Sealed Classes

Changes to the Java® Language Specification • Version 15

This document describes changes to the Java Language Specification to support sealed classes and interfaces, a preview feature of Java SE 15. See JEP 360 for an overview of the feature.

A companion document describes the changes needed to the Java Virtual Machine Specification to support sealed classes and interfaces.

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.

Chapter 3: Lexical Structure

3.8 Identifiers

An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter.

Identifier:
IdentifierChars but not a Keyword or BooleanLiteral or NullLiteral
IdentifierChars:
JavaLetter {JavaLetterOrDigit}
JavaLetter:
any Unicode character that is a "Java letter"
JavaLetterOrDigit:
any Unicode character that is a "Java letter-or-digit"

A "Java letter" is a character for which the method Character.isJavaIdentifierStart(int) returns true.

A "Java letter-or-digit" is a character for which the method Character.isJavaIdentifierPart(int) returns true.

The "Java letters" include uppercase and lowercase ASCII Latin letters A-Z (\u0041-\u005a), and a-z (\u0061-\u007a), and, for historical reasons, the ASCII dollar sign ($, or \u0024) and underscore (_, or \u005f). The dollar sign should be used only in mechanically generated source code or, rarely, to access pre-existing names on legacy systems. The underscore may be used in identifiers formed of two or more characters, but it cannot be used as a one-character identifier due to being a keyword.

The "Java digits" include the ASCII digits 0-9 (\u0030-\u0039).

Letters and digits may be drawn from the entire Unicode character set, which supports most writing scripts in use in the world today, including the large sets for Chinese, Japanese, and Korean. This allows programmers to use identifiers in their programs that are written in their native languages.

An identifier cannot have the same spelling (Unicode character sequence) as a keyword (3.9), boolean literal (3.10.3), or the null literal (3.10.7), or a compile-time error occurs.

Two identifiers are the same only if, after ignoring characters that are ignorable, the identifiers have the same Unicode character for each letter or digit. An ignorable character is a character for which the method Character.isIdentifierIgnorable(int) returns true. Identifiers that have the same external appearance may yet be different.

For example, the identifiers consisting of the single letters LATIN CAPITAL LETTER A (A, \u0041), LATIN SMALL LETTER A (a, \u0061), GREEK CAPITAL LETTER ALPHA (A, \u0391), CYRILLIC SMALL LETTER A (a, \u0430) and MATHEMATICAL BOLD ITALIC SMALL A (a, \ud835\udc82) are all different.

Unicode composite characters are different from their canonical equivalent decomposed characters. For example, a LATIN CAPITAL LETTER A ACUTE (Á, \u00c1) is different from a LATIN CAPITAL LETTER A (A, \u0041) immediately followed by a NON-SPACING ACUTE (´, \u0301) in identifiers. See The Unicode Standard, Section 3.11 "Normalization Forms".

Examples of identifiers are:

The identifiers var, and yield, sealed, and permits are restricted identifiers because they are not allowed in some contexts.

A type identifier is an identifier that is not the character sequence var, or the character sequence yield.

A type identifier is an identifier that is not one of the following character sequences: permits, sealed, var, or yield.

TypeIdentifier:
Identifier but not permits, sealed, var, or yield

Type identifiers are used in certain contexts involving the declaration or use of types. For example, the name of a class must be a TypeIdentifier, so it is illegal to declare a class named permits, sealed, var, or yield (8.1).

An unqualified method identifier is an identifier that is not the character sequence yield.

UnqualifiedMethodIdentifier:
Identifier but not yield

This restriction allows yield to be used in a yield statement (14.21) and still also be used as a (qualified) method name for compatibility reasons.

3.9 Keywords

5152 character sequences, formed from ASCII letters, are reserved for use as keywords and cannot be used as identifiers (3.8).

Keyword:
(one of)
abstract double int strictfp
char for interface super
assert else long switch
boolean enum native synchronized
break extends new this
byte final non-sealed throw
case finally package throws
catch float private transient
class goto protected try
const if public void
continue implements return volatile
default import short while
do instanceof static _ (underscore)

The keywords const and goto are reserved, even though they are not currently used. This may allow a Java compiler to produce better error messages if these C++ keywords incorrectly appear in programs. The keyword _ (underscore) is reserved for possible future use in parameter declarations.

A variety of character sequences are sometimes assumed, incorrectly, to be keywords:

A further ten character sequences are restricted keywords: open, module, requires, transitive, exports, opens, to, uses, provides, and with. These character sequences are tokenized as keywords solely where they appear as terminals in the ModuleDeclaration, ModuleDirective, and RequiresModifier productions (7.7). They are tokenized as identifiers everywhere else, for compatibility with programs written before the introduction of restricted keywords. There is one exception: immediately to the right of the character sequence requires in the ModuleDirective production, the character sequence transitive is tokenized as a keyword unless it is followed by a separator, in which case it is tokenized as an identifier.

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 be a type identifier when the entity being introduced is a class, interface, or type parameter.

A declared entity is one of the following:

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

The declaration of a type which is not generic (class C ...) declares one entity: a non-generic type (C). A non-generic type is not a raw type, despite the syntactic similarity. In contrast, the declaration of a generic type (class C<T> ... or interface C<T> ...) declares two entities: a generic type (C<T>) and a corresponding non-generic type (C). In this case, the meaning of the term C depends on the context where it appears:

The 14 15 non-generic contexts are as follows:

  1. In a uses or provides directive in a module declaration (7.7.1)

  2. In a single-type-import declaration (7.5.1)

  3. To the left of the . in a single-static-import declaration (7.5.3)

  4. To the left of the . in a static-import-on-demand declaration (7.5.4)

  5. In a permits clause of a sealed class or interface declaration (8.1.6, 9.1.4).

5. 6. To the left of the ( in a constructor declaration (8.8)

6. 7. After the @ sign in an annotation (9.7)

7. 8. To the left of .class in a class literal (15.8.2)

8. 9. To the left of .this in a qualified this expression (15.8.4)

9. 10. To the left of .super in a qualified superclass field access expression (15.11.2)

10. 11. To the left of .Identifier or .super.Identifier in a qualified method invocation expression (15.12)

11. 12. To the left of .super:: in a method reference expression (15.13)

12. 13. In a qualified expression name in a postfix expression or a try-with-resources statement (15.14.1, 14.20.3)

13. 14. In a throws clause of a method or constructor (8.4.6, 8.8.5, 9.4)

14. 15. In an exception parameter declaration (14.20)

The first eleven twelve non-generic contexts correspond to the first eleven twelve syntactic contexts for a TypeName in 6.5.1. The twelfth thirteenth non-generic context is where a qualified ExpressionName such as C.x may include a TypeName C to denote static member access. The common use of TypeName in these twelve thirteen contexts is significant: it indicates that these contexts involve a less-than-first-class use of a type. In contrast, the thirteenth fourteenth and fourteenth fifteenth non-generic contexts employ ClassType, indicating that throws and catch clauses use types in a first-class way, in line with, say, field declarations. The characterization of these two contexts as non-generic is due to the fact that an exception type cannot be parameterized (8.1.2).

Note that the ClassType production allows annotations, so it is possible to annotate the use of a type in a throws or catch clause, whereas the TypeName production disallows annotations, so it is not possible to annotate the name of a type in, say, a single-type-import declaration.

...

6.3 Scope of a Declaration

...

The scope of a declaration of a member m declared in or inherited by a class type C (8.1.6 8.1.7) is the entire body of C, including any nested type declarations.

The scope of a declaration of a member m declared in or inherited by an interface type I (9.1.4 9.1.5) is the entire body of I, including any nested type declarations.

...

6.5 Determining the Meaning of a Name

6.5.1 Syntactic Classification of a Name According to Context

A name is syntactically classified as a ModuleName in these contexts:

A name is syntactically classified as a PackageName in these contexts:

A name is syntactically classified as a TypeName in these contexts:

The extraction of a TypeName from the identifiers of a ReferenceType in the 16 contexts above is intended to apply recursively to all sub-terms of the ReferenceType, such as its element type and any type arguments.

For example, suppose a field declaration uses the type p.q.Foo[]. The brackets of the array type are ignored, and the term p.q.Foo is extracted as a dotted sequence of Identifiers to the left of the brackets in an array type, and classified as a TypeName. A later step determines which of p, q, and Foo is a type name or a package name.

As another example, suppose a cast operator uses the type p.q.Foo<? extends String>. The term p.q.Foo is again extracted as a dotted sequence of Identifier terms, this time to the left of the < in a parameterized type, and classified as a TypeName. The term String is extracted as an Identifier in an extends clause of a wildcard type argument of a parameterized type, and classified as a TypeName.

A name is syntactically classified as an ExpressionName in these contexts:

A name is syntactically classified as a MethodName in this context:

A name is syntactically classified as a PackageOrTypeName in these contexts:

A name is syntactically classified as an AmbiguousName in these contexts:

The effect of syntactic classification is to restrict certain kinds of entities to certain parts of expressions:

Chapter 8: Classes

Class declarations define new reference types and describe how they are implemented (8.1).

A top level class is a class that is not a nested class.

A nested class is any class whose declaration occurs within the body of another class or interface.

This chapter discusses the common semantics of all classes - top level (7.6) and nested (including member classes (8.5, 9.5), local classes (14.3) and anonymous classes (15.9.5)). Details that are specific to particular kinds of classes are discussed in the sections dedicated to these constructs.

A named class may be declared abstract (8.1.1.1) and must be declared abstract if it is incompletely implemented; such a class cannot be instantiated, but can be extended by subclasses.

The degree to which a class can be extended can be explicitly controlled (8.1.1.2). A class may be declared sealed, in which case there is a fixed set of classes that directly extend the sealed class. A class may be declared final, in which case it cannot have any subclasses.

If a class is declared public, then it can be referred to from code in any package of its module and potentially from code in other modules. Each class except Object is an extension of (that is, a subclass of) a single existing class (8.1.4) and may implement interfaces (8.1.5). Classes may be generic (8.1.2), that is, they may declare type variables whose bindings may differ among different instances of the class.

Classes may be decorated with annotations (9.7) just like any other kind of declaration.

The body of a class declares members (fields and methods and nested classes and interfaces), instance and static initializers, and constructors (8.1.6 8.1.7). The scope (6.3) of a member (8.2) is the entire body of the declaration of the class to which the member belongs. Field, method, member class, member interface, and constructor declarations may include the access modifiers (6.6) public, protected, or private. The members of a class include both declared and inherited members (8.2). Newly declared fields can hide fields declared in a superclass or superinterface. Newly declared class members and interface members can hide class or interface members declared in a superclass or superinterface. Newly declared methods can hide, implement, or override methods declared in a superclass or superinterface.

Field declarations (8.3) describe class variables, which are incarnated once, and instance variables, which are freshly incarnated for each instance of the class. A field may be declared final (8.3.1.2), in which case it can be assigned to only once. Any field declaration may include an initializer.

Member class declarations (8.5) describe nested classes that are members of the surrounding class. Member classes may be static, in which case they have no access to the instance variables of the surrounding class; or they may be inner classes (8.1.3).

Member interface declarations (8.5) describe nested interfaces that are members of the surrounding class.

Method declarations (8.4) describe code that may be invoked by method invocation expressions (15.12). A class method is invoked relative to the class type; an instance method is invoked with respect to some particular object that is an instance of a class type. A method whose declaration does not indicate how it is implemented must be declared abstract. A method may be declared final (8.4.3.3), in which case it cannot be hidden or overridden. A method may be implemented by platform-dependent native code (8.4.3.4). A synchronized method (8.4.3.6) automatically locks an object before executing its body and automatically unlocks the object on return, as if by use of a synchronized statement (14.19), thus allowing its activities to be synchronized with those of other threads (17).

Method names may be overloaded (8.4.9).

Instance initializers (8.6) are blocks of executable code that may be used to help initialize an instance when it is created (15.9).

Static initializers (8.7) are blocks of executable code that may be used to help initialize a class.

Constructors (8.8) are similar to methods, but cannot be invoked directly by a method call; they are used to initialize new class instances. Like methods, they may be overloaded (8.8.8).

8.1 Class Declarations

A class declaration specifies a new named reference type.

There are two kinds of class declarations: normal class declarations and enum declarations.

ClassDeclaration:
NormalClassDeclaration
EnumDeclaration
NormalClassDeclaration:
{ClassModifier} class TypeIdentifier [TypeParameters]
[Superclass] [Superinterfaces] [PermittedSubclasses]
ClassBody

The rules in this section apply to all class declarations, including enum declarations. However, special rules apply to enum declarations with regard to class modifiers, inner classes, and superclasses; these rules are stated in 8.9.

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.

8.1.1 Class Modifiers

A class declaration may include class modifiers.

ClassModifier:
(one of)
Annotation public protected private
abstract static sealed non-sealed final strictfp

The rules for annotation modifiers on a class declaration are specified in 9.7.4 and 9.7.5.

The access modifier public (6.6) pertains only to top level classes (7.6) and member classes (8.5), not to local classes (14.3) or anonymous classes (15.9.5).

The access modifiers protected and private pertain only to member classes within a directly enclosing class declaration (8.5).

The modifier static pertains only to member classes (8.5.1), not to top level or local or anonymous classes.

It is a compile-time error if the same keyword appears more than once as a modifier for a class declaration, or if a class declaration has more than one of the access modifiers public, protected, and private (6.6).

It is a compile-time error if a class declaration has more than one of the class modifiers sealed, non-sealed and final.

If two or more (distinct) class modifiers appear in a class declaration, then it is customary, though not required, that they appear in the order consistent with that shown above in the production for ClassModifier.

8.1.1.2 sealed and final Classes

Extensibility of class hierarchies is an important feature of object-oriented programming. However, there are circumstances where explicit control of this extensibility is desirable.

A class can be declared sealed when it is useful to restrict its direct subclasses to a fixed set of classes. A sealed class permits a given set of direct subclasses (8.1.6).

A class can be declared final if its definition is complete and no subclasses are desired or required.

It is a compile-time error if the name of a final class appears in the extends clause (8.1.4) of another class declaration; this implies that a final class cannot have any subclasses.

This rule already appears in 8.1.4.

It is a compile-time error if a class is declared both final and abstract, because the implementation of such a class could never be completed (8.1.1.1).

Because a final class never has any subclasses, the methods of a final class are never overridden (8.4.8.1).

It is a compile-time error if a class has a sealed direct superclass or a sealed direct superinterface and is not declared final, sealed, or non-sealed.

An enum type can implement a sealed interface, as an enum type is (implicitly) either final or sealed (8.9).

JEP 384 proposes supporting Records in the Java programming language. A record class is implicitly final, so can implement a sealed interface.

It is a compile-time error if a class has neither a sealed direct superclass nor a sealed direct superinterface but is declared non-sealed.

An effect of the sealed keyword is to force all direct subclasses to explicitly declare whether they are final, sealed, or non-sealed. This helps programmers avoid accidentally exposing a sealed class hierarchy to unwanted subclassing.

8.1.4 Superclasses and Subclasses

The optional extends clause in a normal class declaration specifies the direct superclass of the current class.

Superclass:
extends ClassType

The extends clause must not appear in the definition of the class Object, or a compile-time error occurs, because it is the primordial class and has no direct superclass.

The ClassType must name an accessible class type (6.6), or a compile-time error occurs.

It is a compile-time error if the ClassType names a class that is sealed (8.1.1.2) and the class being declared is not a permitted subclass of the named class (8.1.6).

It is a compile-time error if the ClassType names a class that is final, because final classes are not allowed to have subclasses (8.1.1.2).

It is a compile-time error if the ClassType names the class Enum or any invocation of Enum (8.9).

If the ClassType has type arguments, it must denote a well-formed parameterized type (4.5), and none of the type arguments may be wildcard type arguments, or a compile-time error occurs.

Given a (possibly generic) class declaration C<F1,...,Fn> (n 0, C Object), the direct superclass of the class type C<F1,...,Fn> is the type given in the extends clause of the declaration of C if an extends clause is present, or Object otherwise.

Given a generic class declaration C<F1,...,Fn> (n > 0), the direct superclass of the parameterized class type C<T1,...,Tn>, where Ti (1 i n) is a type, is D<U1 θ,...,Uk θ>, where D<U1,...,Uk> is the direct superclass of C<F1,...,Fn> and θ is the substitution [F1:=T1,...,Fn:=Tn].

A class is said to be a direct subclass of its direct superclass. The direct superclass is the class from whose implementation the implementation of the current class is derived.

The subclass relationship is the transitive closure of the direct subclass relationship. A class A is a subclass of class C if either of the following is true:

Class C is said to be a superclass of class A whenever A is a subclass of C.

Example 8.1.4-1. Direct Superclasses and Subclasses

class Point { int x, y; }
final class ColoredPoint extends Point { int color; }
class Colored3DPoint extends ColoredPoint { int z; }  // error

Here, the relationships are as follows:

The declaration of class Colored3dPoint causes a compile-time error because it attempts to extend the final class ColoredPoint.

Example 8.1.4-2. Superclasses and Subclasses

class Point { int x, y; }
class ColoredPoint extends Point { int color; }
final class Colored3dPoint extends ColoredPoint { int z; }

Here, the relationships are as follows:

A class C directly depends on a type T if T is mentioned in the extends or implements clause of C either as a superclass or superinterface, or as a qualifier in the fully qualified form of a superclass or superinterface name.

A class C depends on a reference type T if any of the following is true:

It is a compile-time error if a class depends on itself.

If circularly declared classes are detected at run time, as classes are loaded, then a ClassCircularityError is thrown (12.2.1).

Example 8.1.4-3. Class Depends on Itself

class Point extends ColoredPoint { int x, y; }
class ColoredPoint extends Point { int color; }

This program causes a compile-time error because class Point depends on itself.

8.1.5 Superinterfaces

The optional implements clause in a class declaration lists the names of interfaces that are direct superinterfaces of the class being declared.

Superinterfaces:
implements InterfaceTypeList
InterfaceTypeList:
InterfaceType {, InterfaceType}

Each InterfaceType must name an accessible interface type (6.6), or a compile-time error occurs.

It is a compile-time error if an InterfaceType names an interface that is sealed (9.1.1.3) and the class being declared is not a permitted subclass of the named interface (9.1.4).

If an InterfaceType has type arguments, it must denote a well-formed parameterized type (4.5), and none of the type arguments may be wildcard type arguments, or a compile-time error occurs.

It is a compile-time error if the same interface is mentioned as a direct superinterface more than once in a single implements clause. This is true even if the interface is named in different ways.

Example 8.1.5-1. Illegal Superinterfaces

class Redundant implements java.lang.Cloneable, Cloneable {
    int x;
}

This program results in a compile-time error because the names java.lang.Cloneable and Cloneable refer to the same interface.

Given a (possibly generic) class declaration C<F1,...,Fn> (n 0, C Object), the direct superinterfaces of the class type C<F1,...,Fn> are the types given in the implements clause of the declaration of C, if an implements clause is present.

Given a generic class declaration C<F1,...,Fn> (n > 0), the direct superinterfaces of the parameterized class type C<T1,...,Tn>, where Ti (1 i n) is a type, are all types I<U1 θ,...,Uk θ>, where I<U1,...,Uk> is a direct superinterface of C<F1,...,Fn> and θ is the substitution [F1:=T1,...,Fn:=Tn].

An interface type I is a superinterface of class type C if any of the following is true:

A class can have a superinterface in more than one way.

A class is said to implement all its superinterfaces.

A class may not at the same time be a subtype of two interface types which are different parameterizations of the same generic interface (9.1.2), or a subtype of a parameterization of a generic interface and a raw type naming that same generic interface, or a compile-time error occurs.

This requirement was introduced in order to support translation by type erasure (4.6).

Example 8.1.5-2. Superinterfaces

interface Colorable {
    void setColor(int color);
    int getColor();
}
enum Finish { MATTE, GLOSSY }
interface Paintable extends Colorable {
    void setFinish(Finish finish);
    Finish getFinish();
}

class Point { int x, y; }
class ColoredPoint extends Point implements Colorable {
    int color;
    public void setColor(int color) { this.color = color; }
    public int getColor() { return color; }
}
class PaintedPoint extends ColoredPoint implements Paintable {
    Finish finish;
    public void setFinish(Finish finish) {
        this.finish = finish;
    }
    public Finish getFinish() { return finish; }
}

Here, the relationships are as follows:

The class PaintedPoint has Colorable as a superinterface both because it is a superinterface of ColoredPoint and because it is a superinterface of Paintable.

Example 8.1.5-3. Illegal Multiple Inheritance of an Interface

interface I<T> {}
class B implements I<Integer> {}
class C extends B implements I<String> {}

Class C causes a compile-time error because it attempts to be a subtype of both I<Integer> and I<String>.

Unless the class being declared is abstract, all the abstract member methods of each direct superinterface must be implemented (8.4.8.1) either by a declaration in this class or by an existing method declaration inherited from the direct superclass or a direct superinterface, because a class that is not abstract is not permitted to have abstract methods (8.1.1.1).

Each default method (9.4.3) of a superinterface of the class may optionally be overridden by a method in the class; if not, the default method is typically inherited and its behavior is as specified by its default body.

It is permitted for a single method declaration in a class to implement methods of more than one superinterface.

Example 8.1.5-3. Implementing Methods of a Superinterface

interface Colorable {
    void setColor(int color);
    int getColor();
}
class Point { int x, y; };
class ColoredPoint extends Point implements Colorable {
    int color;
}

This program causes a compile-time error, because ColoredPoint is not an abstract class but fails to provide an implementation of methods setColor and getColor of the interface Colorable.

In the following program:

interface Fish  { int getNumberOfScales(); }
interface Piano { int getNumberOfScales(); }
class Tuna implements Fish, Piano {
    // You can tune a piano, but can you tuna fish?
    public int getNumberOfScales() { return 91; }
}

the method getNumberOfScales in class Tuna has a name, signature, and return type that matches the method declared in interface Fish and also matches the method declared in interface Piano; it is considered to implement both.

On the other hand, in a situation such as this:

interface Fish       { int    getNumberOfScales(); }
interface StringBass { double getNumberOfScales(); }
class Bass implements Fish, StringBass {
    // This declaration cannot be correct,
    // no matter what type is used.
    public ?? getNumberOfScales() { return 91; }
}

it is impossible to declare a method named getNumberOfScales whose signature and return type are compatible with those of both the methods declared in interface Fish and in interface StringBass, because a class cannot have multiple methods with the same signature and different primitive return types (8.4). Therefore, it is impossible for a single class to implement both interface Fish and interface StringBass (8.4.8).

8.1.6 Permitted Direct Subclasses

This is a new subsection. The existing subsection 8.1.6 "Class Body and Member Declarations" is renumbered to 8.1.7.

The optional permits clause in a class declaration lists the classes that are permitted to directly extend a sealed class (8.1.1.2).

PermittedSubclasses
permits TypeName {, TypeName }

It is a compile-time error if a class declaration has a permits clause but the declared class is not sealed.

Every TypeName in the permits clause of a class declaration must denote an accessible class (6.6); otherwise a compile-time error occurs.

It is a compile-time error if a class is named more than once in a single permits clause. This is true even if the class is named in different ways.

It is a compile-time error if the declaration of a class C names a class in its permits clause, but C is not the direct superclass (8.1.4) of the named class.

If a sealed class C belongs to a named module, then every class named in the permits clause of the declaration of C must belong to the same module than C; otherwise a compile-time error occurs.

If a sealed class C belongs to an unnamed module, then every class named in the permits clause of the declaration of C must belong to the same package than C; otherwise or a compile-time error occurs.

Sealed class hierarchies are not intended to be declared across different maintenance domains. Modules cannot depend on each other in a circular fashion, yet a sealed class and its direct subclasses need to refer to each other in a circular fashion (in permits and extends clauses, respectively). Necessarily, therefore, a sealed class and its direct subclasses must co-exist in the same module. In an unnamed module, a sealed class and its direct subclasses must belong to the same package.

The permitted direct subclasses of a sealed class C are the classes listed by its permits clause, or, if C lacks a permits clause, each top level or nested class declared in the same compilation unit as C (7.3) whose direct superclass is C.

It is a compile-time error if the declaration of a sealed class C lacks a permits clause and no top level or nested class declared in the same compilation unit as C names C as its direct superclass.

8.1.6 8.1.7 Class Body and Member Declarations

A class body may contain declarations of members of the class, that is, fields (8.3), methods (8.4), classes (8.5), and interfaces (8.5).

A class body may also contain instance initializers (8.6), static initializers (8.7), and declarations of constructors (8.8) for the class.

ClassBody:
{ {ClassBodyDeclaration} }
ClassBodyDeclaration:
ClassMemberDeclaration
InstanceInitializer
StaticInitializer
ConstructorDeclaration
ClassMemberDeclaration:
FieldDeclaration
MethodDeclaration
ClassDeclaration
InterfaceDeclaration
;

The scope and shadowing of a declaration of a member m declared in or inherited by a class type C is specified in 6.3 and 6.4.

If C itself is a nested class, there may be definitions of the same kind (variable, method, or type) and name as m in enclosing scopes. (The scopes may be blocks, classes, or packages.) In all such cases, the member m declared in or inherited by C shadows (6.4.1) the other definitions of the same kind and name.

8.2 Class Members

The members of a class type are all of the following:

...

8.5 Member Type Declarations

A member class is a class whose declaration is directly enclosed in the body of another class or interface declaration (8.1.6 8.1.7, 9.1.4 9.1.5).

A member interface is an interface whose declaration is directly enclosed in the body of another class or interface declaration (8.1.6 8.1.7, 9.1.4 9.1.5).

...

8.9 Enum Types

An enum declaration specifies a new enum type, a special kind of class type.

EnumDeclaration:
{ClassModifier} enum TypeIdentifier [Superinterfaces] EnumBody

It is a compile-time error if an enum declaration has the modifier abstract, or final, sealed or non-sealed.

An enum declaration is implicitly final unless it contains at least one enum constant that has a class body (8.9.1).

An enum declaration is either implicitly final or implicitly sealed, as follows:

A nested enum type is implicitly static. It is permitted for the declaration of a nested enum type to redundantly specify the static modifier.

This implies that it is impossible to declare an enum type in the body of an inner class (8.1.3), because an inner class cannot have static members except for constant variables.

It is a compile-time error if the same keyword appears more than once as a modifier for an enum declaration, or if an enum declaration has more than one of the access modifiers public, protected, and private (6.6).

The direct superclass of an enum type E is Enum<E> (8.1.4).

An enum type has no instances other than those defined by its enum constants. It is a compile-time error to attempt to explicitly instantiate an enum type (15.9.1).

In addition to the compile-time error, three further mechanisms ensure that no instances of an enum type exist beyond those defined by its enum constants:

8.9.1 Enum Constants

The body of an enum declaration may contain enum constants. An enum constant defines an instance of the enum type.

EnumBody:
{ [EnumConstantList] [,] [EnumBodyDeclarations] }
EnumConstantList:
EnumConstant {, EnumConstant}
EnumConstant:
{EnumConstantModifier} Identifier [( [ArgumentList] )] [ClassBody]
EnumConstantModifier:
Annotation

The following production from 15.12 is shown here for convenience:

ArgumentList:
Expression {, Expression}

The rules for annotation modifiers on an enum constant declaration are specified in 9.7.4 and 9.7.5.

The Identifier in a EnumConstant may be used in a name to refer to the enum constant.

The scope and shadowing of an enum constant is specified in 6.3 and 6.4.

An enum constant may be followed by arguments, which are passed to the constructor of the enum when the constant is created during class initialization as described later in this section. The constructor to be invoked is chosen using the normal rules of overload resolution (15.12.2). If the arguments are omitted, an empty argument list is assumed.

The optional class body of an enum constant implicitly defines an anonymous class declaration (15.9.5) that extends the immediately enclosing enum type. The optional class body of an enum constant implicitly declares an anonymous class (15.9.5) that (i) is final, and (ii) extends the immediately enclosing sealed enum type. The class body is governed by the usual rules of anonymous classes; in particular it cannot contain any constructors. Instance methods declared in these class bodies may be invoked outside the enclosing enum type only if they override accessible methods in the enclosing enum type (8.4.8).

It is a compile-time error for the class body of an enum constant to declare an abstract method.

Because there is only one instance of each enum constant, it is permitted to use the == operator in place of the equals method when comparing two object references if it is known that at least one of them refers to an enum constant.

The equals method in Enum is a final method that merely invokes super.equals on its argument and returns the result, thus performing an identity comparison.

8.9.2 Enum Body Declarations

In addition to enum constants, the body of an enum declaration may contain constructor and member declarations as well as instance and static initializers.

EnumBodyDeclarations:
; {ClassBodyDeclaration}

The following productions from 8.1.6 8.1.7 are shown here for convenience:

ClassBodyDeclaration:
ClassMemberDeclaration
InstanceInitializer
StaticInitializer
ConstructorDeclaration
ClassMemberDeclaration:
FieldDeclaration
MethodDeclaration
ClassDeclaration
InterfaceDeclaration
;

...

Chapter 9: Interfaces

An interface declaration introduces a new reference type whose members are classes, interfaces, constants, and methods. This type has no instance variables, and typically declares one or more abstract methods; otherwise unrelated classes can implement the interface by providing implementations for its abstract methods. Interfaces may not be directly instantiated.

A nested interface is any interface whose declaration occurs within the body of another class or interface.

A top level interface is an interface that is not a nested interface.

We distinguish between two kinds of interfaces - normal interfaces and annotation types.

This chapter discusses the common semantics of all interfaces - normal interfaces, both top level (7.6) and nested (8.5, 9.5), and annotation types (9.6). Details that are specific to particular kinds of interfaces are discussed in the sections dedicated to these constructs.

Programs can use interfaces to make it unnecessary for related classes to share a common abstract superclass or to add methods to Object.

An interface may be declared to be a direct extension of one or more other interfaces, meaning that it inherits all the member types, instance methods, and constants of the interfaces it extends, except for any members that it may override or hide.

A class may be declared to directly implement one or more interfaces, meaning that any instance of the class implements all the abstract methods specified by the interface or interfaces. A class necessarily implements all the interfaces that its direct superclasses and direct superinterfaces do. This (multiple) interface inheritance allows objects to support (multiple) common behaviors without sharing a superclass.

Unlike a class, an interface cannot be declared final. However, an interface may be declared sealed (9.1.1.3), in which case it specifies a fixed set of classes and interfaces that may directly implement or extend the sealed interface.

A variable whose declared type is an interface type may have as its value a reference to any instance of a class which implements the specified interface. It is not sufficient that the class happen to implement all the abstract methods of the interface; the class or one of its superclasses must actually be declared to implement the interface, or else the class is not considered to implement the interface.

9.1 Interface Declarations

An interface declaration specifies a new named reference type. There are two kinds of interface declarations - normal interface declarations and annotation type declarations (9.6).

InterfaceDeclaration:
NormalInterfaceDeclaration
AnnotationTypeDeclaration
NormalInterfaceDeclaration:
{InterfaceModifier} interface TypeIdentifier [TypeParameters]
[ExtendsInterfaces] [PermittedSubclassesAndSubinterfaces]
InterfaceBody

The TypeIdentifier in an interface declaration specifies the name of the interface.

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

The scope and shadowing of an interface declaration is specified in 6.3 and 6.4.

9.1.1 Interface Modifiers

An interface declaration may include interface modifiers.

InterfaceModifier:
(one of)
Annotation public protected private
abstract static sealed non-sealed strictfp

The rules for annotation modifiers on an interface declaration are specified in 9.7.4 and 9.7.5.

The access modifier public (6.6) pertains to every kind of interface declaration.

The access modifiers protected and private pertain only to member interfaces whose declarations are directly enclosed by a class declaration (8.5.1).

The modifier static pertains only to member interfaces (8.5.1, 9.5), not to top level interfaces (7.6).

It is a compile-time error if the same keyword appears more than once as a modifier for an interface declaration, or if a interface declaration has more than one of the access modifiers public, protected, and private (6.6).

It is a compile-time error if an interface is declared both sealed and non-sealed.

If two or more (distinct) interface modifiers appear in an interface declaration, then it is customary, though not required, that they appear in the order consistent with that shown above in the production for InterfaceModifier.

9.1.1.3 sealed Interfaces

An interface can be declared sealed when it is useful to restrict its direct subclasses and subinterfaces to a fixed set of classes and interfaces. A sealed interface permits a given set of direct subclasses and subinterfaces (9.1.4).

It is a compile-time error if an interface has a sealed direct superinterface and is not declared sealed or non-sealed.

It is a compile-time error if an interface has no sealed direct superinterface but is declared non-sealed.

9.1.3 Superinterfaces and Subinterfaces

If an extends clause is provided, then the interface being declared extends each of the other named interfaces and therefore inherits the member types, instance methods, and constants of each of the other named interfaces.

These other named interfaces are the direct superinterfaces of the interface being declared.

Any class that implements the declared interface is also considered to implement all the interfaces that this interface extends.

ExtendsInterfaces:
extends InterfaceTypeList

The following production from 8.1.5 is shown here for convenience:

InterfaceTypeList:
InterfaceType {, InterfaceType}

Each InterfaceType in the extends clause of an interface declaration must name an accessible interface type (6.6), or a compile-time error occurs.

It is a compile-time error if an InterfaceType names an interface that is sealed (9.1.1.3) and the interface being declared is not a permitted subinterface of the named interface (9.1.4).

If an InterfaceType has type arguments, it must denote a well-formed parameterized type (4.5), and none of the type arguments may be wildcard type arguments, or a compile-time error occurs.

Given a (possibly generic) interface declaration I<F1,...,Fn> (n 0), the direct superinterfaces of the interface type I<F1,...,Fn> are the types given in the extends clause of the declaration of I, if an extends clause is present.

Given a generic interface declaration I<F1,...,Fn> (n > 0), the direct superinterfaces of the parameterized interface type I<T1,...,Tn>, where Ti (1 i n) is a type, are all types J<U1 θ,...,Uk θ>, where J<U1,...,Uk> is a direct superinterface of I<F1,...,Fn> and θ is the substitution [F1:=T1,...,Fn:=Tn].

The superinterface relationship is the transitive closure of the direct superinterface relationship. An interface K is a superinterface of interface I if either of the following is true:

Interface I is said to be a subinterface of interface K whenever K is a superinterface of I.

While every class is an extension of class Object, there is no single interface of which all interfaces are extensions.

An interface I directly depends on a type T if T is mentioned in the extends clause of I either as a superinterface or as a qualifier in the fully qualified form of a superinterface name.

An interface I depends on a reference type T if any of the following is true:

It is a compile-time error if an interface depends on itself.

If circularly declared interfaces are detected at run time, as interfaces are loaded, then a ClassCircularityError is thrown (12.2.1).

9.1.4 Permitted Direct Subclasses and Subinterfaces

This is a new subsection. The existing subsection 9.1.4 "Interface Body and Member Declarations" is renumbered to 9.1.5.

The optional permits clause in an interface declaration lists the classes and interfaces that are permitted to directly implement or extend a sealed interface (9.1.1.3).

PermittedSubclassesAndSubinterfaces
permits TypeName {, TypeName }

It is a compile-time error if an interface declaration has a permits clause but the declared interface is not sealed.

Every TypeName in the permits clause of an interface declaration must denote an accessible class or interface (6.6); otherwise a compile-time error occurs.

It is a compile-time error if a class or interface is named more than once in a single permits clause. This is true even if the class or interface is named in different ways.

It is a compile-time error if the declaration of an interface I names a class or interface in its permits clause, but I is not a direct superinterface (8.1.5, 9.1.3) of the named class or interface.

If a sealed interface I belongs to a named module, then every class or interface named in the permits clause of the declaration of I must belong to the same module as I; otherwise a compile-time error occurs.

If a sealed interface I belongs to an unnamed module, then every class or interface named in the permits clause of the declaration of I must belong to the same package as I; otherwise a compile-time error occurs.

Sealed interface hierarchies are not intended to be declared across different maintenance domains. Modules cannot depend on each other in a circular fashion, yet a sealed interface and its direct subclasses and subinterfaces need to refer to each other in a circular fashion (in permits, implements and extends clauses, respectively). Necessarily, therefore, a sealed interface and its direct subclasses and subinterfaces must co-exist in the same module. In an unnamed module, a sealed interface and its direct subclasses and subinterfaces must belong to the same package.

The permitted direct subclasses and subinterfaces of a selaed interface I are the classes and interfaces listed by its permits clause, or, if I lacks a permits clause, each top level or nested class or interface declared in the same compilation unit as I (7.3) whose direct superinterfaces include I.

It is a compile-time error if the declaration of a sealed interface I lacks a permits clause and no top level or nested class or interface declared in the same compilation unit as I names I as a direct superinterface.

9.1.4 9.1.5 Interface Body and Member Declarations

The body of an interface may declare members of the interface, that is, fields (9.3), methods (9.4), classes (9.5), and interfaces (9.5).

InterfaceBody:
{ {InterfaceMemberDeclaration} }
InterfaceMemberDeclaration:
ConstantDeclaration
InterfaceMethodDeclaration
ClassDeclaration
InterfaceDeclaration
;

The scope of a declaration of a member m declared in or inherited by an interface type I is specified in 6.3.

9.2 Interface Members

The members of an interface type are:

...

9.6 Annotation Types

An annotation type declaration specifies a new annotation type, a special kind of interface type. To distinguish an annotation type declaration from a normal interface declaration, the keyword interface is preceded by an at-sign (@).

AnnotationTypeDeclaration:
{InterfaceModifier} @ interface TypeIdentifier AnnotationTypeBody

Note that the at-sign (@) and the keyword interface are distinct tokens. It is possible to separate them with whitespace, but this is discouraged as a matter of style.

The rules for annotation modifiers on an annotation type declaration are specified in 9.7.4 and 9.7.5.

It is a compile-time error if an annotation type declaration has the modifier sealed (9.1.1.3).

The TypeIdentifier in an annotation type declaration specifies the name of the annotation type.

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

The direct superinterface of every annotation type is java.lang.annotation.Annotation.

By virtue of the AnnotationTypeDeclaration syntax, an annotation type declaration cannot be generic, and no extends clause is permitted.

A consequence of the fact that an annotation type cannot explicitly declare a superclass or superinterface is that a subclass or subinterface of an annotation type is never itself an annotation type. Similarly, java.lang.annotation.Annotation is not itself an annotation type.

An annotation type inherits several members from java.lang.annotation.Annotation, including the implicitly declared methods corresponding to the instance methods of Object, yet these methods do not define elements of the annotation type (9.6.1).

Because these methods do not define elements of the annotation type, it is illegal to use them in annotations of that type (9.7). Without this rule, we could not ensure that elements were of the types representable in annotations, or that accessor methods for them would be available.

Unless explicitly modified herein, all of the rules that apply to normal interface declarations apply to annotation type declarations.

For example, annotation types share the same namespace as normal class and interface types; and annotation type declarations are legal wherever interface declarations are legal, and have the same scope and accessibility.

9.8 Functional Interfaces

A functional interface is an interface that is not declared sealed that has just one abstract method (aside from the methods of Object), and thus represents a single function contract. This "single" method may take the form of multiple abstract methods with override-equivalent signatures inherited from superinterfaces; in this case, the inherited methods logically represent a single method.

For an interface I that is not declared sealed, let M be the set of abstract methods that are members of I that do not have the same signature as any public instance method of the class Object (4.3.2). Then, I is a functional interface if there exists a method m in M for which both of the following are true:

In addition to the usual process of creating an interface instance by declaring and instantiating a class (15.9), instances of functional interfaces can be created with method reference expressions and lambda expressions (15.13, 15.27).

The definition of functional interface excludes methods in an interface that are also public methods in Object. This is to allow functional treatment of an interface like java.util.Comparator<T> that declares multiple abstract methods of which only one is really "new" - int compare(T,T). The other - boolean equals(Object) - is an explicit declaration of an abstract method that would otherwise be implicitly declared in the interface (9.2) and automatically implemented by every class that implements the interface.

Note that if non-public methods of Object, such as clone(), are explicitly declared in an interface as public, they are not automatically implemented by every class that implements the interface. The implementation inherited from Object is protected while the interface method is public, so the only way to implement the interface would be for a class to override the non-public Object method with a public method.

Example 9.8-1. Functional Interfaces

A simple example of a functional interface is:

interface Runnable {
    void run();
}

The following interface is not functional because it declares nothing which is not already a member of Object:

interface NonFunc {
    boolean equals(Object obj);
}

However, its subinterface can be functional by declaring an abstract method which is not a member of Object:

interface Func extends NonFunc {
    int compare(String o1, String o2);
}

Similarly, the well known interface java.util.Comparator<T> is functional because it has one abstract non-Object method:

interface Comparator<T> {
    boolean equals(Object obj);
    int compare(T o1, T o2);
}

The following interface is not functional because while it only declares one abstract method which is not a member of Object, it declares two abstract methods which are not public members of Object:

interface Foo {
    int m();
    Object clone();
}

Example 9.8-2. Functional Interfaces and Erasure

In the following interface hierarchy, Z is a functional interface because while it inherits two abstract methods which are not members of Object, they have the same signature, so the inherited methods logically represent a single method:

interface X { int m(Iterable<String> arg); }
interface Y { int m(Iterable<String> arg); }
interface Z extends X, Y {}

Similarly, Z is a functional interface in the following interface hierarchy because Y.m is a subsignature of X.m and is return-type-substitutable for X.m:

interface X { Iterable m(Iterable<String> arg); }
interface Y { Iterable<String> m(Iterable arg); }
interface Z extends X, Y {}

The definition of functional interface respects the fact that an interface cannot have two members which are not subsignatures of each other, yet have the same erasure (9.4.1.2). Thus, in the following three interface hierarchies where Z causes a compile-time error, Z is not a functional interface: (because none of its abstract members are subsignatures of all other abstract members)

interface X { int m(Iterable<String> arg); }
interface Y { int m(Iterable<Integer> arg); }
interface Z extends X, Y {}

interface X { int m(Iterable<String> arg, Class c); }
interface Y { int m(Iterable arg, Class<?> c); }
interface Z extends X, Y {}

interface X<T> { void m(T arg); }
interface Y<T> { void m(T arg); }
interface Z<A, B> extends X<A>, Y<B> {}

Similarly, the definition of "functional interface" respects the fact that an interface may only have methods with override-equivalent signatures if one is return-type-substitutable for all the others. Thus, in the following interface hierarchy where Z causes a compile-time error, Z is not a functional interface: (because none of its abstract members are return-type-substitutable for all other abstract members)

interface X { long m(); }
interface Y { int  m(); }
interface Z extends X, Y {}

In the following example, the declarations of Foo<T,N> and Bar are legal: in each, the methods called m are not subsignatures of each other, but do have different erasures. Still, the fact that the methods in each are not subsignatures means Foo<T,N> and Bar are not functional interfaces. However, Baz is a functional interface because the methods it inherits from Foo<Integer,Integer> have the same signature and so logically represent a single method.

interface Foo<T, N extends Number> {
    void m(T arg);
    void m(N arg);
}
interface Bar extends Foo<String, Integer> {}
interface Baz extends Foo<Integer, Integer> {}

Finally, the following examples demonstrate the same rules as above, but with generic methods:

interface Exec { <T> T execute(Action<T> a); }
  // Functional

interface X { <T> T execute(Action<T> a); }
interface Y { <S> S execute(Action<S> a); }
interface Exec extends X, Y {}
  // Functional: signatures are logically "the same"

interface X { <T>   T execute(Action<T> a); }
interface Y { <S,T> S execute(Action<S> a); }
interface Exec extends X, Y {}
  // Error: different signatures, same erasure

Example 9.8-3. Generic Functional Interfaces

Functional interfaces can be generic, such as java.util.function.Predicate<T>. Such a functional interface may be parameterized in a way that produces distinct abstract methods - that is, multiple methods that cannot be legally overridden with a single declaration. For example:

interface I    { Object m(Class c); }
interface J<S> { S m(Class<?> c); }
interface K<T> { T m(Class<?> c); }
interface Functional<S,T> extends I, J<S>, K<T> {}

Functional<S,T> is a functional interface - I.m is return-type-substitutable for J.m and K.m - but the functional interface type Functional<String,Integer> clearly cannot be implemented with a single method. However, other parameterizations of Functional<S,T> which are functional interface types are possible.

The declaration of a functional interface allows a functional interface type to be used in a program. There are four kinds of functional interface type:

In special circumstances, it is useful to treat an intersection type as a functional interface type. Typically, this will look like an intersection of a functional interface type with one or more marker interface types, such as Runnable & java.io.Serializable. Such an intersection can be used in casts (15.16) that force a lambda expression to conform to a certain type. If one of the interface types in the intersection is java.io.Serializable, special run-time support for serialization is triggered (15.27.4).

Chapter 13: Binary Compatibility

13.4 Evolution of Classes

13.4.2 sealed, non-sealed and final Classes

The discussion about final classes has been moved to a new subsection 13.4.2.3.

13.4.2.1 sealed Classes

If a class that was not declared sealed is changed to be declared sealed, then an IncompatibleClassChangeError is thrown if a binary of a pre-existing subclass of this class is loaded that is not contained in its permits clause; such a change is not recommended for widely distributed classes.

Changing a class that is declared sealed to no longer be declared sealed does not break compatibility with pre-existing binaries.

13.4.2.2 non-sealed Classes

Changing a class that is not declared non-sealed to be declared non-sealed does not break compatibility with pre-existing binaries.

Changing a class that is declared non-sealed to no longer be declared non-sealed does not break compatibility with pre-existing binaries.

13.4.2.3 final Classes

If a class that was not declared final is changed to be declared final, then a VerifyError is thrown if a binary of a pre-existing subclass of this class is loaded, because final classes can have no subclasses; such a change is not recommended for widely distributed classes.

Changing a class that is declared final to no longer be declared final does not break compatibility with pre-existing binaries.

13.4.4 Superclasses and Superinterfaces

A ClassCircularityError is thrown at load time if a class would be a superclass of itself. Changes to the class hierarchy that could result in such a circularity when newly compiled binaries are loaded with pre-existing binaries are not recommended for widely distributed classes.

Changing the direct superclass or the set of direct superinterfaces of a class type will not break compatibility with pre-existing binaries, provided that the total set of superclasses or superinterfaces, respectively, of the class type loses no members.

If a change to the direct superclass or the set of direct superinterfaces results in any class or interface no longer being a superclass or superinterface, respectively, then linkage errors may result if pre-existing binaries are loaded with the binary of the modified class. Such changes are not recommended for widely distributed classes.

Example 13.4.4-1. Changing A Superclass

Suppose that the following test program:

class Hyper { char h = 'h'; } 
class Super extends Hyper { char s = 's'; }
class Test extends Super {
    public static void printH(Hyper h) {
        System.out.println(h.h);
    }
    public static void main(String[] args) {
        printH(new Super());
    }
}

is compiled and executed, producing the output:

h

Suppose that a new version of class Super is then compiled:

class Super { char s = 's'; }

This version of class Super is not a subclass of Hyper. If we then run the existing binaries of Hyper and Test with the new version of Super, then a VerifyError is thrown at link time. The verifier objects because the result of new Super() cannot be passed as an argument in place of a formal parameter of type Hyper, because Super is not a subclass of Hyper.

It is instructive to consider what might happen without the verification step: the program might run and print:

s

This demonstrates that without the verifier, the Java type system could be defeated by linking inconsistent binary files, even though each was produced by a correct Java compiler.

The lesson is that an implementation that lacks a verifier or fails to use it will not maintain type safety and is, therefore, not a valid implementation.

The requirement that alternatives in a multi-catch clause (14.20) not be subclasses or superclasses of each other is only a source restriction. Assuming the following client code is legal:

try {
    throwAorB();
} catch(ExceptionA | ExceptionB e) {
    ...
}

where ExceptionA and ExceptionB do not have a subclass/superclass relationship when the client is compiled, it is binary compatible with respect to the client for ExceptionA and ExceptionB to have such a relationship when the client is executed.

This is analogous to other situations where a class transformation that is binary compatible for a client might not be source compatible for the same client.

Changing the set of permitted direct subclasses of a class (8.1.6) or permitted direct subclasses and subinterfaces of an interface (9.1.4) may result in linkage errors if pre-existing binaries are loaded with the binary of the modified class or interface. Such changes are not recommended for widely distributed classes or interfaces.

13.5 Evolution of Interfaces

Existing sections 13.5.2 - 13.5.7 are renumbered as 13.5.3 - 13.5.8.

13.5.2 sealed and non-sealed Interfaces

If an interface that was not declared sealed is changed to be declared sealed, then an IncompatibleClassChangeError is thrown if a binary of a pre-existing subclass or subinterface of this interface is loaded that is not contained in its permits clause; such a change is not recommended for widely distributed classes.

Changing an interface that is declared sealed to no longer be declared sealed does not break compatibility with pre-existing binaries.

Changing an interface that is not declared non-sealed to be declared non-sealed does not break compatibility with pre-existing binaries.

Changing an interface that is declared non-sealed to no longer be declared non-sealed does not break compatibility with pre-existing binaries.

Chapter 14: Blocks and Statements

14.3 Local Class Declarations

A local class is a nested class (8) that is not a member of any class and that has a name (6.2, 6.7).

All local classes are inner classes (8.1.3).

Every local class declaration statement is immediately contained by a block (14.2). Local class declaration statements may be intermixed freely with other kinds of statements in the block.

It is a compile-time error if a local class declaration contains any of the access modifiers public, protected, or private (6.6), or the modifier any of the modifiers static (8.1.1), sealed or non-sealed (8.1.1.2).

It is a compile-time error if the direct superclass or direct superinterface of a local class is sealed (8.1.1.2).

The scope and shadowing of a local class declaration is specified in 6.3 and 6.4.

Example 14.3-1. Local Class Declarations

Here is an example that illustrates several aspects of the rules given above:

class Global {
    class Cyclic {}

    void foo() {
        new Cyclic(); // create a Global.Cyclic
        class Cyclic extends Cyclic {} // circular definition

        {
            class Local {}
            {
                class Local {} // compile-time error
            }
            class Local {} // compile-time error
            class AnotherLocal {
                void bar() {
                    class Local {} // ok
                }
            }
        }
        class Local {} // ok, not in scope of prior Local
    }
}

The first statement of method foo creates an instance of the member class Global.Cyclic rather than an instance of the local class Cyclic, because the statement appears prior to the scope of the local class declaration.

The fact that the scope of a local class declaration encompasses its whole declaration (not only its body) means that the definition of the local class Cyclic is indeed cyclic because it extends itself rather than Global.Cyclic. Consequently, the declaration of the local class Cyclic is rejected at compile time.

Since local class names cannot be redeclared within the same method (or constructor or initializer, as the case may be), the second and third declarations of Local result in compile-time errors. However, Local can be redeclared in the context of another, more deeply nested, class such as AnotherLocal.

The final declaration of Local is legal, since it occurs outside the scope of any prior declaration of Local.

Chapter 15: Expressions

15.9 Class Instance Creation Expressions

15.9.1 Determining the Class being Instantiated

If ClassOrInterfaceTypeToInstantiate ends with TypeArguments (rather than <>), then ClassOrInterfaceTypeToInstantiate must denote a well-formed parameterized type (4.5), or a compile-time error occurs.

If ClassOrInterfaceTypeToInstantiate ends with <>, but the type denoted by the Identifier in ClassOrInterfaceTypeToInstantiate is not generic, then a compile-time error occurs.

If the class instance creation expression ends in a class body, then the class being instantiated is an anonymous class. Then:

If a class instance creation expression does not declare an anonymous class, then:

15.9.5 Anonymous Class Declarations

An anonymous class is implicitly declared by a class instance creation expression and also by an enum constant with a class body (8.9.1).

An anonymous class is never abstract (8.1.1.1).

An anonymous class is never final (8.1.1.2) unless it is declared by an enum constant with a class body (8.9.1).

An anonymous class is never sealed (8.1.1.2).

The fact that an anonymous class is not final is relevant in casting, in particular the narrowing reference conversion allowed for the cast operator (5.5). It is also of interest in subclassing, in that it is impossible to declare a subclass of an anonymous class, despite an anonymous class being non-final, because an anonymous class cannot be named by an extends clause (8.1.4).

An anonymous class is always an inner class (8.1.3); it is never static (8.1.1, 8.5.1).

The superclass or superinterface of an anonymous class is given by the class instance creation expression (15.9.1), with type arguments inferred as necessary while choosing a constructor (15.9.3).

If the class instance creation expression uses <> with an anonymous class, then for all non-private methods declared in the anonymous class body, it is as if the method declaration is annotated with @Override (9.6.4.4).

When <> is used, the inferred type arguments may not be as anticipated by the programmer. Consequently, the supertype of the anonymous class may not be as anticipated, and methods declared in the anonymous class may not override supertype methods as intended. Treating such methods as if annotated with @Override (if they are not explicitly annotated with @Override) helps avoid silently incorrect programs.