Java 25 makes it easier than ever to write small, focused programs without ceremony. Thanks to JEP 512: Compact Source Files and Instance Main Methods (finalized in Java 25), you can skip the enclosing class declaration entirely and still define multiple methods and fields in a single source file. This is perfect for scripts, learning exercises, and quick utilities.

Let me walk you through the best practices for organizing methods in these compact files.

1. The Basic Structure

A compact source file has an implicit top-level class. You just write your main method (instance-style, no static required) and add helper methods around it:

void main() {
    println("Welcome!");
    greet("Alice");
    println("Sum = " + add(3, 4));
}

void greet(String name) {
    println("Hello, " + name + "!");
}

int add(int a, int b) {
    return a + b;
}

No public class Foo { ... } wrapper is needed. The compiler generates it for you.

2. Put main First (or Make It Easy to Find)

For readability, keep the entry point at the top of the file so a reader immediately sees the program’s flow:

void main() {
    var user = askName();
    var total = computeTotal(10, 20, 30);
    printReport(user, total);
}

// --- helpers below ---

String askName() {
    return "Guest";
}

int computeTotal(int... values) {
    int sum = 0;
    for (int v : values) sum += v;
    return sum;
}

void printReport(String user, int total) {
    println("User : " + user);
    println("Total: " + total);
}

This mirrors how many scripting languages read: entry point first, details after.

3. Group Related Methods Together

When your file grows, cluster methods by responsibility and separate the groups with comment banners:

void main() {
    var nums = List.of(1, 2, 3, 4, 5);
    println("Sum      = " + sum(nums));
    println("Average  = " + average(nums));
    println("Uppercase: " + shout("hello"));
}

// ---------- Math helpers ----------

int sum(List<Integer> xs) {
    return xs.stream().mapToInt(Integer::intValue).sum();
}

double average(List<Integer> xs) {
    return xs.stream().mapToInt(Integer::intValue).average().orElse(0);
}

// ---------- String helpers ----------

String shout(String s) {
    return s.toUpperCase() + "!";
}

If a group becomes large, that is a strong signal it should be extracted into its own class or file.

4. Use Instance Fields for Shared State

Compact files support instance fields, so you don’t need to pass configuration through every method call:

final String appName = "DemoApp";
final int maxRetries = 3;

void main() {
    banner();
    run();
}

void banner() {
    println("=== " + appName + " ===");
}

void run() {
    for (int i = 1; i <= maxRetries; i++) {
        println("Attempt " + i);
    }
}

Prefer final fields to keep the file predictable and easy to reason about.

5. Leverage the Auto-Imported java.base and IO Methods

Java 25 auto-imports common utilities and gives you top-level print, println, and readln (from java.lang.IO). This keeps helper methods short:

void main() {
    var name = readln("Your name: ");
    println(greeting(name));
}

String greeting(String name) {
    return "Hello, " + (name.isBlank() ? "stranger" : name) + "!";
}

No import statements, no System.out.println — the file stays compact.

6. Keep Methods Small and Single-Purpose

Because there is no class boundary to hide behind, discipline matters more. Follow these guidelines:

7. Order Methods by “Newspaper Style”

Arrange methods so the reader moves from high-level to low-level, like a newspaper article:

void main() {           // headline: what the program does
    processOrder();
}

void processOrder() {   // section: main steps
    validate();
    charge();
    ship();
}

void validate() { /* ... */ }   // details
void charge()   { /* ... */ }
void ship()     { /* ... */ }

Readers rarely need to jump around — they simply scroll down for more detail.

8. When to Stop Using a Compact File

Compact source files shine for small, self-contained programs. Migrate to a regular class (or multiple classes) when you notice:

• Multiple unrelated groups of methods
• Need for multiple types (records, enums beyond simple helpers)
• Reuse from other files
• Unit tests targeting individual methods

You can promote a compact file by simply wrapping everything in public class Name { ... } and adding public static void main(String[] args) — the migration is mechanical.

Complete Example

Here is a compact file that puts all the tips together:

final String title = "Tip Calculator";

void main() {
    banner();
    double bill = 84.50;
    double tipPct = 0.18;

    double tip = tipAmount(bill, tipPct);
    double total = bill + tip;

    printLine("Bill",  bill);
    printLine("Tip",   tip);
    printLine("Total", total);
}

// ---------- domain logic ----------

double tipAmount(double bill, double pct) {
    return round2(bill * pct);
}

double round2(double v) {
    return Math.round(v * 100.0) / 100.0;
}

// ---------- output helpers ----------

void banner() {
    println("=== " + title + " ===");
}

void printLine(String label, double value) {
    println(String.format("%-6s: %8.2f", label, value));
}

Run it directly with:

java TipCalculator.java

Summary

• Put main first, helpers below.
• Group related methods and separate groups with comment banners.
• Use instance fields for shared, mostly-final state.
• Rely on auto-imports and top-level IO methods to stay concise.
• Follow newspaper order — general to specific.
• Graduate to a regular class once the file grows beyond a single concern.

Compact source files let you write real, multi-method programs with almost zero boilerplate — as long as you keep the file focused and well-organized.

Java 25 finalizes JEP 512: Compact Source Files and Instance Main Methods, which was previewed in earlier JDK releases. This feature makes Java far more approachable for beginners and reduces boilerplate for small programs and scripts.

What Changed?

Traditionally, every Java program required this ceremony:

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

With Java 25, the main method no longer needs to be:

• public
• static
• Declared with a String[] args parameter

The New Rules

The JVM launcher now looks for a main method in this order of preference:

1. static void main(String[] args) — traditional form
2. static void main() — no parameters
3. void main(String[] args) — instance method with args
4. void main() — instance method, no args simplest form

If an instance main method is found, the JVM will implicitly create an instance of the enclosing class using its no-argument constructor, then invoke main on it.

Example 1: The Simplest Instance main

public class Hello {
    void main() {
        System.out.println("Hello from an instance main!");
    }
}

That’s the entire program. No static, no String[] args, no public.

Run it with:

java Hello.java

Example 2: Instance main With Fields and Helper Methods

Because main is now an instance method, it can freely use instance fields and call other instance methods without needing static everywhere:

public class Greeter {
    private final String greeting = "Hello";

    void main() {
        greet("Java 25");
        greet("Developers");
    }

    void greet(String name) {
        System.out.println(greeting + ", " + name + "!");
    }
}

Notice that greet is also a plain instance method — no static modifier needed.

Example 3: Combined With Compact Source Files

Java 25 also allows you to omit the enclosing class entirely (Compact Source File):

void main() {
    System.out.println("No class declaration required!");
}

Save this as Demo.java and run:

java Demo.java

The compiler implicitly wraps the code in a synthetic class for you.

Example 4: Instance main With Arguments

If you still need command-line arguments, just declare them:

public class Echo {
    void main(String[] args) {
        for (String arg : args) {
            System.out.println("Argument: " + arg);
        }
    }
}

Requirements & Caveats

• The enclosing class must have an accessible no-argument constructor (the default one is fine if you don’t declare any constructor).
• The class cannot be abstract.
• If both a static and an instance main exist, the static one wins (per the resolution order above).
• The instance main method cannot be private. It must be at least package-private.
• To run a single source file directly (java Foo.java), you don’t need to compile first — the launcher handles it.

Why This Matters

• Lower barrier for beginners — no need to explain public, static, String[] args, or classes on day one.
• Cleaner scripts — small utilities and experiments become much more concise.
• Smoother learning curve — students can gradually introduce classes, static, and access modifiers as they progress, rather than all at once.

Quick Comparison

Instance main methods, combined with compact source files, make Java 25 one of the most beginner-friendly releases in the language’s history — while remaining fully backward compatible with every existing Java program.

Java 25 makes it official: you can now write and run a program without declaring a class, without public static void main(String[] args), and even without import statements for common APIs. This feature is called Compact Source Files and Instance Main Methods (JEP 512, finalized in Java 25).

Let’s walk through it step by step.

1. Prerequisites

• JDK 25 installed and available on your PATH
• Verify with:

java --version
javac --version

Both should report version 25.

2. Write the Program

Create a plain text file named Hello.java. That’s it — no class, no public static, no ceremony:

void main() {
    IO.println("Hello, Java 25!");
}

A few things worth noticing:

• There is no class declaration. The compiler wraps the code in an implicitly declared class for you.
• main is an instance method (not static) and takes no arguments (the String[] args parameter is optional now).
• IO.println(...) comes from the new java.io.IO class, which is auto-imported in compact source files — no System.out.println and no import needed.

3. Run It Directly with java

Since JDK 11, you can run a single-file source program directly. In Java 25, this works beautifully with the new compact form:

java Hello.java

Expected output:

Hello, Java 25!

No javac step is required. The launcher compiles the file in memory and runs it.

4. A Slightly Richer Example

You can still read input, do logic, and use any Java API — just without the boilerplate:

void main() {
    var name = IO.readln("What is your name? ");
    IO.println("Welcome, " + name + "!");

    for (int i = 1; i <= 3; i++) {
        IO.println("Count: " + i);
    }
}

Run it the same way:

java Hello.java

5. When You Outgrow It

Compact source files are meant for learning, scripting, and quick experiments. When your program grows, you can gradually add:

1. A String[] args parameter to main when you need CLI arguments.
2. Additional methods and fields directly in the file (they become members of the implicit class).
3. Finally, an explicit class declaration — at which point you have a regular Java source file.

The transition is smooth because the language rules are a strict superset of traditional Java.

6. Common Pitfalls

Summary

To run your first Java 25 program without creating a class:

1. Install JDK 25.
2. Create Hello.java containing just a void main() method.
3. Use IO.println(...) — no imports needed.
4. Run it with java Hello.java.

This is the shortest path from “I have JDK installed” to “my program is running” that Java has ever offered — perfect for beginners and for quick prototypes alike.