JEP 512: Compact Source Files and Instance Main Methods

Summary

Evolve the Java programming language so that beginners can write their first programs without needing to understand language features designed for large programs. Far from using a separate dialect of the language, beginners can write streamlined declarations for single-class programs and then seamlessly expand their programs to use more advanced features as their skills grow. Experienced developers can likewise enjoy writing small programs succinctly, without the need for constructs intended for programming in the large.

History

This feature was first proposed for preview by JEP 445 (JDK 21) and subsequently improved and refined by JEP 463 (JDK 22), JEP 477 (JDK 23), and JEP 495 (JDK 24). We here propose to finalize the feature in JDK 25, renaming simple source files to compact source files, with several minor improvements based upon experience and feedback:

• The new IO class for basic console I/O is now in the java.lang package rather than the java.io package. Thus it is implicitly imported by every source file.

• The static methods of the IO class are no longer implicitly imported into compact source files. Thus invocations of these methods must name the class, e.g., IO.println("Hello, world!"), unless the methods are explicitly imported.

• The implementation of the IO class is now based upon System.out and System.in rather than the java.io.Console class.

Goals

• Offer a smooth on-ramp to Java programming, so that instructors can introduce concepts in a gradual manner.

• Help students write simple programs in a concise manner, and grow their code gracefully as their skills grow.

• Reduce the ceremony of writing other kinds of small programs, such as scripts and command-line utilities.

• Do not introduce a separate dialect of the Java language.

• Do not introduce a separate toolchain. Small Java programs should be compiled and run with the same tools as large programs.

Motivation

The Java programming language excels for large, complex applications developed and maintained by large teams over many years. It has rich features for data hiding, reuse, access control, namespace management, and modularity which allow components to be cleanly composed while being developed and maintained independently. With these features, components can expose well-defined interfaces for their interaction with other components and, at the same time, hide internal implementation details so as to permit the independent evolution of each. Indeed, the object-oriented paradigm is, fundamentally, about plugging together components that interact through well-defined protocols while abstracting away implementation details. This composition of large components is called programming in the large.

The Java programming language is also, however, intended to be a first language. When programmers first start out they do not write large programs, in a team — they write small programs, alone. They have no need for encapsulation and namespaces, useful to separately evolve components written by different people. When teaching programming, instructors start with the basic programming in the small concepts of variables, control flow, and subroutines. At that stage there is no need for the programming-in-the-large concepts of classes, packages, and modules. Making the language more welcoming to newcomers is in the interest of Java veterans, but they, too, may enjoy writing small programs more concisely, without any programming-in-the-large constructs.

Consider the classic Hello, World! example that is often a beginner's first program:

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

There is too much clutter here — too much code, too many concepts, and too many constructs — for what the program does.

• The class declaration and the mandatory public access modifier are programming-in-the-large constructs. They are useful when encapsulating code with a well-defined interface to external components, but pointless in this little example.

• The String[] args parameter also exists to connect the code to an external component, in this case the operating system's shell. It is mysterious and unhelpful here, especially since it is not used in small programs such as HelloWorld.

• The static modifier is part of the language's class-and-object model. For the beginner, static is not just mysterious, but harmful: To add more methods or fields to this program, the beginner must either declare them all as static — thereby propagating an idiom which is neither common nor a good habit — or else confront the difference between static and instance members and learn how to instantiate an object.

• The beginner may be further puzzled by the mysterious incantation System.out.println, and wonder why a simple function call does not suffice. Even in first-week programs, the beginner may be forced to learn how to import basic utility classes for essential functionality, and wonder why they could not be provided automatically.

The new programmer encounters these concepts at the worst possible time, before they learn about variables and control flow, and when they cannot appreciate the utility of programming-in-the-large constructs for keeping a large program well organized. Instructors often offer the admonition, "don't worry about that, you'll understand it later." This is unsatisfying to they and their students alike, and leaves students with the enduring impression that the language is complicated.

The motivation for this work is not merely to reduce ceremony. We aim to help programmers that are new to the Java language, or to programming in general, learn the language in a manner that introduces concepts in the right order: Start with the fundamental programming-in-the-small concepts, such as doing simple textual I/O and processing arrays with for loops, and proceed to advanced programming-in-the-large concepts only when they are actually beneficial and can be more easily grasped.

The motivation for this work is, moreover, not only to help beginning programmers. We aim to help everyone who writes small programs, whether they be students, system administrators writing command-line utilities, or domain experts prototyping core algorithms that will eventually be used in enterprise-scale software systems.

We propose to make it easier to write small programs not by changing the structure of the Java language — code is still enclosed in methods, which are enclosed in classes, which are enclosed in packages, which are enclosed in modules — but by hiding such details until they are useful.

Description

First, we allow main methods to omit the infamous boilerplate of public static void main(String[] args), which simplifies the Hello, World! program to:

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

Second, we introduce a compact form of source file that lets developers get straight to the code, without a superfluous class declaration:

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

Third, we add a new class in the java.lang package that provides basic line-oriented I/O methods for beginners, thereby replacing the mysterious System.out.println with a simpler form:

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

Finally, for programs that go beyond Hello, World! and need, for example, basic data structures or file I/O, in compact source files we automatically import a range of standard APIs beyond the java.lang package.

These changes combine to offer an on-ramp, that is, a gradual incline that merges gracefully onto the highway. As beginners move on to larger programs, they need not discard what they learned in the early stages, but, rather, they see how it all fits within the larger picture. As experienced developers proceed from prototype to production, they can smoothly grow their code into components of larger programs.

Instance main methods

In order to write and run programs, beginners will learn about the entry point of a program. The current Java Language Specification (JLS) explains that the entry point of a Java program is a method called main (§12.1):

The Java Virtual Machine starts execution by invoking the method main of some specified class or interface, passing it a single argument which is an array of strings.

The JLS further states (§12.1.4):

The method main must be declared public, static, and void. It must specify a formal parameter whose declared type is array of String.

These requirements on the declaration of main are historical and unnecessary. We can streamline the entry point of a Java program in two ways: Allow main to be non-static, and drop the requirements for public and an array parameter. These changes allow us to write Hello, World! with no public modifier, no static modifier, and no String[] parameter, postponing the introduction of those constructs until they are needed:

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

Assuming this program is in the file HelloWorld.java, we can run it directly with the source-code launcher:

$ java HelloWorld.java

Alternatively, we can compile it explicitly and then run it:

$ javac HelloWorld.java
$ java HelloWorld

Either way, the launcher starts the Java Virtual Machine and then chooses and invokes a main method of the specified class:

1. If the class declares or inherits a main method with a String[] parameter then the launcher chooses that method.

   Otherwise, if the class declares or inherits a main method with no parameters then the launcher chooses that method.

   Otherwise, the launcher reports an error and terminates.

2. If the chosen method is static then the launcher invokes it.

   Otherwise, the chosen method is an instance main method. The class must have a non-private constructor with no parameters. The launcher invokes that constructor and then invokes the chosen main method of the resulting object. If there is no such constructor then the launcher reports an error and terminates.

Any main method that can be chosen and invoked under this protocol is known as a launchable main method. For example, the HelloWorld class has one launchable main method, namely void main().

Compact source files

In the Java language, every class resides in a package and every package resides in a module. Modules and packages provide namespacing and encapsulation for classes, but small programs that consist of just a few classes do not need these concepts. Accordingly, developers can omit package and module declarations, and their classes will reside in an unnamed package of an unnamed module.

Classes provide namespacing and encapsulation for fields and methods, but small programs that consist of just a few fields and methods do not need these concepts. We should not require beginners to understand these concepts before they are comfortable with the basic building blocks of variables, control flow, and subroutines. Accordingly, we can stop requiring class declarations for small programs that consist of a few fields and methods, just as we do not require package or module declarations.

Henceforth, if the Java compiler encounters a source file with fields and methods that are not enclosed in a class declaration, it will consider the source file to implicitly declare a class whose members are the unenclosed fields and methods. Such a source file is called a compact source file.

With this change, we can write Hello, World! as a compact source file:

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

The implicitly declared class of a compact source file

• Is a final top-level class in the unnamed package;
• Extends java.lang.Object and does not implement any interfaces;
• Has a default constructor with no parameters, and no other constructors;
• Has, as its members, the fields and methods in the compact source file; and
• Must have a launchable main method; if it does not, a compile-time error is reported.

Since the fields and methods declared in a compact source file are interpreted as members of the implicitly declared class, we can write Hello, World! by calling a method declared nearby:

String greeting() { return "Hello, World!"; }

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

or by accessing a field:

String greeting = "Hello, World!";

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

A compact source file declares a class implicitly, so the class does not have a name that can be used in source code. The Java compiler generates a class name when compiling a compact source file, but that name is implementation-specific and should not be relied upon in any source code — not even source code in the compact source file itself.

We can refer to the current instance of the class via this, either explicitly or, as above, implicitly, but we cannot instantiate the class with the new operator. This reflects an important tradeoff: If beginners have not yet learned object-oriented concepts such as classes, then writing code in a compact source file should not require a class declaration — which is what would give the class a name usable with new.

A compact source file is just another single-file source-code program. As shown earlier, we can run a compact source file directly with the source-code launcher, or we can compile it explicitly and then run it.

The javadoc tool can generate documentation from a compact source file, even though the implicitly declared class should not be referenced by other classes and thus cannot be used to define an API. Documenting the members of the implicitly declared class may be useful for beginners learning about javadoc, and for experienced developers prototyping code intended to be used in larger programs.

Interacting with the console

Beginners frequently write programs that interact with the console. Writing to the console ought to be straightforward, but traditionally it requires calling the inscrutable System.out.println method. To beginners, this is deeply mysterious: What is System? What is out?

Even worse is reading from the console which, again, ought to be a straightforward method call. Since writing to the console involves using System.out, it seems reasonable that reading would involve using System.in, but getting a String from System.in requires a lot of code, such as:

try {
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
    String line = reader.readLine();
    ...
} catch (IOException ioe) {
    ...
}

Experienced developers are used to this sort of boilerplate, but for beginners this code contains yet more mysteries, leading to a plethora of questions: What are try and catch, what is a BufferedReader, what is an InputStreamReader, and whatever is an IOException? There are other approaches, but none is significantly better, especially for the beginner.

To simplify the writing of interactive programs, we add a new class, java.lang.IO, which declares five static methods:

public static void print(Object obj);
public static void println(Object obj);
public static void println();
public static String readln(String prompt);
public static String readln();

A beginner can now write Hello, World! as:

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

They can then easily move on to the simplest of interactive programs:

void main() {
    String name = IO.readln("Please enter your name: ");
    IO.print("Pleased to meet you, ");
    IO.println(name);
}

Beginners do need to learn that the qualifier IO is required for these basic line-oriented I/O methods, but that not an undue pedagogical burden. They are likely to learn about such qualifiers soon anyway; e.g,. the qualifier Math for mathematical functions such as Math.sin(x).

Since the IO class resides in the java.lang package, it can be used without an import in any Java program. This applies to all programs, not just those in compact source files or those that declare instance main methods; for example:

class Hello {
    public static void main(String[] args) {
        String name = IO.readln("Please enter your name: ");
        IO.print("Pleased to meet you, ");
        IO.println(name);
    }
}

Automatic import of the java.base module

Many other classes in the Java Platform API are useful in small programs. They can be imported explicitly at the start of a compact source file:

import java.util.List;

void main() {
    var authors = List.of("James", "Bill", "Guy", "Alex", "Dan", "Gavin");
    for (var name : authors) {
        IO.println(name + ": " + name.length());
    }
}

Experienced developers will find this natural, though for convenience some might be inclined to use import-on-demand declarations (i.e., import java.util.*). For beginners, however, any form of import is another source of mystery, requiring an understanding of the package hierarchy of the Java API.

To further simplify the writing of small programs, we make all of the public top-level classes and interfaces of the packages exported by the java.base module available for use in compact source files, as if they were imported on demand. Popular classes and interfaces in commonly used packages such as java.io, java.math, and java.util are thus immediately usable. In the example above, import java.util.List can be removed since List will be imported automatically.

A companion JEP proposes a new kind of import declaration, import module M, which imports, on demand, all of the public top-level class and interfaces of the packages exported by module M. Every compact source file is considered to import the java.base module automatically, as if the declaration

import module java.base;

appears at the start of every compact source file.

Growing a program

A small program in a compact source file is focused on what the program does, omitting concepts and constructs it does not need. Even so, all members are interpreted as in an ordinary class. To evolve a compact source file into a ordinary source file, all we need to do is wrap its fields and methods in an explicit class declaration and add an import declaration. For example, this compact source file:

void main() {
    var authors = List.of("James", "Bill", "Guy", "Alex", "Dan", "Gavin");
    for (var name : authors) {
        IO.println(name + ": " + name.length());
    }
}

can be evolved into an ordinary source file that declares a single class:

import module java.base;

class NameLengths {
    void main() {
        var authors = List.of("James", "Bill", "Guy", "Alex", "Dan", "Gavin");
        for (var name : authors) {
            IO.println(name + ": " + name.length());
        }
    }
}

The main method does not change in any way. Thus turning a small program into a class that can serve as a component in a larger program is always straightforward.

Alternatives

Automatically import the console I/O methods

In earlier previews of this feature, we explored the possibility of compact source files automatically importing the static methods of the new IO class. Thus developers could write println(...) in compact source files instead of IO.println(...).

This had the pleasing effect of making the methods in IO appear to be built-in to the Java language, but it added a speed bump to the on-ramp: To evolve a compact source file into an ordinary source file, a beginner would have to add a static import declaration — another advanced concept. This runs contrary to our second goal, namely that that beginners should be able to grow their code gracefully. This design would also create a long-term burden of reviewing a likely endless stream of proposals to add additional methods to the IO class.

Automatically import fewer packages

Rather than automatically importing all 54 packages from the java.base module into compact source files, we could instead import only some of them. But, which ones?

Every reader will have suggestions for which packages to auto-import into every small program: java.io and java.util would be near-universal suggestions; java.util.stream and java.util.function would be common; and java.math, java.net, and java.time would each have supporters. For the JShell tool, we managed to find ten java.* packages which are broadly useful when experimenting with one-off Java code, but it is difficult to see which subset of java.* packages deserves to be permanently and automatically imported into every compact source file. The list would, moreover, change as the Java Platform evolves; e.g., java.util.stream and java.util.function were introduced only in Java 8. Developers would likely become reliant on IDEs to remind them of which automatic imports are in effect — an undesirable outcome.

Importing all of the packages exported by the java.base module is a consistent and reasonable choice for the classes implicitly declared by compact source files.

Allow top-level statements

An alternative design would allow statements to appear directly in a compact source file, removing the need to declare a main method. This design would interpret the entire compact source file as the body of an implicitly declared main method of an implicitly declared class.

Unfortunately, this design would be limiting because it would be impossible to declare methods in a compact source file. Such methods would be interpreted as appearing in the body of an invisible main method, but that would make them illegal since methods cannot be declared within methods. Compact source files could only represent linear programs that consist of one statement after another, without the power to abstract repeated computations into subroutines.

Furthermore, in this design, all variable declarations would be interpreted as local variables of the invisible main method. This would be limiting because local variables can only be accessed from lambda expressions when they are effectively final, which is an advanced concept. Writing lambda expressions in compact source files would be error-prone and confusing.

We believe that the desire to write statements directly in a compact source file, outside of a method body, is largely driven by the pain of writing public static void main(String[] args). Having made main methods easier to declare, we believe it best for compact source files to be composed of methods and fields rather than statements.

Extend JShell

JShell is an interactive tool for executing Java code immediately. It offers an incremental environment for programming, allowing beginners to experiment without a lot of ceremony.

An alternative design would extend JShell to achieve our goals. While an attractive idea in theory, it is less appealing in practice.

A JShell session is not a Java program, but, rather, a series of code snippets. Snippets are executed one at a time, but they are not independent: The execution of the current snippet depends on the results of executing all of the previous snippets, so values and declarations appear to evolve over time. At any one moment, there is a notion of the current state of the program under development, but there is no actual textual representation of the program. This works well for experimentation — the primary use case for JShell — but it is not a realistic foundation for helping beginners to write real programs.

At a more technical level, all declarations in a JShell session are interpreted as static members of an unspecified class, and all statements are executed in a context in which all previous declarations are in scope. If we were to interpret a compact source file as a series of code snippets then the file could only express classes whose methods and fields are static, in effect introducing a Java dialect. Evolving a compact source file into an ordinary source file would involve adding static modifiers to every method and field declaration, hindering the graceful evolution of small programs to larger ones.

Introduce a new dialect of the Java language

A radically different design would define a different dialect of the language for use in compact source files. This would allow all sorts of things to be removed in the pursuit of brevity. For example, we could drop the requirement that main methods explicitly be declared void. Unfortunately, this would prevent the graceful evolution of small programs to larger ones, which is a more important goal. We prefer an on-ramp to a cliff edge.