JEP 512 Explained: Compact Source Files and Simpler Java Programs

JEP 512 Explained: Compact Source Files and Simpler Java Programs explores how Java 25 reduces boilerplate using compact source files, instance #main() methods, automatic imports, and simpler console I/O—making Java easier to learn and write while preserving its core design principles.

• Introduction to JEP 512
• What are compact source files in Java 25?
• Instance #main() methods: A new way to start Java programs
• What do the compiler and JVM do behind the scenes?
• Console I/O made simpler with java.lang.IO
• Design choices behind JEP 512: Imports, growth, and trade-offs
• Rules, limitations, and when not to use JEP 512
• When should we use JEP 512?
• Conclusion

1. Introduction to JEP 512

For decades, Java has been known for its explicit and structured nature. While this has always been one of Java’s biggest strengths—especially for large, maintainable systems—it has also made the language feel verbose, particularly for very small programs.

Even the simplest Java program requires multiple mandatory elements: a class, a main method, and boilerplate syntax. This is not wrong, but it does create friction in certain situations.

In classic Java, every executable program must be wrapped inside a class and must declare a public static void main(String[] args) method.

A minimal program looks like this:

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

This structure enforces discipline and clarity, but it also means:

1. We write more syntax than logic

2. Beginners must understand multiple concepts upfront:

• Classes
• Access modifiers
• Static methods
• Method signatures

For experienced developers, this is routine. For newcomers, it often feels overwhelming.

1.1. The “Hello World” problem for beginners

The first program a learner writes is supposed to build confidence. However, in Java, the very first step already introduces several advanced ideas at once.

From a beginner’s perspective:

• Why do we need a class?
• What does static mean?
• Why is String[] args required?

None of these are needed to understand printing a line to the console, yet they are unavoidable. This is commonly referred to as the “Hello World problem”—the gap between what we want to do and what Java forces us to learn first.

1.2. Goal of JEP 512 — In simple words

JEP 512 addresses this exact pain point. The goal is not to replace Java’s object-oriented model or remove structure. Instead, it aims to:

1. Make small programs simpler
2. Reduce boilerplate for entry-level code
3. Improve Java’s usability for:

• Beginners
• Teaching
• Quick experiments
• Scripts and demos

In simple terms: JEP 512 allows us to write small Java programs without immediately worrying about classes and static main methods—while keeping Java’s core principles intact.

2. What are compact source files in Java 25?

Compact Source Files are a source-level enhancement introduced by JEP 512 to make Java simpler for small programs. The idea is straightforward: when a program is small and self-contained, Java should not force developers—especially beginners—to deal with concepts that are not immediately necessary.

In the Java language, every class resides in a package, and every package belongs to a module. Packages and modules exist to provide namespacing and encapsulation, which are critical for building large, maintainable systems. However, small programs that consist of just a few lines of code do not benefit much from these concepts. Recognizing this, Java has always allowed developers to omit package and module declarations, placing such code in the unnamed package of the unnamed module.

JEP 512 extends this same philosophy to class declarations.

Traditionally, Java required every field and method to be declared inside a class. While this is essential for object-oriented design, it introduces additional mental overhead for beginners. Before printing a single line, learners are forced to encounter ideas such as classes, access modifiers, and static methods—concepts that are not required to understand variables, control flow, or simple logic.

With Compact Source Files, we can now write small Java programs that focus purely on the fundamentals:

• Variables and expressions
• Conditional logic and loops
• Simple method-like behavior without a full class structure

A compact source file is simply a .java file that contains top-level code, without any explicit class declaration. For example:

void main() {
    int x = 5;
    int y = 10;
    System.out.println(x + y);
}
Output: 15

This is a valid Java program in Java 25. The code is direct, readable, and free from structural boilerplate that adds little value at this scale.

Compact source files are intentionally designed for small programs. They work best for:

• Learning and teaching Java
• Writing short examples or demos
• Quick experiments and throwaway utilities

They are not meant to replace traditional Java structure. As applications grow, proper use of classes, packages, and modules remains essential for maintainability and clarity.

In essence, compact source files recognize a simple reality: small programs do not need big abstractions on day one. They allow us to start simple and adopt Java’s full structural power only when the code truly needs it.

3. Instance #main() methods: A new way to start Java programs

With compact source files, Java removes the need for an explicit class declaration—but a Java program still needs a clear entry point. This is where instance #main() methods come in. JEP 512 introduces a simplified way to define the starting point of a program without relying on the traditional public static void main(String[] args) signature.

Instead of treating main as a static method tied to a class, Java now allows #main() to be written as an instance method. This keeps Java’s execution model intact while significantly reducing boilerplate for small programs.

In this new model, the program starts by implicitly creating an instance and then invoking its main() method. From a developer’s perspective, this feels more natural—especially for beginners—because it avoids concepts like static, access modifiers, and method signatures that are not immediately relevant when learning basic programming.

Key characteristics of instance #main() methods include:

• The method name must still be main
• The method can be non-static
• No command-line arguments are required
• The return type is void

This allows us to write programs that focus directly on logic, without structural noise.

Because the execution still begins at a method boundary, Java preserves:

• A well-defined program entry point
• Predictable stack frames
• Compatibility with debugging and tooling models

In other words, Java becomes simpler at the surface while remaining disciplined underneath.

Instance #main() methods are especially useful for:

• Learning and teaching Java fundamentals
• Writing short, self-contained programs
• Experimenting with language features

At the same time, they do not replace the traditional main method for real-world applications. As programs grow and require configuration, arguments, or a clearer structure, the classic public static void main(String[] args) remains the preferred approach.

Instance #main() methods strike a careful balance: they reduce ceremony without compromising Java’s execution principles.

4. What do the compiler and JVM do behind the scenes?

Even with the simplifications introduced by JEP 512, Java’s startup model remains firmly grounded in its existing rules. The Java launcher still starts the JVM and then looks for a launchable main method in the target class. What changes with JEP 512 is how that class comes into existence and how the launcher chooses which main method to invoke.

When a compact source file is compiled, the compiler implicitly declares a class on our behalf. This class is not visible in source code, has no usable name, and exists only so that the JVM and launcher can operate exactly as they always have.

From the launcher’s perspective, program startup follows a strict and well-defined protocol:

• If a class declares or inherits a main(String[] args) method, that method is chosen.
• Otherwise, if the class declares or inherits a no-argument main() method, that method is chosen.
• If neither exists, the launcher reports an error and terminates.

Once a main method is chosen, invocation depends on whether the method is static or instance-based. If the method is static, the launcher invokes it directly. If the method is an instance #main(), the launcher first creates an object and then invokes the method on that object. For this to work, the class must have a non-private, no-argument constructor; otherwise, startup fails.

Any main method that can be selected and invoked under these rules is known as a launchable main method.

4.1. The Implicit Class of a compact source file

A compact source file always declares a class implicitly. Although we never write the class declaration, the compiler guarantees that this implicit class has a predictable shape.

The implicitly declared class:

• Is a final top-level class in the unnamed package
• Extends java.lang.Object and implements no interfaces
• Has a default no-argument constructor and no other constructors
• Contains all fields and methods written in the compact source file
• Must define a launchable main method, or compilation fails

Because this class has no source-level name, the compiler generates a class name internally. That name is implementation-specific and must never be relied upon—not even inside the compact source file itself.

4.2. How instance #main() works at runtime

Consider this compact source file:

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

At runtime, the launcher:

• Starts the JVM
• Identifies the implicit class generated by the compiler
• Detects that the class has a no-argument instance main()
• Creates an instance using the default constructor
• Invokes #main() on that instance

This explains why instance #main() methods work without changing the JVM. The launcher already knows how to invoke instance methods—it simply follows existing rules.

4.3. Fields, methods, and this in compact source files

Because all fields and methods belong to the implicitly declared class, they behave exactly like instance members. This allows us to write code that feels simple and local, without introducing object-oriented syntax explicitly.

For example, we can call a nearby method:

String hello() {
    return "Java, HandsOn!";
}

void main() {
    System.out.println(hello());
}
Output: Java, HandsOn!

Or access a field directly:

String hello = "Java, HandsOn!";

void main() {
    System.out.println(hello);
}
Output: Java, HandsOn!

In all cases, this refers to the current instance of the implicit class. We may use this explicitly or implicitly, but we cannot instantiate the class using the new keyword because the class has no source-level name.

This reflects a deliberate design choice: if beginners have not yet learned about classes, object creation should not be required just to write simple programs.

4.4. Why this design matters

JEP 512 carefully balances simplicity with discipline:

• The JVM and launcher remain unchanged
• Startup rules stay predictable and debuggable
• Java avoids becoming a script-based language
• Beginners can focus on logic before structure

In short, compact source files and instance main() methods simplify how we write Java programs, while preserving exactly how Java programs are launched and executed.

5. Console I/O made simpler with java.lang.IO

Writing programs that interact with the console is often the very first step for beginners. Printing output or reading user input should be simple and intuitive. However, traditional Java has never made this easy. Even something as basic as printing a line requires calling System.out.println, which immediately raises questions for new learners: What is System? What is out? Why is printing not just a simple method call?

Reading from the console is even more intimidating. While it seems natural to expect input to come from System.in, obtaining a single line of text requires a significant amount of boilerplate code. Developers must deal with streams, readers, exception handling, and checked exceptions—concepts that are far removed from the goal of simply reading a string from the user.

Experienced developers are accustomed to this complexity, but for beginners, it introduces unnecessary friction and confusion at the very moment when they should be building confidence.

To address this, JEP 512 introduces a new class: java.lang.IO.

The IO class provides a small set of straightforward, line-oriented methods designed specifically for basic console interaction. Instead of navigating streams and exception handling, we can now express our intent directly.

The class defines the following static methods:

• IO.print(Object obj)
• IO.println(Object obj)
• IO.println()
• IO.readln(String prompt)
• IO.readln()

These methods cover the most common console I/O needs while keeping the API intentionally minimal.

With java.lang.IO, printing to the console looks exactly like what beginners expect it to look like:

void main() {
    IO.println("Java, HandsOn!");
}
Output: Java, HandsOn!

There is no indirection, no mysterious fields, and no streams involved. We are simply printing a line of text.

Reading user input is now equally straightforward. Instead of multiple classes and a try-catch block, we can read a line of input with a single method call:

void main() {
    String name = IO.readln("Please enter website name: ");
    IO.print("Great website to learn java, ");
    IO.println(name);
}
Output:
Please enter website name: https://javahandson.com/
Great website to learn java, https://javahandson.com/

This code is easy to read and easy to explain. The intent is immediately clear, even to someone who has just started learning Java.

5.1. Why the IO qualifier is a good thing

Beginners do need to learn that the methods are accessed through the IO qualifier, but this is not an unreasonable burden. They will encounter qualified method calls very early anyway—such as Math.sin(x) for mathematical operations.

In this sense, IO.print and IO.readln fit naturally into Java’s existing style, without introducing new concepts or special syntax.

5.2. Available everywhere, not just in compact source files

The IO class resides in the java.lang package, which means it is automatically available without any import statement. This applies to all Java programs, not just compact source files or those using instance main() methods.

For example, the same simplified I/O works perfectly in traditional Java code:

class Hello {
    public static void main(String[] args) {
        String name = IO.readln("Please enter website name: ");
		IO.print("Great website to learn java, ");
		IO.println(name);
    }
}

This makes java.lang.IO a general improvement to Java’s usability, not a feature limited to beginners or new syntax styles.

By introducing java.lang.IO, Java removes one of the most common early stumbling blocks:

• No streams for basic input/output
• No checked exceptions for simple console interaction
• Clear, readable code from the very first program

Together with compact source files and instance #main() methods, simplified console I/O helps Java feel approachable without being diluted.

6. Design choices behind JEP 512: Imports, growth, and trade-offs

6.1. Automatic imports in compact source files

One of the subtle but powerful usability improvements in JEP 512 is the introduction of automatic imports for compact source files. This change targets a common pain point for beginners: understanding and managing import statements before they even know which classes exist in the Java API.

In traditional Java programs, using a class such as List requires knowing both the class name and the package it belongs to. Even in a small program, this often forces beginners to write import declarations without really understanding what they are importing or why. While experienced developers find this natural, for newcomers, it becomes yet another early obstacle.

To reduce this friction, Java 25 treats every compact source file as if it automatically imports the java.base module.

Conceptually, it is as though the following declaration appears at the top of every compact source file:

import module java.base;

As a result, all public top-level classes and interfaces from the packages exported by java.base become available on demand, without requiring explicit import statements.

This has a very practical effect. For example, a compact source file can use List directly, without importing it:

void main() {
    var names = List.of("Suraj", "Shweta", "Iqbal", "Reshma", "Kartik", "Shrunalini");
    for (var name : names) {
        IO.println(name + ": " + name.length());
    }
}
Output:
Suraj: 5
Shweta: 6
Iqbal: 5
Reshma: 6
Kartik: 6
Shrunalini: 10

Note* IDE’s may not have the capability to run them directly, so use the below

C:\my-space\src> javac --enable-preview --source 25 Sum.java

C:\my-space\src> java Sum.java

Here, the List is available automatically because it belongs to the java.util package, which is exported by the java.base module. The same applies to commonly used classes in packages such as java.io, java.math, and java.time.

This design has a few important benefits:

• Beginners are not forced to understand the Java package hierarchy upfront
• Small programs stay focused on what they do, not on setup code
• Commonly used platform classes are immediately usable

It is important to note that this automatic import applies only to compact source files. Ordinary Java source files continue to require explicit import declarations, preserving clarity and control in larger applications.

By choosing to automatically import the entire java.base module—rather than a selective subset of packages—Java avoids inconsistency and future maintenance problems. As the platform evolves, new packages are added to java.base becomes available naturally, without changing the rules or surprising developers.

Overall, automatic imports complement compact source files by removing unnecessary ceremony at the very start of the Java learning journey, while still allowing programs to grow cleanly into fully structured Java code when needed.

6.2. Growing a compact program into a regular Java program

Compact source files are intentionally designed as an on-ramp, not a dead end. While they remove unnecessary ceremony for small programs, they do not introduce a separate programming model. Everything written in a compact source file is interpreted as belonging to an ordinary Java class—just one that is declared implicitly.

This design makes it easy to evolve a small program into a regular Java program when the need arises. As requirements grow, we do not have to rewrite logic or rethink structure. The only change required is to make the implicit structure explicit.

Consider a compact source file:

void main() {
    var names = List.of("Suraj", "Shweta", "Iqbal", "Reshma", "Kartik", "Shrunalini");
    for (var name : names) {
        IO.println(name + ": " + name.length());
    }
}

To turn this into an ordinary Java source file, we simply wrap the existing code inside a class declaration and add any required imports:

import module java.base;

class NameLengths {
    void main() {
        var names = List.of("Suraj", "Shweta", "Iqbal", "Reshma", "Kartik", "Shrunalini");
		for (var name : names) {
			IO.println(name + ": " + name.length());
		}
    }
}

Notably, the #main() method itself does not change. There is no need to add static, introduce command-line arguments, or restructure the logic. This ensures that beginners can grow their programs gradually, learning new concepts only when the program genuinely needs them.

This growth model has important advantages:

• Compact source files remain real Java programs, not a simplified dialect
• Code written for learning transitions naturally into production-style code
• There is no “rewrite cliff” when structure becomes necessary

By allowing a smooth transition from compact to ordinary source files, Java preserves continuity in both learning and development.

6.3. Why Java chose this design (and rejected alternatives)

The design of JEP 512 reflects a series of deliberate trade-offs. Rather than pursuing maximum brevity at any cost, Java prioritizes graceful growth and long-term consistency. Several alternative designs were considered and rejected for this reason.

One possibility was to allow top-level statements, treating the entire source file as the body of an implicit main method. While this would remove even the need to declare main(), it would severely limit expressiveness. Methods could not be declared, repeated logic could not be abstracted, and all variables would become local to a hidden method—creating problems with lambdas and effectively final variables. Java deliberately avoids this restriction.

Another idea was to automatically import the IO methods statically, allowing calls like println() instead of IO.println(). Although appealing at first glance, this approach would make it harder to evolve a compact source file into a regular one, because a static import would suddenly be required. It would also create long-term pressure to treat IO as a growing set of built-in language features.

The option of importing only a subset of packages automatically was also rejected. Choosing which packages deserve special treatment would be arbitrary, fragile, and difficult to evolve as the platform grows. Importing the entire java.base module provides a consistent and future-proof rule.

Extending JShell was considered as well. While JShell is excellent for experimentation, it is not a representation of a complete Java program. Its incremental, stateful execution model does not translate well into teaching how to write real, standalone applications that can be compiled, versioned, and maintained.

Finally, Java deliberately avoided introducing a new dialect for compact source files. A dialect might allow even more brevity, but it would create a sharp divide between “learning Java” and “real Java”. JEP 512 instead ensures that compact source files are simply another way to write standard Java code.

In summary, Java chose a design that favors:

• Familiar execution semantics
• A smooth learning curve
• Easy evolution from small programs to larger ones
• Long-term maintainability over short-term convenience

This philosophy explains why JEP 512 simplifies Java’s surface syntax without compromising its core principles.

7. Rules, limitations, and when not to use JEP 512

JEP 512 makes Java easier to approach, but it does not remove the language’s fundamental structure or discipline. Compact source files and instance #main() methods are designed with clear boundaries, ensuring that simplicity does not turn into ambiguity or misuse. Understanding these boundaries helps us use the feature confidently and appropriately.

At the most basic level, every compact source file must define a launchable main method. This can either be a traditional main(String[] args) or a no-argument #main(). Java does not allow free-floating statements at the top level; execution must still begin inside a method. If a compact source file does not contain a valid main method, the compiler reports an error.

Although we do not declare a class explicitly, a compact source file always corresponds to an implicit class. All fields and methods written in the file become members of that class. We can refer to the current instance using this, but the class has no source-level name and therefore cannot be instantiated using the new operator. This limitation is intentional and reinforces the idea that compact source files are about using Java, not defining object models.

JEP 512 is introduced as a preview feature in Java 25, which means it is not enabled by default. Compilation and execution require preview flags, and the feature may still evolve in future Java releases. In practice, tooling support may also be incomplete. IDEs and build tools may continue to assume traditional Java structure unless explicitly configured, which can lead to warnings or confusing suggestions for beginners.

From a design perspective, compact source files are deliberately not suitable for large or long-lived applications. As a program grows, the absence of explicit class names, package organization, and clear API boundaries becomes a disadvantage rather than a convenience. Java’s traditional structure exists for a reason, and JEP 512 does not attempt to replace it.

Where JEP 512 works well is in scenarios where structure would otherwise overshadow intent. It is well-suited for learning, teaching, quick experiments, short examples, and small utilities. In these cases, reducing boilerplate allows readers and learners to focus on logic, control flow, and problem-solving rather than syntax.

On the other hand, we should avoid using JEP 512 when building production systems, libraries, frameworks, or shared codebases. In such environments, explicit classes and packages provide clarity, maintainability, and consistency that compact syntax intentionally omits.

The right way to think about JEP 512 is not as a replacement for traditional Java, but as a starting point. It gives us a simpler entry into the language while preserving a clear and natural path toward fully structured Java code as programs grow in size and responsibility.

8. When should we use JEP 512?

JEP 512 is meant to make Java easier to start with, not to change how Java works at its core. It removes extra syntax that is not needed for very small programs, while keeping Java’s usual rules and behavior the same behind the scenes.

We should use JEP 512 when we want to focus on learning or explaining logic, not on structure. It works very well for beginners, tutorials, blog examples, classroom teaching, and quick experiments. In these cases, compact source files help us write clean and readable code without worrying about classes, static, or long method signatures.

JEP 512 is also useful when we want to show a single idea clearly. Short examples become easier to read and easier to understand, which is especially helpful when introducing Java to new developers.

At the same time, JEP 512 is not meant for everything. When a program starts growing, or when we are building real applications, libraries, or shared codebases, traditional Java structure becomes important. Classes, packages, and clear entry points help keep large codebases organized and maintainable.

The right way to think about JEP 512 is this: it helps us start small and grow naturally. We can begin with compact source files and later move to regular Java code without rewriting our logic.

In short, JEP 512 makes Java more friendly at the beginning, while still encouraging good structure as programs become larger. Used in the right places, it makes Java simpler without making it weaker.

9. Conclusion

JEP 512 Explained: Compact Source Files and Simpler Java Programs shows a clear shift in how Java approaches beginners and small programs. Instead of forcing every program to start with classes, static, and boilerplate, Java now allows us to begin with simpler code while still following the same execution rules behind the scenes.

Compact source files, instance main() methods, simplified console I/O, and automatic access to core platform classes all work together to reduce friction at the starting point. These features do not turn Java into a scripting language, nor do they introduce a separate dialect. They simply remove concepts that are not immediately necessary when writing small programs.

At the same time, JEP 512 carefully preserves Java’s biggest strength: its ability to grow. Code written using compact source files can easily evolve into regular Java programs by adding explicit classes and structure when needed. There is no rewrite, no loss of meaning, and no change in execution behavior.

In the end, JEP 512 is about balance. It makes Java easier to start, easier to teach, and easier to explain—without weakening the language or breaking its core principles. Used in the right places, it helps Java feel more approachable while remaining just as powerful and reliable as ever.