JEP 477: Implicitly Declared Classes and Instance Main Methods (Third Preview)

AuthorsRon Pressler, Jim Laskey, & Gavin Bierman
OwnerGavin Bierman
TypeFeature
ScopeSE
StatusTargeted
Release23
Componentspecification / language
Discussionamber dash dev at openjdk dot org
EffortS
DurationS
Relates toJEP 463: Implicitly Declared Classes and Instance Main Methods (Second Preview)
JEP 476: Module Import Declarations (Preview)
Reviewed byBrian Goetz
Endorsed byBrian Goetz
Created2024/01/09 17:02
Updated2024/05/22 12:07
Issue8323335

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 language feature.

History

Implicitly declared classes and instance main methods were first proposed as a preview feature by JEP 445, delivered in JDK 21. The feature was previewed again by JEP 463, with significant changes based on feedback, and delivered in JDK 22.

We here propose to preview the feature for a third time, with two additions:

Goals

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! 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 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 them 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. We 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.

Description

First, we enhance the protocol by which Java programs are launched to allow instance main methods. Such methods are not static, need not be public, and need not have a String[] parameter. Then we can simplify the Hello, World! program to:

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

Second, we allow a compilation unit, i.e., a source file, to implicitly declare a class:

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

Third, in implicitly declared classes we automatically import useful methods for textual 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 implicitly declared classes we automatically import, on demand, all of the public top-level classes and interfaces of the packages exported by the java.base module.

This is a preview language feature, disabled by default

To try the examples below in JDK 23 you must enable preview features:

The launch protocol

New programmers just want to write and run a computer program. The Java Language Specification (JLS), however, focuses on defining the meaning of a compilation unit, that is, a source file with a package declaration, import declarations, and class declarations. All that the JLS has to say about a Java program is this:

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 says:

The manner in which the initial class or interface is specified to the Java Virtual Machine is beyond the scope of this specification, but it is typical, in host environments that use command lines, for the fully qualified name of the class or interface to be specified as a command line argument and for following command line arguments to be used as strings to be provided as the argument to the method main.

The actions of choosing the class containing the main method, assembling its dependencies in the form of a module path or a class path (or both), loading the class, initializing it, and invoking the main method with its arguments constitute the launch protocol. In the JDK it is implemented by the launcher, i.e., the java executable.

A flexible launch protocol

We enhance the launch protocol to offer more flexibility in the declaration of a program's entry point and, in particular, to allow instance main methods, as follows:

These changes allow us to write Hello, World! with no access modifiers, no static modifiers, and no String[] parameter, so the introduction of these constructs can be postponed until they are needed:

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

Implicitly declared classes

In the Java language, every class resides in a package and every package resides in a module. These namespacing and encapsulation constructs apply to all code, but small programs that do not need them can omit them. A program that does not need class namespaces can omit the package statement, making its classes implicit members of the unnamed package; classes in the unnamed package cannot be referenced explicitly by classes in named packages. A program that does not need to encapsulate its packages can omit the module declaration, making its packages implicit members of the unnamed module; packages in the unnamed module cannot be referenced explicitly by packages in named modules.

Before classes serve their main purpose as templates for the construction of objects, they serve only as namespaces for methods and fields. We should not require beginners to confront the concept of classes before they are comfortable with the more basic building blocks of variables, control flow, and subroutines, before they embark on learning object orientation, and when they are still writing simple, single-file programs. Even though every method resides in a class, we can stop requiring explicit class declarations for code that does not need it — just as we do not require explicit package or module declarations for code that does not need them.

Henceforth, if the Java compiler encounters a source file with a method that is not enclosed in a class declaration then it will consider that method, any other such methods, and any unenclosed fields and any classes in the file to form the body of an implicitly declared class.

An implicitly declared class (or implicit class, for short) is a final, top-level class. It extends Object and does not implement any interfaces. An implicit class cannot be referenced by name, so there can be no method references to its static methods; the this keyword can still be used, however, as can method references to instance methods.

Instances of an implicit class cannot be constructed directly, since the code of an implicit class cannot reference the class by name. Such a class is useful only as a standalone program or as an entry point to a program. Thus an implicit class must have a main method that can be launched as described above. This requirement is enforced by the Java compiler.

An implicit class resides in the unnamed package, and the unnamed package resides in the unnamed module. While there can be only one unnamed package (barring multiple class loaders) and only one unnamed module, there can be multiple implicit classes in the unnamed module. Every implicit class contains a main method and so represents a program, thus multiple such classes in the unnamed package represent multiple programs.

An implicit class is almost exactly like an explicitly declared class. Its members can have the same modifiers (e.g., private and static) and the modifiers have the same defaults (e.g., package access and instance membership). One key difference is that while an implicit class has a default zero-parameter constructor, it can have no other constructor.

With these changes we can now write Hello, World! as:

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

Top-level members are interpreted as members of the implicit class, so we can also write the program as:

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

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

or, using a field, as:

String greeting = "Hello, World!";

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

If an implicit class has an instance main method rather than a static main method then launching it is equivalent to embedding it in an anonymous class declaration:

new Object() {
    // the implicit class's body
}.main();

A source file named HelloWorld.java containing an implicit class can be launched with the source-code launcher, like so:

$ java HelloWorld.java

The Java compiler will compile the file to the class file HelloWorld.class, and then the launcher will launch that file. The compiler chooses HelloWorld for the class name as an implementation detail, but that name still cannot be used directly in source code.

The javadoc tool can generate documentation for an implicit class, even though an implicit class cannot be referenced by name from other classes and thus cannot be used to define an API. Still, the ability to generate documentation for the members of an implicit class may be useful, both for beginners learning to document their code, and for experienced developers prototyping code intended to be used in a larger program, or writing a reusable script for execution by the source code launcher.

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 in reality 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 to try reading from System.in. But reading 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 for, why both BufferedReader and InputStreamReader, and what 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 three methods available for use in the body of every implicit class:

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

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);
}

We achieve this effect by declaring a new top-level class in the java.io package named, simply, IO. It declares the above three static methods for textual I/O with the console, and nothing else. Every implicitly declared class automatically imports these static methods, as if the declaration

import static java.io.IO.*;

appears at the start of every source file containing an implicit class.

The new class java.io.IO is a preview API in JDK 23.

Automatic import of the java.base module

Many other classes declared in the Java API are useful in small programs. They can, of course, be imported explicitly at the start of the 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 the body of every implicit class, 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, without any fuss. In the above example, the import java.util.List declaration can be removed since the interface will be imported automatically.

JEP 476, Module Import Declarations, 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. Thus every implicitly declared class can be considered to implicitly import the java.base module, as if the declaration

import module java.base;

appears at the start of every source file containing an implicit class.

Growing a program

A small program written as an implicit class is much more focused on what the program actually does, omitting concepts and constructs it does not need. Even so, all members are interpreted just as in an ordinary class. To evolve an implicit class into an ordinary class, all we need to do is wrap its declaration, excluding any import declarations, inside an explicit class declaration, and add the automatic imports. For example, this implicit class:

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

can be grown into this top-level class:

import static java.io.IO.*;
import java.util.List;          // alternatively: 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. Turning a small program into one that can serve as a component in a larger program is straightforward.

Eliminating the main method altogether may seem like the next natural step, but it would work against the goal of gracefully evolving small Java programs to larger ones and would impose some non-obvious restrictions (see below). Dropping the void method result would, similarly, create a distinct Java dialect.

Alternatives