1. Java Language Updates
  2. Implicitly Declared Classes and Instance Main Methods

5 Implicitly Declared Classes and Instance Main Methods

The addition of two preview features, Instance Main Methods and Implicitly Declared Classes, to the Java language by JEP 463 enables students to write their first programs without needing to understand the full set of language features designed for large programs.

Note:

This is a preview feature. A preview feature is a feature whose design, specification, and implementation are complete, but is not permanent. A preview feature may exist in a different form or not at all in future Java SE releases. To compile and run code that contains preview features, you must specify additional command-line options. See Preview Language and VM Features.

For background information about instance main methods and implicitly declared classes, see JEP 463.

The Java programming language excels in developing 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. The composition of large components is called programming-in-the-large.

However, the Java programming language is also intended to be a first language and offers many constructs that are useful for programming-in-the-small (everything that is internal to a component). When programmers first start out, they do not write large programs in a team — they write small programs by themselves. At this stage, there is no need for the programming-in-the-large concepts of classes, packages, and modules.

When teaching programming, instructors start with the basic programming-in-the-small concepts of variables, control flow, and subroutines. There is no need for the programming-in-the-large concepts of classes, packages, and modules. Students who are learning to program have no need for encapsulation and namespaces which are useful later to separately evolve components written by different people.

To enhance the Java programming language's support for programming in the small, JEP 463 adds two preview features, instance main methods and implicitly declared classes that accomplish the following:
  • Enhances the protocol by which Java programs are launched by allowing instance main methods that are not static, need not be public, and need not have a String[] parameter. See Flexible Launch Protocol.
  • Allows a compilation unit (a source file) to implicitly declare a class. See the section "Compilation Units" in the chapter "Packages and Modules" in the Java Language Specification and Implicitly Declared Classes.
Consider the classic HelloWorld program that is often used as the first program for Java students: 
public class HelloWorld {
  public static void main(String[] args) {
    System.out.println("Hello, World!");
  }
}

In this first program:

  • 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 rather 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 simple programs like HelloWorld.
  • The static modifier is part of Java's class-and-object model. For the novice, static is not just mysterious but also harmful. To add more methods or fields that main can call and use, the student must either declare them all as static (propagating an idiom which is neither a common nor a good habit) or else confront the difference between static and instance members and learn how to instantiate an object.
The new programmer encounters these programming-in-the-large constructs before they learn about variables and control flow and before they can appreciate the utility of programming-in-the-large constructs for keeping a large program well organized. Educators often offer the admonition, "Don’t worry about that. You’ll understand it later." This is unsatisfying to them and to their students. It leaves students with the enduring impression that the Java language is overly complicated.

The preview language features in JEP 463, instance main methods and implicitly declared classes, reduce the complexity of writing simple programs such as HelloWorld by enabling programmers to write programs without using access modifiers, static modifiers, or the String[] parameter. Far from being a separate dialect, students can now use the Java language to write streamlined declarations for single-class programs and then later seamlessly expand their beginning programs to include more advanced features as their skills grow. Java veterans might also find that instance main methods and implicitly declared classes are useful features when writing simple Java programs that do not require the programming-in-the-large scaffolding of the Java language. The introduction of programming-in-the-large constructs can be postponed by instructors until they are needed.

Topics

Flexible Launch Protocol

In the JDK, the launch protocol is implemented by the command-line tool as the java executable.

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. With JEP 463, the launch protocol has been enhanced to offer more flexibility in the declaration of a program's entry point and, in particular, to allow instance main methods, as follows:

  • Allows the main method of a launched class to have public, protected, or default (such as package) access.
  • If a launched class contains a main method with a String[] parameter, then it chooses that method.
  • If a launched class contains a main method without parameters, then it chooses that method.
  • In either case, if the chosen main method is static then it invokes it.

  • Otherwise, the chosen main method is an instance main method and the launched class must have a zero-parameter, non-private constructor (public, protected, or package access). It invokes that constructor and then invokes the main method of the resulting object. If there is no such constructor, then it reports an error and terminates.

  • If there is no suitable main method then it reports an error and terminates.

By using instance main methods, we can simplify the HelloWorld program presented in Implicitly Declared Classes and Instance Main Methods to:
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. However, small programs that don't need them can omit them.

A program that doesn't 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 doesn't 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 as namespaces for methods and fields. We should not require students to confront the concept of classes:

  • Before they are comfortable with the 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 doesn't need it — just as we don't require explicit package or module declarations for code that don't need them.

Beginning with JEP 463, when the Java compiler encounters a source file containing a method not enclosed in a class declaration, it considers that method, any similar methods, and any unenclosed fields and classes in the file to form the body of an implicitly declared top-level class.

An implicitly declared class (also known as an implicit class) is always a member of the unnamed package. It is also final and doesn't implement any interface or extend any class other than Object. An implicit class can't be referenced by name, so there can be no method references to its static methods. However, the this keyword can still be used, as well as method references to instance methods.

The code of an implicit class can't refer to the implicit class by name, so instances of an implicit class can't be constructed directly. Such a class is useful only as a standalone program or as an entry point to a program. Therefore, an implicit class must have a main method that can be launched as described in Flexible Launch Protocol. 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 only be 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 represents a program. Consequently, multiple implicit classes in an unnamed package represent multiple programs.

An implicit class is similar to an explicitly declared class. Its members can have the same modifiers (such as private and static) and the modifiers have the same defaults (such as 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 the HelloWorld program as:
void main() {
    System.out.println("Hello, World!");
}
Because top-level members are interpreted as members of the implicit class, we can also write the program as:
String greeting() { return "Hello, World!"; }

void main() {
    System.out.println(greeting());
}
Or, by using a field, we can write the program as:
String greeting = "Hello, World!";

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

You can launch a source file named HelloWorld.java that contains an implicit class with the java command-line tool as follows:
$ java HelloWorld.java

The Java compiler compiles that file to the launchable class file HelloWorld.class. In this case, the compiler chooses HelloWorld for the class name as an implementation detail. However, that name still can't be used directly in Java source code.

At this time, the javadoc tool can't generate API documentation for an implicit class because implicit classes don't define an API that is accessible from other classes. However, fields and methods of an implicit class can generate API documentation.

Growing a Program

By omitting the concepts and constructs it doesn't need, a HelloWorld program written as an implicit class is more focused on what the program actually does. Even so, all members continue to be interpreted just as they are in an ordinary class.

Concepts and constructs can easily be added to an implicit class as needed by the program. To evolve an implicit class into an ordinary class, all we need to do is wrap its declaration, excluding import statements, inside an explicit class declaration.