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. This is a

preview<br /> language feature

.

History

This feature was first proposed for preview by JEP 445 (JDK 21) and subsequently improved and refined by JEP 463 (JDK 22) and JEP 477 (JDK 23). We here propose to preview it for a fourth time, with new terminology and a revised title but otherwise unchanged, in order to gain additional experience and feedback.

Goals

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

• Help students write basic 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 over many years by large teams. 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 while hiding internal implementation details so as to permit the independent evolution of each. Indeed, the object-oriented paradigm itself is designed for plugging together pieces that interact through well-defined protocols and abstract 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!|https://en.wikipedia.org/wiki/%22Hello,_World!%22_program]

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 a code unit with a well-defined interface to external components, but pointless in this little example.

• The String[] args parameter also exists to interface the code with 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 the heart of an enterprise-scale software system.

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 simple 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, in simple source files we automatically import a few useful methods for console input and output, thereby avoiding the mysterious System.out.println:

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

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

This is a preview language feature, disabled by default

To try the examples below in JDK 24 you must enable preview features:

• Compile the program with javac --release 24 --enable-preview Main.java and run it with java --enable-preview Main; or,

• When using the source code launcher, run the program with java --enable-preview Main.java; or,

• When using <code class="prettyprint" data-shared-secret="1734795671586-0.12203506911106787">jshell</code>, start it with jshell --enable-preview.

Instance main methods

In order to write and run programs, beginners will learn about the entry point of a program. The Java Language Specification (JLS) explains that

the entry<br /> point of a Java<br /> program

is a method called main:

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:

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

The launcher compiles HelloWorld.java in memory, then finds and invokes a main method:

• If the HelloWorld class contains a main method with a String[] parameter then the launcher chooses that method.

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

  Otherwise, the launcher reports an error and terminates.

• 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 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 invoked under this protocol is known as a launchable main method. For example, the HelloWorld class has one launchable main method, namely void main().

Simple 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 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. 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 simple source file.

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

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

The implicitly declared class of a simple 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 simple 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 simple 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);
}

Since a simple source file declares a class implicitly, the class does not have a name that can be used in code. 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 new. This reflects an important tradeoff: If beginners have not yet learned object-oriented concepts such as classes, then writing code in a simple source file should not require a class declaration — which is what would give the class a name usable with new.

Assuming our simple source file is called HelloWorld.java, we can run it directly with the source-code launcher:

$ java HelloWorld.java

The launcher compiles HelloWorld.java in memory, treating its fields and methods as if they are members of a class called HelloWorld, deriving the name of the class from the name of the file. The launcher then finds and invokes a main method, as described earlier.

If a simple source file has a launchable main method that is an instance method, then running that file with the java launcher is equivalent to embedding it in an anonymous class declaration, instantiating the anonymous class, and invoking the launchable main method:

new Object() {

    String greeting = "Hello, World!";

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

}.main();

The javadoc tool can generate documentation from a simple source file, even though the implicitly declared class cannot 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 a larger program.

Interacting with the console

Many beginner programs need to interact with the console. Writing to the console ought to be a simple method invocation, but traditionally it requires using the qualified name System.out.println. This is mildly painful for experienced developers, but deeply mysterious to the beginner: What is System, what is out, and what are the dots for?

Even worse is reading from the console which, again, ought to be a simple method invocation. Since writing to the console involves System.out, it seems reasonable that reading would involve System.in, but getting a String from System.in requires all this code:

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

Experienced developers are used to this boilerplate, but for the beginner this code contains yet more mysterious concepts, 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 small programs, we make five methods available for use in simple source files:

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

A beginner can now write Hello, World! as:

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

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

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

The five static methods above are declared in the new class java.io.IO, which is a preview API in JDK 24. Every simple source file imports these static methods automatically, as if the declaration

import static java.io.IO.*;

appears at the start of every simple source file.

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 simple source file:

import java.util.List;

void main() {
    var authors = List.of("James", "Bill", "Guy", "Alex", "Dan", "Gavin");
    for (var name : authors) {
        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 simple source files, as if they were imported on demand. Popular APIs 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 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 simple 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 simple source file.

Growing a program

A small program in a simple 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 simple 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 the automatic imports. For example, this simple source file:

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

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

import static java.io.IO.*;
import module java.base;

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

• _Use blank" rel="nofollow noopener" data-shared-secret="1734795671586-0.12203506911106787">JShell for small programs — A JShell session is not a program, but a sequence of code snippets. Declarations typed into jshell are implicitly viewed as static members of some unspecified class, with some unspecified access level, and statements execute in a context in which all previous declarations are in scope.

  This is convenient for experimentation — which is the primary use case for JShell — but not a good model for writing small programs. Evolving a batch of working declarations in JShell into a real program leads to a non-idiomatic style of code because it declares each method, class, and variable as static. JShell is a great tool for exploration and debugging, but it is not the on-ramp programming model we are looking for.

• Interpret code units as static members — Methods and fields are non-static by default. Interpreting top level members in a simple source file as static would change the meaning of the code units in such a class — introducing, in effect, a distinct Java dialect. To preserve the meaning of such members when we evolve a simple source file into an ordinary source file we would have to add explicit static modifiers. When we scale up from a handful of methods to a simple class we want to use classes as classes, not merely containers of static members.

• Interpret the simple source file as a method body — Rather than treat a simple source file as the body of an implicitly declared class, we could treat it as the body of a main method of an implicitly declared class. In other words, a simple source file could contain only statements, local class or interface declarations, or local variable declarations, without the header of the main method.

  This approach would be limiting since there would be no way to declare auxiliary methods; we could only write linear programs, without the power to abstract repeating computations into subroutines. Moreover, there would be no way to declare fields; all variable declarations would be treated as local variable declarations. This is a limitation because local variables can only be accessed from inside lambda bodies or inner classes when they are effectively final, whereas fields do not have that constraint.

  The design we propose above supports both auxiliary methods and field declarations. The burden of writing a main method header, especially with the relaxed requirements on this method in a simple source file, is not onerous, even for beginners.

• Introduce package-level methods and fields — We could achieve a user experience similar to that shown above by allowing package-level methods and fields to be declared in a file without an explicit package or class declaration. However, such a feature would have a far wider impact on how Java code is written in general.

• Different automatic imports — Rather than have the class implicitly declared by a simple source file import all 54 packages in the java.base module on demand, we could instead have it import some subset of those packages. But, which subset should that be?

  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 small program. 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 classes implicitly declared by simple source files.

• Introduce a new dialect of the Java language — A radically different design would define a different dialect of the language for use in simple 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 be explicitly 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.