JEP 477: Implicitly Declared Classes and Instance Main Methods (Third Preview)JEP 477: Implicitly Declared Classes and Instance Main Methods (Third Preview)AuthorsRon Pressler, Jim Laskey, & Gavin BiermanOwnerGavin BiermanTypeFeatureScopeSEStatusClosed / DeliveredRelease23Componentspecification / languageDiscussionamber dash dev at openjdk dot orgEffortSDurationSRelates toJEP 463: Implicitly Declared Classes and Instance Main Methods (Second Preview)JEP 476: Module Import Declarations (Preview)Reviewed byBrian GoetzEndorsed byBrian GoetzCreated2024/01/09 17:02Updated2024/07/08 15:12Issue8323335Summary
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:


Implicitly declared classes automatically import three static methods for
simple textual I/O with the console. These methods are declared in the new
top-level class java.io.IO.


Implicitly declared classes automatically import, on demand, all of the
public top-level classes and interfaces of the packages exported by the
java.base module.


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! 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 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:


Compile the program with javac --release 23 --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 jshell, start it with
jshell --enable-preview.


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:


Allow the main method of a launched class to have public, protected,
or default (i.e., package) access.


If the launched class contains a main method with a String[] parameter
then choose that method.


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


In either case, if the chosen method is static then simply invoke it.


Otherwise, the chosen method is an instance method and the launched class
must have a zero-parameter, non-private constructor (i.e., of public,
protected, or package access). Invoke that constructor and then invoke the
main method of the resulting object. If there is no such constructor then
report an error and terminate.


If there is no suitable main method then report an error and terminate.


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


Use 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 an implicit class
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 an implicit class into an ordinary class 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 code units as locals — We can already declare
local variables within methods. Assume that we could also declare local
methods, i.e., methods within other methods. Then we could interpret the
body of a small program as the body of a main method, with variables
interpreted as local variables rather than fields, and methods interpreted
as local methods rather than class members. This would allow us to eschew
the main method altogether and write top-level statements.
The problem with this approach is that, in the Java language, locals behave
differently from fields, and in a more restricted way to boot: Locals can
only be accessed from inside lambda bodies or inner classes when they are
effectively final. The proposed design allows us to separate locals and
fields in the same manner as they have always been. The burden of writing a
main method 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 implicit classes import
all 54 packages in the java.base module on demand, we could instead
have them 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 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 implicit classes.


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