Java SE >Java SE Specifications >Java Language Specification
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Programs are organized as sets of packages. Each package has its own set of names for types, which helps to prevent name conflicts.
A top level type is accessible (§6.6) outside the package that declares it only if the type is declaredpublic.
The naming structure for packages is hierarchical (§7.1). The members of a package are class and interface types (§7.6), which are declared in compilation units of the package, and subpackages, which may contain compilation units and subpackages of their own.
A package can be stored in a file system or in a database (§7.2). Packages that are stored in a file system may have certain constraints on the organization of their compilation units to allow a simple implementation to find classes easily.
A package consists of a number of compilation units (§7.3). A compilation unit automatically has access to all types declared in its package and also automatically imports all of thepublic types declared in the predefined packagejava.lang.
For small programs and casual development, a package can be unnamed (§7.4.2) or have a simple name, but if code is to be widely distributed, unique package names should be chosen using qualified names. This can prevent the conflicts that would otherwise occur if two development groups happened to pick the same package name and these packages were later to be used in a single program.
The members of a package are its subpackages and all the top level class types (§7.6,§8 (Classes)) and top level interface types (§9 (Interfaces)) declared in all the compilation units (§7.3) of the package.
For example, in the Java SE platform API:
The packagejava has subpackagesawt,applet,io,lang,net, andutil, but no compilation units.
The packagejava.awt has a subpackage namedimage, as well as a number of compilation units containing declarations of class and interface types.
If the fully qualified name (§6.7) of a package isP, andQ is a subpackage ofP, thenP.Q is the fully qualified name of the subpackage, and furthermore denotes a package.
A package may not contain two members of the same name, or a compile-time error results.
Here are some examples:
Because the packagejava.awt has a subpackageimage, it cannot (and does not) contain a declaration of a class or interface type namedimage.
If there is a package namedmouse and a member typeButton in that package (which then might be referred to asmouse.Button), then there cannot be any package with the fully qualified namemouse.Button ormouse.Button.Click.
Ifcom.nighthacks.java.jag is the fully qualified name of a type, then there cannot be any package whose fully qualified name is eithercom.nighthacks.java.jag orcom.nighthacks.java.jag.scrabble.
It is however possible for members of different packages to have the same simple name. For example, it is possible to declare a package:
package vector;public class Vector { Object[] vec; }that has as a member apublic class namedVector, even though the packagejava.util also declares a class namedVector. These two class types are different, reflected by the fact that they have different fully qualified names (§6.7). The fully qualified name of this exampleVector isvector.Vector, whereasjava.util.Vector is the fully qualified name of theVector class included in the Java SE platform. Because the packagevector contains a class namedVector, it cannot also have a subpackage namedVector.
The hierarchical naming structure for packages is intended to be convenient for organizing related packages in a conventional manner, but has no significance in itself other than the prohibition against a package having a subpackage with the same simple name as a top level type (§7.6) declared in that package.
For example, there is no special access relationship between a package namedoliver and another package namedoliver.twist, or between packages namedevelyn.wood andevelyn.waugh. That is, the code in a package namedoliver.twist has no better access to the types declared within packageoliver than code in any other package.
Each host system determines how packages and compilation units are created and stored.
Each host system also determines which compilation units are observable (§7.3) in a particular compilation. The observability of compilation units in turn determines which packages are observable, and which packages are in scope.
In simple implementations of the Java SE platform, packages and compilation units may be stored in a local file system. Other implementations may store them using a distributed file system or some form of database.
If a host system stores packages and compilation units in a database, then the database must not impose the optional restrictions (§7.6) on compilation units permissible in file-based implementations.
For example, a system that uses a database to store packages may not enforce a maximum of one public class or interface per compilation unit.
Systems that use a database must, however, provide an option to convert a program to a form that obeys the restrictions, for purposes of export to file-based implementations.
As an extremely simple example of storing packages in a file system, all the packages and source and binary code in a project might be stored in a single directory and its subdirectories. Each immediate subdirectory of this directory would represent a top level package, that is, one whose fully qualified name consists of a single simple name. Each further level of subdirectory would represent a subpackage of the package represented by the containing directory, and so on.
The directory might contain the following immediate subdirectories:
comglsjagjavawnj
where directoryjava would contain the Java SE platform packages; the directoriesjag,gls, andwnj might contain packages that three of the authors of this specification created for their personal use and to share with each other within this small group; and the directorycom would contain packages procured from companies that used the conventions described in§6.1 to generate unique names for their packages.
Continuing the example, the directoryjava would contain, among others, the following subdirectories:
appletawtiolangnetutil
corresponding to the packagesjava.applet,java.awt,java.io,java.lang,java.net, andjava.util that are defined as part of the Java SE platform API.
Still continuing the example, if we were to look inside the directoryutil, we might see the following files:
BitSet.java Observable.javaBitSet.class Observable.classDate.java Observer.javaDate.class Observer.class...
where each of the.java files contains the source for a compilation unit (§7.3) that contains the definition of a class or interface whose binary compiled form is contained in the corresponding.class file.
Under this simple organization of packages, an implementation of the Java SE platform would transform a package name into a pathname by concatenating the components of the package name, placing a file name separator (directory indicator) between adjacent components.
For example, if this simple organization were used on an operating system where the file name separator is/, the package name:
jag.scrabble.board
would be transformed into the directory name:
jag/scrabble/board
A package name component or class name might contain a character that cannot correctly appear in a host file system's ordinary directory name, such as a Unicode character on a system that allows only ASCII characters in file names. As a convention, the character can be escaped by using, say, the@ character followed by four hexadecimal digits giving the numeric value of the character, as in the\uxxxx escape (§3.3).
Under this convention, the package name:
children.activities.crafts.papierM\u00e2ch\u00e9
which can also be written using full Unicode as:
children.activities.crafts.papierMâché
might be mapped to the directory name:
children/activities/crafts/papierM@00e2ch@00e9
If the@ character is not a valid character in a file name for some given host file system, then some other character that is not valid in a identifier could be used instead.
CompilationUnit is the goal symbol (§2.1) for the syntactic grammar (§2.3) of Java programs. It is defined by the following productions:
Acompilation unit consists of three parts, each of which is optional:
Apackage declaration (§7.4), giving the fully qualified name (§6.7) of the package to which the compilation unit belongs.
A compilation unit that has nopackage declaration is part of an unnamed package (§7.4.2).
import declarations (§7.5) that allow types from other packages andstatic members of types to be referred to using their simple names.
Top level type declarations (§7.6) of class and interface types.
Every compilation unit implicitly imports everypublic type name declared in the predefined packagejava.lang, as if the declarationimport java.lang.*; appeared at the beginning of each compilation unit immediately after anypackage statement. As a result, the names of all those types are available as simple names in every compilation unit.
All the compilation units of the predefined packagejava and its subpackageslang andio are alwaysobservable.
For all other packages, the host system determines which compilation units are observable.
The observability of a compilation unit influences the observability of its package (§7.4.3).
Types declared in different compilation units can depend on each other, circularly. A Java compiler must arrange to compile all such types at the same time.
Apackage declaration appears within a compilation unit to indicate the package to which the compilation unit belongs.
Apackage declaration in a compilation unit specifies the name (§6.2) of the package to which the compilation unit belongs.
The package name mentioned in apackage declaration must be the fully qualified name of the package (§6.7).
The scope and shadowing of a package declaration is specified in§6.3 and§6.4.
The rules for annotation modifiers on a package declaration are specified in§9.7.4 and§9.7.5.
At most one annotatedpackage declaration is permitted for a given package.
The manner in which this restriction is enforced must, of necessity, vary from implementation to implementation. The following scheme is strongly recommended for file-system-based implementations: The sole annotatedpackage declaration, if it exists, is placed in a source file calledpackage-info.java in the directory containing the source files for the package. This file does not contain the source for a class calledpackage-info.java; indeed it would be illegal for it to do so, aspackage-info is not a legal identifier. Typicallypackage-info.java contains only apackage declaration, preceded immediately by the annotations on the package. While the file could technically contain the source code for one or more classes with package access, it would be very bad form.
It is recommended thatpackage-info.java, if it is present, take the place ofpackage.html forjavadoc and other similar documentation generation systems. If this file is present, the documentation generation tool should look for the package documentation comment immediately preceding the (possibly annotated)package declaration inpackage-info.java. In this way,package-info.java becomes the sole repository for package-level annotations and documentation. If, in future, it becomes desirable to add any other package-level information, this file should prove a convenient home for this information.
A compilation unit that has nopackage declaration is part of anunnamed package.
Unnamed packages are provided by the Java SE platform principally for convenience when developing small or temporary applications or when just beginning development.
An unnamed package cannot have subpackages, since the syntax of apackage declaration always includes a reference to a named top level package.
An implementation of the Java SE platform must support at least one unnamed package. An implementation may support more than one unnamed package, but is not required to do so. Which compilation units are in each unnamed package is determined by the host system.
Example 7.4.2-1.
The compilation unit:
class FirstCall { public static void main(String[] args) { System.out.println("Mr. Watson, come here. " + "I want you."); }}defines a very simple compilation unit as part of an unnamed package.
In implementations of the Java SE platform that use a hierarchical file system for storing packages, one typical strategy is to associate an unnamed package with each directory; only one unnamed package is observable at a time, namely the one that is associated with the "current working directory". The precise meaning of "current working directory" depends on the host system.
A package isobservable if and only if either:
A compilation unit containing a declaration of the package is observable (§7.3).
The packagesjava,java.lang, andjava.io are always observable.
One can conclude this from the rule above and from the rules of observable compilation units, as follows. The predefined packagejava.lang declares the classObject, so the compilation unit forObject is always observable (§7.3). Hence, thejava.lang package is observable (§7.4.3), and thejava package also. Furthermore, sinceObject is observable, the array typeObject[] implicitly exists. Its superinterfacejava.io.Serializable (§10.1) also exists, hence thejava.io package is observable.
Animport declaration allows a named type or astatic member to be referred to by a simple name (§6.2) that consists of a single identifier.
Without the use of an appropriate import declaration, the only way to refer to a type declared in another package, or astatic member of another type, is to use a fully qualified name (§6.7).
A single-type-import declaration (§7.5.1) imports a single named type, by mentioning its canonical name (§6.7).
A type-import-on-demand declaration (§7.5.2) imports all the accessible types (§6.6) of a named type or named package as needed, by mentioning the canonical name of a type or package.
A single-static-import declaration (§7.5.3) imports all accessiblestatic members with a given name from a type, by giving its canonical name.
A static-import-on-demand declaration (§7.5.4) imports all accessiblestatic members of a named type as needed, by mentioning the canonical name of a type.
The scope and shadowing of a type or member imported by these declarations is specified in§6.3 and§6.4.
Animport declaration makes types or members available by their simple names only within the compilation unit that actually contains theimport declaration. The scope of the type(s) or member(s) introduced by animport declaration specifically does not include other compilation units in the same package, otherimport declarations in the current compilation unit, or apackage declaration in the current compilation unit (except for the annotations of apackage declaration).
Asingle-type-import declaration imports a single type by giving its canonical name, making it available under a simple name in the class and interface declarations of the compilation unit in which the single-type-import declaration appears.
TheTypeName must be the canonical name of a class type, interface type, enum type, or annotation type (§6.7).
The name must be qualified (§6.5.5.2), or a compile-time error occurs.
It is a compile-time error if the named type is not accessible (§6.6).
If two single-type-import declarations in the same compilation unit attempt to import types with the same simple name, then a compile-time error occurs, unless the two types are the same type, in which case the duplicate declaration is ignored.
If the type imported by the single-type-import declaration is declared in the compilation unit that contains theimport declaration, theimport declaration is ignored.
If a single-type-import declaration imports a type whose simple name isn, and the compilation unit also declares a top level type (§7.6) whose simple name isn, a compile-time error occurs.
If a compilation unit contains both a single-type-import declaration that imports a type whose simple name isn, and a single-static-import declaration (§7.5.3) that imports a type whose simple name isn, a compile-time error occurs.
Example 7.5.1-1. Single-Type-Import
import java.util.Vector;
causes the simple nameVector to be available within the class and interface declarations in a compilation unit. Thus, the simple nameVector refers to the type declarationVector in the packagejava.util in all places where it is not shadowed (§6.4.1) or obscured (§6.4.2) by a declaration of a field, parameter, local variable, or nested type declaration with the same name.
Note that the actual declaration ofjava.util.Vector is generic (§8.1.2). Once imported, the nameVector can be used without qualification in a parameterized type such asVector<String>, or as the raw typeVector. A related limitation of theimport declaration is that a nested type declared inside a generic type declaration can be imported, but its outer type is always erased.
Example 7.5.1-2. Duplicate Type Declarations
This program:
import java.util.Vector;class Vector { Object[] vec; }causes a compile-time error because of the duplicate declaration ofVector, as does:
import java.util.Vector;import myVector.Vector;
wheremyVector is a package containing the compilation unit:
package myVector;public class Vector { Object[] vec; }Example 7.5.1-3. No Import of a Subpackage
Note that animport statement cannot import a subpackage, only a type.
For example, it does not work to try to importjava.util and then use the nameutil.Random to refer to the typejava.util.Random:
import java.util;class Test { util.Random generator; } // incorrect: compile-time errorExample 7.5.1-4. Importing a Type Name that is also a Package Name
Package names and type names are usually different under the naming conventions described in§6.1. Nevertheless, in a contrived example where there is an unconventionally-named packageVector, which declares a public class whose name isMosquito:
package Vector;public class Mosquito { int capacity; }and then the compilation unit:
package strange;import java.util.Vector;import Vector.Mosquito;class Test { public static void main(String[] args) { System.out.println(new Vector().getClass()); System.out.println(new Mosquito().getClass()); }}the single-type-import declaration importing classVector from packagejava.util does not prevent the package nameVector from appearing and being correctly recognized in subsequentimport declarations. The example compiles and produces the output:
class java.util.Vectorclass Vector.Mosquito
Atype-import-on-demand declaration allows all accessible types of a named package or type to be imported as needed.
ThePackageOrTypeName must be the canonical name (§6.7) of a package, a class type, an interface type, an enum type, or an annotation type.
If thePackageOrTypeName denotes a type (§6.5.4), then the name must be qualified (§6.5.5.2), or a compile-time error occurs.
It is a compile-time error if the named package or type is not accessible (§6.6).
It is not a compile-time error to name eitherjava.lang or the named package of the current compilation unit in a type-import-on-demand declaration. The type-import-on-demand declaration is ignored in such cases.
Two or more type-import-on-demand declarations in the same compilation unit may name the same type or package. All but one of these declarations are considered redundant; the effect is as if that type was imported only once.
If a compilation unit contains both a type-import-on-demand declaration and a static-import-on-demand declaration (§7.5.4) that name the same type, the effect is as if thestatic member types of that type (§8.5,§9.5) were imported only once.
Example 7.5.2-1. Type-Import-on-Demand
import java.util.*;
causes the simple names of allpublic types declared in the packagejava.util to be available within the class and interface declarations of the compilation unit. Thus, the simple nameVector refers to the typeVector in the packagejava.util in all places in the compilation unit where that type declaration is not shadowed (§6.4.1) or obscured (§6.4.2).
The declaration might be shadowed by a single-type-import declaration of a type whose simple name isVector; by a type namedVector and declared in the package to which the compilation unit belongs; or any nested classes or interfaces.
The declaration might be obscured by a declaration of a field, parameter, or local variable namedVector.
(It would be unusual for any of these conditions to occur.)
Asingle-static-import declaration imports all accessiblestatic members with a given simple name from a type. This makes thesestatic members available under their simple name in the class and interface declarations of the compilation unit in which the single-static-import declaration appears.
TheTypeName must be the canonical name (§6.7) of a class type, interface type, enum type, or annotation type.
The name must be qualified (§6.5.5.2), or a compile-time error occurs.
It is a compile-time error if the named type is not accessible (§6.6).
TheIdentifier must name at least onestatic member of the named type. It is a compile-time error if there is nostatic member of that name, or if all of the named members are not accessible.
It is permissible for one single-static-import declaration to import several fields or types with the same name, or several methods with the same name and signature.
If a single-static-import declaration imports a type whose simple name isn, and the compilation unit also declares a top level type (§7.6) whose simple name isn, a compile-time error occurs.
If a compilation unit contains both a single-static-import declaration that imports a type whose simple name isn, and a single-type-import declaration (§7.5.1) that imports a type whose simple name isn, a compile-time error occurs.
Astatic-import-on-demand declaration allows all accessiblestatic members of a named type to be imported as needed.
TheTypeName must be the canonical name (§6.7) of a class type, interface type, enum type, or annotation type.
The name must be qualified (§6.5.5.2), or a compile-time error occurs.
It is a compile-time error if the named type is not accessible (§6.6).
Two or more static-import-on-demand declarations in the same compilation unit may name the same type; the effect is as if there was exactly one such declaration.
Two or more static-import-on-demand declarations in the same compilation unit may name the same member; the effect is as if the member was imported exactly once.
It is permissible for one static-import-on-demand declaration to import several fields or types with the same name, or several methods with the same name and signature.
If a compilation unit contains both a static-import-on-demand declaration and a type-import-on-demand declaration (§7.5.2) that name the same type, the effect is as if thestatic member types of that type (§8.5,§9.5) were imported only once.
Atop level type declaration declares a top level class type (§8 (Classes)) or a top level interface type (§9 (Interfaces)).
Extra ";" tokens appearing at the level of type 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.
In the absence of an access modifier, a top level type has package access: it is accessible only within compilation units of the package in which it is declared (§6.6.1). A type may be declaredpublic to grant access to the type from code in other packages.
It is a compile-time error if a top level type declaration contains any one of the following access modifiers:protected,private, orstatic.
It is a compile-time error if the name of a top level type appears as the name of any other top level class or interface type declared in the same package.
The scope and shadowing of a top level type is specified in§6.3 and§6.4.
The fully qualified name of a top level type is specified in§6.7.
Example 7.6-1. Conflicting Top Level Type Declarations
package test;import java.util.Vector;class Point { int x, y;}interface Point { // compile-time error #1 int getR(); int getTheta();}class Vector { Point[] pts; } // compile-time error #2Here, the first compile-time error is caused by the duplicate declaration of the namePoint as both a class and an interface in the same package. A second compile-time error is the attempt to declare the nameVector both by a class type declaration and by a single-type-import declaration.
Note, however, that it is not an error for the name of a class to also name a type that otherwise might be imported by a type-import-on-demand declaration (§7.5.2) in the compilation unit (§7.3) containing the class declaration. Thus, in this program:
package test;import java.util.*;class Vector {} // not a compile-time errorthe declaration of the classVector is permitted even though there is also a classjava.util.Vector. Within this compilation unit, the simple nameVector refers to the classtest.Vector, not tojava.util.Vector (which can still be referred to by code within the compilation unit, but only by its fully qualified name).
Example 7.6-2. Scope of Top Level Types
package points;class Point { int x, y; // coordinates PointColor color; // color of this point Point next; // next point with this color static int nPoints;}class PointColor { Point first; // first point with this color PointColor(int color) { this.color = color; } private int color; // color components}This program defines two classes that use each other in the declarations of their class members. Because the class typesPoint andPointColor have all the type declarations in packagepoints, including all those in the current compilation unit, as their scope, this program compiles correctly. That is, forward reference is not a problem.
Example 7.6-3. Fully Qualified Names
class Point { int x, y; }In this code, the classPoint is declared in a compilation unit with nopackage statement, and thusPoint is its fully qualified name, whereas in the code:
package vista;class Point { int x, y; }the fully qualified name of the classPoint isvista.Point. (The package namevista is suitable for local or personal use; if the package were intended to be widely distributed, it would be better to give it a unique package name (§6.1).)
An implementation of the Java SE platform must keep track of types within packages by their binary names (§13.1). Multiple ways of naming a type must be expanded to binary names to make sure that such names are understood as referring to the same type.
For example, if a compilation unit contains the single-type-import declaration (§7.5.1):
import java.util.Vector;
then within that compilation unit, the simple nameVector and the fully qualified namejava.util.Vector refer to the same type.
If and only if packages are stored in a file system (§7.2), the host system may choose to enforce the restriction that it is a compile-time error if a type is not found in a file under a name composed of the type name plus an extension (such as.java or.jav) if either of the following is true:
This restriction implies that there must be at most one such type per compilation unit. This restriction makes it easy for a Java compiler to find a named class within a package. In practice, many programmers choose to put each class or interface type in its own compilation unit, whether or not it ispublic or is referred to by code in other compilation units.
For example, the source code for apublic typewet.sprocket.Toad would be found in a fileToad.java in the directorywet/sprocket, and the corresponding object code would be found in the fileToad.class in the same directory.