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JEP
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Resolution: Delivered
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P3
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Ron Pressler, Jim Laskey, & Gavin Bierman
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Feature
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Open
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SE
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S
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S
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477
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:
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 [<code class="prettyprint" >java.io.IO</code>].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!|https://en.wikipedia.org/wiki/%22Hello,_World!%22_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 mandatorypublic
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 asHelloWorld
.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 asstatic
— 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
, 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 withjava --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="1733127385265-0.08563332299066462">jshell</code>, start it with
jshell --enable-preview
.
The launch protocol
New programmers just want to write and run a computer program. The
<em>Java<br /> Language<br /> Specification</em>
(JLS), however, focuses on defining the meaning of a compilation unit, that is, a source file with apackage
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 havepublic
,protected
, or default (i.e., package) access.If the launched class contains a
main
method with aString[]
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., ofpublic
,protected
, or package access). Invoke that constructor and then invoke themain
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
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
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<br /> 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, [<code class="prettyprint" >IO</code>][<code class="prettyprint" >java.io.IO</code>]. 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 [<code class="prettyprint" >java.io.IO</code>] 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 blank" rel="nofollow noopener" data-shared-secret="1733127385265-0.08563332299066462">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 asstatic
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 explicitstatic
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 themain
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
orclass
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
andjava.util
would be near-universal suggestions;java.util.stream
andjava.util.function
would be common; andjava.math
,java.net
, andjava.time
would each have supporters. For the JShell tool, we managed to find tenjava.*
packages which are broadly useful when experimenting with one-off Java code, but it is difficult to see which subset ofjava.*
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
andjava.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.
- relates to
-
JDK-8305457 Implement java.io.IO
- Resolved
-
JDK-8315398 JEP 463: Implicitly Declared Classes and Instance Main Methods (Second Preview)
- Closed
-
JDK-8315129 JEP 476: Module Import Declarations (Preview)
- Closed
-
JDK-8324859 Improve error recovery
- Resolved
-
JDK-8335984 JEP 495: Simple Source Files and Instance Main Methods (Fourth Preview)
- Completed