Implicitly Declared Classes and Instance main Methods (Third Preview)

Changes to the Java® Language Specification • Version 23+36-2368

This document describes changes to the Java Language Specification to support Implicitly Declared Classes and Instance main Methods, which is a preview feature of Java SE 23. See JEP 477 for an overview of the feature.

This feature is co-developed with another feature, Module Import Declarations. See JEP 476 for an overview of this feature. This document assumes that changes described in a companion document have already been applied to the Java Language Specification.

A companion document describes the changes needed to the Java Virtual Machine Specification to support Implicitly Declared Classes and Instance main Methods.

Changes are described with respect to existing sections of the JLS. New text is indicated like this and deleted text is indicated like this. Explanation and discussion, as needed, is set aside in grey boxes.

Changelog:

2024-05: Adapter class renamed from java.io.Console.Basic to java.io.IO.

2024-03: First draft of third preview. Main changes to second preview:

  1. Within every simple compilation unit the three static methods declared in the new class Basic that is nested in the class java.io.Console, can be used using a simple name, as if they have been imported, to enable the compact expression of simple textual I/O with the console.

  2. Within every simple compilation unit the public top level classes and interfaces from all the packages exported by the module java.base can be used using a simple name, as if they have all been imported on demand.

Chapter 6: Names

6.1 Declarations

A declaration introduces an entity into a program and includes an identifier (3.8) that can be used in a name to refer to this entity. The identifier is constrained to avoid certain contextual keywords when the entity being introduced is a class, interface, or type parameter.

A declared entity is one of the following:

Constructors (8.8, 8.10.4) are also introduced by declarations, but use the name of the class in which they are declared rather than introducing a new name.

The rest of this section is unchanged.

6.3 Scope of a Declaration

The scope of a declaration is the region of the program within which the entity declared by the declaration can be referred to using a simple name, provided it is not shadowed (6.4.1).

A declaration is said to be in scope at a particular point in a program if and only if the declaration's scope includes that point.

The scope of the declaration of an observable top level package (7.4.3) is all observable compilation units associated with modules to which the package is uniquely visible (7.4.3).

The declaration of a package that is not observable is never in scope.

The declaration of a subpackage is never in scope.

The package java is always in scope.

The scope of a class or interface imported by a single-type-import declaration (7.5.1) or a type-import-on-demand declaration (7.5.2) is the module declaration (7.7) and all the class and interface declarations (8.1, 9.1) of the compilation unit in which the import declaration appears, as well as any annotations on the module declaration or package declaration of the compilation unit.

The scope of a member imported by a single-static-import declaration (7.5.3) or a static-import-on-demand declaration (7.5.4) is the module declaration and all the class and interface declarations of the compilation unit in which the import declaration appears, as well as any annotations on the module declaration or package declaration of the compilation unit.

The scope of a top level class or interface (7.6) declared in an ordinary compilation unit is all class and interface declarations in the package in which the top level class or interface is declared.

The implicit declaration of a top level class in a simple compilation unit (7.3) is never in scope.

The scope of a declaration of a member m declared in or inherited by a class or interface C (8.2, 9.2) is the entire body of C, including any nested class or interface declarations. If C is a record class, then the scope of m additionally includes the header of the record declaration of C.

The rest of this section is unchanged.

6.7 Fully Qualified Names and Canonical Names

Every primitive type, named package, top level class, and top level interface has a fully qualified name:

Each member class, member interface, and array type may have a fully qualified name:

A local class, local interface, or anonymous class does not have a fully qualified name.

Every primitive type, named package, top level class, and top level interface has a canonical name:

Each member class, member interface, and array type may have a canonical name:

A local class, local interface, or anonymous class does not have a canonical name.

Example 6.7-1. Fully Qualified Names

In the code:

package points;
class Point    { int x, y; }
class PointVec { Point[] vec; }

the fully qualified name of the type Point is "points.Point"; the fully qualified name of the type PointVec is "points.PointVec"; and the fully qualified name of the type of the field vec of class PointVec is "points.Point[]".

Example 6.7-2. Fully Qualified Names v. Canonical Name

The difference between a fully qualified name and a canonical name can be seen in code such as:

package p;
class O1 { class I {} }
class O2 extends O1 {}

Both p.O1.I and p.O2.I are fully qualified names that denote the member class I, but only p.O1.I is its canonical name.

Chapter 7: Packages and Modules

7.3 Compilation Units

CompilationUnit is the goal symbol (2.1) for the syntactic grammar (2.3) of Java programs. It is defined by the following production:

CompilationUnit:
OrdinaryCompilationUnit
SimpleCompilationUnit
ModularCompilationUnit
OrdinaryCompilationUnit:
[PackageDeclaration] {ImportDeclaration} {TopLevelClassOrInterfaceDeclaration}
SimpleCompilationUnit:
{ImportDeclaration} {ClassMemberDeclarationNoMethod} MethodDeclaration {ClassMemberDeclaration}
ClassMemberDeclarationNoMethod:
FieldDeclaration
ClassDeclaration
InterfaceDeclaration
;
ModularCompilationUnit:
{ImportDeclaration} ModuleDeclaration

An ordinary compilation unit consists of three parts, each of which is optional:

A simple compilation unit consists of at least one method declaration (8.4), optionally preceded by import declarations. Declarations of methods, fields, classes, and interfaces are permitted in any order after the import declarations (if any), as they would be in a class body (8.1.7).

This means that the following compilation unit is unambiguously an ordinary compilation unit:

import p.*;
class Test { ... }

whereas the following is unambiguously a simple compilation unit:

import p.*;
static void main(){ ... }
class Test { ... }

A simple compilation unit implicitly declares a top level class (7.6) whose members are the methods, fields, classes, and interfaces declared in the simple compilation unit. The details of the class are specified in 8.1.8.

Every simple compilation unit implicitly imports on demand every public class or interface declared in all the packages exported by the predefined module java.base, as if the declaration import module java.base; appeared at the beginning of each simple compilation unit (7.5.5). As a result, the names of all those classes and interfaces are available as simple names in every simple compilation unit.

The module import declaration feature is proposed by JEP 476 and specified in a companion document

Every simple compilation unit implicitly imports all the static members from the class java.io.IO, as if the declaration import static java.io.IO.*; appeared at the beginning of the simple compilation unit immediately after the declaration that imports module java.base;. As a result, the names of all those static members are available as simple names in every simple compilation unit.

The java.io.IO class declares the following three static methods that provide for the most basic textual I/O with the console:

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

This allows code in a simple compilation unit to easily perform basic interaction with the console without any import declarations. The simple compilation unit corresponding to the famous "Hello World" program is as follows:

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

and the simplest of interactive programs can be written as the following simple compilation unit:

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

A modular compilation unit consists of a module declaration (7.7), optionally preceded by import declarations. The import declarations allow classes and interfaces from packages in this module and other modules, as well as static members of classes and interfaces, to be referred to using their simple names within the module declaration.

Every ordinary and modular compilation unit implicitly imports every public class or interface declared in the predefined package java.lang, as if the declaration import java.lang.*; appeared at the beginning of each compilation unit immediately after any package declaration. As a result, the names of all those classes and interfaces are available as simple names in every ordinary and modular compilation unit.

The host system determines which compilation units are observable, except for the compilation units in the predefined package java and its subpackages lang and io, which are all always observable.

Each observable compilation unit may be associated with a module, as follows:

The observability of a compilation unit influences the observability of its package (7.4.3), while the association of an observable compilation unit with a module influences the observability of that module (7.7.6).

When compiling the modular and ordinary compilation units associated with a module M, the host system must respect the dependencies specified in M's declaration. Specifically, the host system must limit the ordinary compilation units that would otherwise be observable, to only those that are visible to M. The ordinary compilation units that are visible to M are the observable ordinary compilation units associated with the modules that are read by M. The modules read by M are given by the result of resolution, as described in the java.lang.module package specification, with M as the only root module. The host system must perform resolution to determine the modules read by M; it is a compile-time error if resolution fails for any of the reasons described in the java.lang.module package specification.

The readability relation is reflexive, so M reads itself, and thus all of the modular and ordinary compilation units associated with M are visible to M.

The modules read by M drive the packages that are uniquely visible to M (7.4.3), which in turn drives both the top level packages in scope and the meaning of package names for code in the modular and ordinary compilation units associated with M (6.3, 6.5.3, 6.5.5).

The rules above ensure that package and type names used in annotations in a modular compilation unit (in particular, annotations applied to the module declaration) are interpreted as if they appeared in an ordinary compilation unit associated with the module.

Classes and interfaces declared in different ordinary compilation units can refer to each other, circularly. A Java compiler must arrange to compile all such classes and interfaces at the same time.

7.6 Top Level Class and Interface Declarations

A top level class or interface declaration declares a top level class (8.1) or a top level interface (9.1).

TopLevelClassOrInterfaceDeclaration:
ClassDeclaration
InterfaceDeclaration
;

Extra ";" tokens appearing at the level of class and interface declarations in a compilation unit have no effect on the meaning of the compilation unit. Stray semicolons are permitted in the Java programming language solely as a concession to C++ programmers who are used to placing ";" after a class declaration. They should not be used in new Java code.

A top level class may also be implicitly declared (8.1.8).

In the absence of an access modifier, a top level class or interface has package access: it is accessible only within ordinary compilation units of the package in which it is declared (6.6.1). A class or interface may be declared public to grant access to the class or interface from code in other packages of the same module, and potentially from code in packages of other modules.

It is a compile-time error if a top level class or interface declaration contains any one of the following access modifiers: protected, private, or static.

It is a compile-time error if the name of a top level class or interface appears as the name of any other top level class or interface declared in the same package.

The scope and shadowing of a top level class or interface is specified in 6.3 and 6.4.

The fully qualified name of a top level class or interface is specified in 6.7.

The rest of this section is unchanged.

Chapter 8: Classes

8.1 Class Declarations

A class declaration specifies a class.

There are three kinds of class declarations: normal class declarations, enum declarations (8.9), and record declarations (8.10).

ClassDeclaration:
NormalClassDeclaration
EnumDeclaration
RecordDeclaration
NormalClassDeclaration:
{ClassModifier} class TypeIdentifier [TypeParameters]
[ClassExtends] [ClassImplements] [ClassPermits] ClassBody

A class is also implicitly declared by a class instance creation expression (15.9.5) and an enum constant that ends with a class body (8.9.1).

Some classes are implicitly declared by other constructs (8.1.8).

The TypeIdentifier in a class declaration specifies the name of the class.

It is a compile-time error if a class has the same simple name as any of its enclosing classes or interfaces.

The scope and shadowing of a class declaration is specified in 6.3 and 6.4.1.

8.1.8 Implicitly Declared Classes

Not all classes are specified by a class declaration. The following constructs implicitly declare classes:

In all cases, the members of any implicitly declared class, including any implicitly declared members, are subject to the usual rules for member declarations in a class.

The following production from 7.3 is shown here for convenience:

SimpleCompilationUnit:
{ImportDeclaration} {ClassMemberDeclarationNoMethod} MethodDeclaration {ClassMemberDeclaration}
ClassMemberDeclarationNoMethod:
FieldDeclaration
ClassDeclaration
InterfaceDeclaration
;

The class implicitly declared by a simple compilation unit satisfies the following properties:

It is a compile-time error if this class does not declare a candidate main method (12.1.4).

Note that an unnamed package may have multiple implicitly declared classes as members.

Chapter 12: Execution

This chapter specifies activities that occur during execution of a program. It is organized around the life cycle of the Java Virtual Machine and of the classes, interfaces, and objects that form a program.

The Java Virtual Machine starts up by loading a specified class or interface, then invoking the a method main in this specified class or interface. Section 12.1 outlines the loading, linking, and initialization steps involved in executing main, as an introduction to the concepts in this chapter. Further sections specify the details of loading (12.2), linking (12.3), and initialization (12.4).

The chapter continues with a specification of the procedures for creation of new class instances (12.5); and finalization of class instances (12.6). It concludes by describing the unloading of classes (12.7) and the procedure followed when a program exits (12.8).

12.1 Java Virtual Machine Startup

The Java Virtual Machine starts execution by invoking the a method main of some specified class or interface. If this main method has a formal parameter, it is passed passing it a single argument which is an array of strings. In the examples in this specification, this first class is typically called Test.

The precise semantics of Java Virtual Machine startup are given in Chapter 5 of The Java Virtual Machine Specification, Java SE 22 Edition. Here we present an overview of the process from the viewpoint of the Java programming language.

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 initial 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. If the original compilation unit was a simple compilation unit (7.3), then the name of the file that contained the compilation unit, minus any extension, is typically used to specify the name of the initial class or interface.

For example, in a UNIX implementation, the command line:

java Test reboot Bob Dot Enzo

will typically start a Java Virtual Machine by invoking method main of class Test (a class in an unnamed package), passing it an argument array containing the four strings "reboot", "Bob", "Dot", and "Enzo".

Whereas if the file HelloWorld.java contained the following simple compilation unit:

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

which has been compiled, then the command line:

java HelloWorld

will typically start a Java Virtual Machine by invoking the main method of the implicitly declared class (8.1.8) producing the output:

Hello, World!

We now outline the steps the Java Virtual Machine may take to execute Test the initial class or interface, as an example of the loading, linking, and initialization processes that are described further in later sections.

12.1.1 Load the Initial Class or Interface Test

The initial attempt to execute the a method main of the initial class or interface Test discovers that the class Test it is not loaded - that is, that the Java Virtual Machine does not currently contain a binary representation for this class or interface. The Java Virtual Machine then uses a class loader to attempt to find such a binary representation. If this process fails, then an error is thrown. This loading process is described further in 12.2.

12.1.2 Link the Initial Class or Interface Test: Verify, Prepare, (Optionally) Resolve

After the class or interface Test is loaded, it must be initialized before a method main can be invoked. And Test, like all classes and interfaces, it must be linked before it is initialized. Linking involves verification, preparation, and (optionally) resolution. Linking is described further in 12.3.

Verification checks that the loaded representation of the class or interface Test is well-formed, with a proper symbol table. Verification also checks that the code that implements the class or interface Test obeys the semantic requirements of the Java programming language and the Java Virtual Machine. If a problem is detected during verification, then an error is thrown. Verification is described further in 12.3.1.

Preparation involves allocation of static storage and any data structures that are used internally by the implementation of the Java Virtual Machine, such as method tables. Preparation is described further in 12.3.2.

Resolution is the process of checking symbolic references from the class or interface Test to other classes and interfaces, by loading the other classes and interfaces that are mentioned and checking that the references are correct.

The resolution step is optional at the time of initial linkage. An implementation may resolve symbolic references from a class or interface that is being linked very early, even to the point of resolving all symbolic references from the classes and interfaces that are further referenced, recursively. (This resolution may result in errors from these further loading and linking steps.) This implementation choice represents one extreme and is similar to the kind of "static" linkage that has been done for many years in simple implementations of the C language. (In these implementations, a compiled program is typically represented as an "a.out" file that contains a fully-linked version of the program, including completely resolved links to library routines used by the program. Copies of these library routines are included in the "a.out" file.)

An implementation may instead choose to resolve a symbolic reference only when it is actively used; consistent use of this strategy for all symbolic references would represent the "laziest" form of resolution. In this case, if the class or interface Test had several symbolic references to another class, then the references might be resolved one at a time, as they are used, or perhaps not at all, if these references were never used during execution of the program.

The only requirement on when resolution is performed is that any errors detected during resolution must be thrown at a point in the program where some action is taken by the program that might, directly or indirectly, require linkage to the class or interface involved in the error. Using the "static" example implementation choice described above, loading and linkage errors could occur before the program is executed if they involved a class or interface mentioned in the initial class or interface Test or any of the further, recursively referenced, classes and interfaces. In a system that implemented the "laziest" resolution, these errors would be thrown only when an incorrect symbolic reference is actively used.

The resolution process is described further in 12.3.3.

12.1.3 Initialize the Initial Class or Interface Test: Execute Initializers

In our continuing example, the Java Virtual Machine is still trying to execute the a method main of the initial class or interfaceTest. This is permitted only if the class has been initialized (12.4.1).

Initialization consists of execution of any class variable initializers and static initializers of the initial class or interface Test, in textual order. But before it Test can be initialized, its direct superclass must be initialized, as well as the direct superclass of its direct superclass, and so on, recursively. In the simplest case, the initial class or interface Test has Object as its implicit direct superclass; if class Object has not yet been initialized, then it must be initialized before the initial class or interface Test is initialized. Class Object has no superclass, so the recursion terminates here.

If the initial class or interface Test has another class Super as its superclass, then Super must be initialized before the initial class or interfaceTest. This requires loading, verifying, and preparing Super if this has not already been done and, depending on the implementation, may also involve resolving the symbolic references from Super and so on, recursively.

Initialization may thus cause loading, linking, and initialization errors, including such errors involving other classes and interfaces.

The initialization process is described further in 12.4.

12.1.4 Invoke Test.main a main method

Finally, after completion of the initialization for the initial class or interface Test (during which other consequential loading, linking, and initializing may have occurred), the a main method main of declared in or inherited by the initial class or interface Test is invoked.

The method main must be declared public, static, and void. It must specify a formal parameter (8.4.1) whose declared type is array of String. Therefore, either of the following declarations is acceptable:

public static void main(String[] args)

public static void main(String... args)

A method of the initial class or interface is a candidate if it is named main and one of the following applies:

Note that a candidate method may be either a static or an instance method. A candidate method may also have a throws clause (8.4.6).

The form of a main method expanded significantly in Java SE 22. Prior to that, it had to be public static and have a single formal parameter; the only variation possible was String[] versus String... for the type of the single formal parameter. In Java 22 and above, there are twelve possible forms, depending on the access modifier, static modifier, and formal parameter. This number increases to 18 if String[] is distinguished from String... in the type of the single formal parameter.

Note that it is not a compile-time error if the initial class or interface counts more than one candidate method among its members.

The presence of a candidate method in a class or interface may not be immediately apparent because a main method may be inherited. For example, a default method in an interface is an instance method (9.4), so may be a candidate when inherited by a class that implements the interface. Development tools are encouraged to highlight when a class or interface has a member main method that could serve as the start of the program.

A main method of the initial class or interface is invoked, as if by application of the following rules:

Note that it is not possible for a class to have both a static and an instance method with the same signature (8.4.2).

The behavior of an implementation if there is no candidate method to invoke, or if there is no suitable constructor in the initial class when invoking an instance candidate method, is beyond the scope of this specification.

Chapter 13: Binary Compatibility

13.1 The Form of a Binary

Programs must be compiled either into the class file format specified by The Java Virtual Machine Specification, Java SE 22 Edition, or into a representation that can be mapped into that format by a class loader written in the Java programming language.

A class file corresponding to a class or interface declaration must have certain properties. A number of these properties are specifically chosen to support source code transformations that preserve binary compatibility. The required properties are:

  1. The class or interface must be named by its binary name, which must meet the following constraints:

    • The binary name of a top level class or interface (7.6) is its canonical name (6.7).

      Note that the canonical name of the top level class implicitly declared by a simple compilation unit is determined by the host system (8.1.8).

    • The binary name of a member class or interface (8.5, 9.5) consists of the binary name of its immediately enclosing class or interface, followed by $, followed by the simple name of the member.

    • The binary name of a local class or interface (14.3) consists of the binary name of its immediately enclosing class or interface, followed by $, followed by a non-empty sequence of digits, followed by the simple name of the local class.

    • The binary name of an anonymous class (15.9.5) consists of the binary name of its immediately enclosing class or interface, followed by $, followed by a non-empty sequence of digits.

    • The binary name of a type variable declared by a generic class or interface (8.1.2, 9.1.2) is the binary name of its immediately enclosing class or interface, followed by $, followed by the simple name of the type variable.

    • The binary name of a type variable declared by a generic method (8.4.4) is the binary name of the class or interface declaring the method, followed by $, followed by the descriptor of the method (JVMS §4.3.3), followed by $, followed by the simple name of the type variable.

    • The binary name of a type variable declared by a generic constructor (8.8.4) is the binary name of the class declaring the constructor, followed by $, followed by the descriptor of the constructor (JVMS §4.3.3), followed by $, followed by the simple name of the type variable.

  2. A reference to another class or interface must be symbolic, using the binary name of the class or interface.

  3. A reference to a field that is a constant variable (4.12.4) must be resolved at compile time to the value V denoted by the constant variable's initializer.

    If such a field is static, then no reference to the field should be present in the code in a binary file, including the class or interface which declared the field. Such a field must always appear to have been initialized (12.4.2); the default initial value for the field (if different than V) must never be observed.

    If such a field is non-static, then no reference to the field should be present in the code in a binary file, except in the class containing the field. (It will be a class rather than an interface, since an interface has only static fields.) The class should have code to set the field's value to V during instance creation (12.5).

  4. Given a legal expression denoting a field access in a class C, referencing a field named f that is not a constant variable and is declared in a (possibly distinct) class or interface D, we define the qualifying class or interface of the field reference as follows:

    • If the expression is referenced by a simple name, then if f is a member of the current class or interface, C, then let Q be C. Otherwise, let Q be the innermost lexically enclosing class or interface declaration of which f is a member. In either case, Q is the qualifying class or interface of the reference.

    • If the reference is of the form TypeName.f, where TypeName denotes a class or interface, then the class or interface denoted by TypeName is the qualifying class or interface of the reference.

    • If the expression is of the form ExpressionName.f or Primary.f, then:

      • If the compile-time type of ExpressionName or Primary is an intersection type V1 & ... & Vn (4.9), then the qualifying class or interface of the reference is the erasure (4.6) of V1.

      • Otherwise, the erasure of the compile-time type of ExpressionName or Primary is the qualifying class or interface of the reference.

    • If the expression is of the form super.f, then the superclass of C is the qualifying class or interface of the reference.

    • If the expression is of the form TypeName.super.f, then the superclass of the class denoted by TypeName is the qualifying class or interface of the reference.

    The reference to f must be compiled into a symbolic reference to the qualifying class or interface of the reference, plus the simple name of the field, f.

    The reference must also include a symbolic reference to the erasure of the declared type of the field, so that the verifier can check that the type is as expected.

  5. Given a method invocation expression or a method reference expression in a class or interface C, referencing a method named m declared (or implicitly declared (9.2)) in a (possibly distinct) class or interface D, we define the qualifying class or interface of the method invocation as follows:

    • If D is Object then the qualifying class or interface of the method invocation is Object.

    • Otherwise:

      • If the method is referenced by a simple name, then if m is a member of the current class or interface C, let Q be C; otherwise, let Q be the innermost lexically enclosing class or interface declaration of which m is a member. In either case, Q is the qualifying class or interface of the method invocation.

      • If the expression is of the form TypeName.m or ReferenceType::m, then the class or interface denoted by TypeName, or the erasure of ReferenceType, is the qualifying class or interface of the method invocation.

      • If the expression is of the form ExpressionName.m or Primary.m or ExpressionName::m or Primary::m, then:

        • If the compile-time type of ExpressionName or Primary is an intersection type V1 & ... & Vn, then the qualifying class or interface of the method invocation is the erasure of V1.

        • Otherwise, the erasure of the compile-time type of ExpressionName or Primary is the qualifying class or interface of the method invocation.

      • If the expression is of the form super.m or super::m, then the superclass of C is the qualifying class or interface of the method invocation.

      • If the expression is of the form TypeName.super.m or TypeName.super::m, then if TypeName denotes a class X, the superclass of X is the qualifying class or interface of the method invocation; if TypeName denotes an interface X, X is the qualifying class or interface of the method invocation.

    A reference to a method must be resolved at compile time to a symbolic reference to the qualifying class or interface of the method invocation, plus the erasure of the declared signature (8.4.2) of the method. The signature of a method must include all of the following as determined by 15.12.3:

    • The simple name of the method

    • The number of parameters to the method

    • A symbolic reference to the type of each parameter

    A reference to a method must also include either a symbolic reference to the erasure of the return type of the denoted method or an indication that the denoted method is declared void and does not return a value.

  6. Given a class instance creation expression (15.9) or an explicit constructor invocation statement (8.8.7.1) or a method reference expression of the form ClassType :: new (15.13) in a class or interface C, referencing a constructor m declared in a (possibly distinct) class or interface D, we define the qualifying class of the constructor invocation as follows:

    • If the expression is of the form new D(...) or ExpressionName.new D(...) or Primary.new D(...) or D :: new, then the qualifying class of the constructor invocation is D.

    • If the expression is of the form new D(...){...} or ExpressionName.new D(...){...} or Primary.new D(...){...}, then the qualifying class of the constructor invocation is the anonymous class declared by the expression.

    • If the expression is of the form super(...) or ExpressionName.super(...) or Primary.super(...), then the qualifying class of the constructor invocation is the direct superclass of C.

    • If the expression is of the form this(...), then the qualifying class of the constructor invocation is C.

    A reference to a constructor must be resolved at compile time to a symbolic reference to the qualifying class of the constructor invocation, plus the declared signature of the constructor (8.8.2). The signature of a constructor must include both:

    • The number of parameters of the constructor

    • A symbolic reference to the type of each formal parameter

A binary representation for a class or interface must also contain all of the following:

  1. If it is a class and is not Object, then a symbolic reference to the direct superclass of this class.

  2. A symbolic reference to each direct superinterface, if any.

  3. A specification of each field declared in the class or interface, given as the simple name of the field and a symbolic reference to the erasure of the type of the field.

  4. If it is a class, then the erased signature of each constructor, as described above.

  5. For each method declared in the class or interface (excluding, for an interface, its implicitly declared methods (9.2)), its erased signature and return type, as described above.

  6. The code needed to implement the class or interface:

    • For an interface, code for the field initializers and the implementation of each method with a block body (9.4.3).

    • For a class, code for the field initializers, the instance and static initializers, the implementation of each method with a block body (8.4.7), and the implementation of each constructor.

  7. Every class or interface must contain sufficient information to recover its canonical name (6.7).

  8. Every member class or interface must have sufficient information to recover its source-level access modifier (6.6).

  9. Every nested class or interface must have a symbolic reference to its immediately enclosing class or interface (8.1.3).

  10. Every class or interface must contain symbolic references to all of its member classes and interfaces (8.5, 9.5), and to all other nested classes and interfaces declared within its body.

  11. A construct emitted by a Java compiler must be marked as synthetic if it does not correspond to a construct declared explicitly or implicitly in source code, unless the emitted construct is a class initialization method (JVMS §2.9).

  12. A construct emitted by a Java compiler must be marked as mandated if it corresponds to a formal parameter declared implicitly in source code (8.8.1, 8.8.9, 8.9.3, 15.9.5.1).

The following formal parameters are declared implicitly in source code:

For reference, the following constructs are declared implicitly in source code, but are not marked as mandated because only formal parameters and modules can be so marked in a class file (JVMS §4.7.24, JVMS §4.7.25):

A class file corresponding to a module declaration must have the properties of a class file for a class whose binary name is module-info and which has no superclass, no superinterfaces, no fields, and no methods. In addition, the binary representation of the module must contain all of the following:

The following sections discuss changes that may be made to class and interface declarations without breaking compatibility with pre-existing binaries. Under the translation requirements given above, the Java Virtual Machine and its class file format support these changes. Any other valid binary format, such as a compressed or encrypted representation that is mapped back into class files by a class loader under the above requirements, will necessarily support these changes as well.