    System.out.println(message);
}

You can run these programs directly from the terminal using:

java HelloWorld.java

The java launcher automatically compiles and executes the compact source file in one step. There is no need to compile it first with javac.

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Instance Main Methods

Traditionally, the entry point for every Java application has been a static method:

public static void main(String[] args)

This requirement made beginners question the meaning of static and String[] args before they even learned what a variable was.

With Java 25, this restriction is relaxed. You can now use an instance main method, which does not need to be static, public, or even have parameters. This means you can write a simple main method like:

void main() {
    System.out.println("Learning Java 25 is simple!");
}

If you prefer, you can still use a parameterized version:

void main(String[] args) {
    System.out.println("Hello, " + args[0]);
}

When running your compact source file, Java automatically creates an instance of the implicit class and invokes its main method. You no longer need to manually manage static contexts or class instantiation.

This improvement allows beginners to start writing Java code that feels more natural and consistent with how methods work in general.

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Simplified Console I/O with java.lang.IO

To complement these changes, Java 25 introduces a new utility class called java.lang.IO. It provides a few easy-to-use methods for basic console input and output, serving as a simpler alternative to System.out.println.

Here’s an example:

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

Common Methods in the IO Class

• IO.print(Object) – prints a value without a new line.
• IO.println(Object) – prints a value followed by a new line.
• IO.readln(String prompt) – prints a prompt and reads a line of user input.

Although this feature is optional, it helps simplify the code further and makes it easier for beginners to perform basic input and output operations without learning about streams and buffers right away.

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Automatic Import of the java.base Module

Compact source files automatically import all public classes and interfaces from the java.base module. This module includes frequently used classes such as List, Map, Stream, Collectors, and Function.

As a result, you can use these classes directly without writing any import statements.

For example:

void main() {
    String[] fruits = { "apple", "banana", "cherry" };
    var map = Stream.of(fruits)
        .collect(Collectors.toMap(
            s -> s.substring(0, 1).toUpperCase(),
            Function.identity()
        ));
    map.forEach((k, v) -> IO.println(k + " = " + v));
}

This automatic import makes compact source files cleaner and ensures that common Java classes are readily available to beginners.

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Running Compact Source Files

There are two simple ways to run a compact source file:

1. Direct Execution

Run the file directly from the command line:

java HelloWorld.java

Java compiles and runs the file in one step.

2. Compile and Run Separately

You can also compile and run it the traditional way:

javac HelloWorld.java
java HelloWorld

In both cases, the Java runtime looks for a launchable main method in the file. It checks the following in order:

1. A main(String[] args) method if it exists.
2. Otherwise, a main() method with no parameters.
3. If neither is found, it reports an error.

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Gradually Growing a Program

The best part about compact source files is that they are still fully compatible with standard Java syntax. You can start with a compact file and later expand it into a traditional class as your program grows.

For instance, this compact version:

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

can later evolve into:

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

This progression makes it easier for educators to teach and for learners to build confidence before diving into advanced topics like object-oriented programming, packages, and modules.

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Conclusion

Java 25 marks an important step in making the language more accessible to beginners while keeping its professional capabilities intact. The introduction of compact source files and instance main methods reduces unnecessary complexity, allowing new programmers to focus on learning logic, syntax, and problem-solving before exploring advanced concepts.

These enhancements also benefit experienced developers who want to write quick scripts or prototype ideas without creating full project structures.

By simplifying the entry point into the language, Java 25 ensures that learning Java is not only easier but also more enjoyable. It bridges the gap between beginner-friendly languages like Python and Java’s robust enterprise capabilities—making it a better fit for both education and rapid development.

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