CHAPTER 7

Packages

Java programs are organized as sets of packages. Each package has its own set of names for types, which helps to prevent name conflicts. A type is accessible (§6.6) outside the package that declares it only if the type is declared public.

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 (§7.2.1) or in a database (§7.2.2). Packages that are stored in a file system have certain constraints on the organization of their compilation units to allow a simple implementation to find classes easily. In either case, the set of packages available to a Java program is determined by the host system, but must always include at least the three standard packages java.lang, java.util, and java.io as specified in Chapters 20, 21, and 22. In most host environments, the standard packages java.applet, java.awt, and java.net, which are not described in this specification, are also available to Java programs.

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 each of the types declared in the predefined package java.lang.

A compilation unit has three parts, each of which is optional:

• A package declaration (§7.4), giving the fully qualified name (§6.7) of the package to which the compilation unit belongs
• import declarations (§7.5) that allow types from other packages to be referred to using their simple names
• Type declarations (§7.6) of class and interface types

7.1 Package Members

• Subpackages of the package
• Types declared in the compilation units (§7.3) of the package

• The package java has subpackages awt, applet, io, lang, net, and util, but no compilation units.
• The package java.awt has a subpackage named image, as well as a number of compilation units containing declarations of class and interface types.

The subpackages of package java named lang, util, and io (whose fully qualified package names are therefore java.lang, java.util, and java.io) are a standard part of every Java implementation and are specified in Chapters 20, 21, and 22. Many Java implementations will include the entire set of java packages defined in the series of books The Java Application Programming Interface.

A package may not contain a type declaration and a subpackage of the same name, or a compile-time error results. Here are some examples:

• Because the package java.awt has a subpackage image, it cannot (and does not) contain a declaration of a class or interface type named image.
• If there is a package named mouse and a type Button in that package (which then might be referred to as mouse.Button), then there cannot be any package with the fully qualified name mouse.Button or mouse.Button.Click.
• If COM.Sun.java.jag is the fully qualified name of a type, then there cannot be any package whose fully qualified name is either COM.Sun.java.jag or COM.Sun.java.jag.scrabble.

7.2 Host Support for Packages

The packages may be stored in a local file system in simple implementations of Java. Other implementations may use a distributed file system or some form of database to store Java source and/or binary code.

7.2.1 Storing Packages in a File System

COM
gls
jag
java
wnj

Continuing the example, the directory java would probably contain at least the following subdirectories:

applet	
awt
io
lang
net
util

Still continuing the example, if we were to look inside the directory util, we might see the following files:

BitSet.java										Observable.java
BitSet.class										Observable.class
Date.java										Observer.java
Date.class										Observer.class
Dictionary.java										Properties.java
Dictionary.class										Properties.class
EmptyStackException.java										Random.java
EmptyStackException.class										Random.class
Enumeration.java										Stack.java
Enumeration.class										Stack.class
Hashtable.java										StringTokenizer.java
Hashtable.class										StringTokenizer.class
NoSuchElementException.java										Vector.java
NoSuchElementException.class										Vector.class

Under this simple organization of packages, an implementation of Java 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 a UNIX system, where the file name separator is /, the package name:

jag.scrabble.board

jag/scrabble/board

COM.Sun.sunsoft.DOE

COM/Sun/sunsoft/DOE

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), so that the package name:

children.activities.crafts.papierM\u00e2ch\u00e9

children.activities.crafts.papierMâché

children/activities/crafts/papierM@00e2ch@00e9

7.2.2 Storing Packages in a Database

Java allows such a database to relax the restrictions (§7.6) on compilation units in file-based implementations. For example, a system that uses a database to store packages need 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 Java program to a form that obeys the restrictions, for purposes of export to file-based implementations.

7.3 Compilation Units

CompilationUnit:

	PackageDeclarationopt ImportDeclarationsopt TypeDeclarationsopt

ImportDeclarations:

	ImportDeclaration

	ImportDeclarations ImportDeclaration

TypeDeclarations:

	TypeDeclaration

	TypeDeclarations TypeDeclaration

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

• A package declaration (§7.4), giving the fully qualified name (§6.7) of the package to which the compilation unit belongs
• import declarations (§7.5) that allow types from other packages to be referred to using their simple names
• Type declarations (§7.6) of class and interface types

7.4 Package Declarations

7.4.1 Named Packages

PackageDeclaration:

	package PackageName ;

If a type named T is declared in a compilation unit of a package whose fully qualified name is P, then the fully qualified name of the type is P.T; thus in the example:

package wnj.points;
class Point { int x, y; }

7.4.2 Unnamed Packages

class FirstCall {
	public static void main(String[] args) {
		System.out.println("Mr. Watson, come here. "
									+ "I want you.");
	}
}

A Java system must support at least one unnamed package; it 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.

In Java systems 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 available 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.

Unnamed packages are provided by Java principally for convenience when developing small or temporary applications or when just beginning development.

Caution must be taken when using unnamed packages. It is possible for a compilation unit in a named package to import a type from an unnamed package, but the compiled version of this compilation unit will likely then work only when that particular unnamed package is "current." For this reason, it is strongly recommended that compilation units of named packages never import types from unnamed packages. It is also recommended that any type declared in an unnamed package not be declared public, to keep them from accidentally being imported by a named package.

It is recommended that a Java system provide safeguards against unintended consequences in situations where compilation units of named packages import types from unnamed packages. One strategy is to provide a way to associate with each named package at most one unnamed package, and then to detect and warn about situations in which a named package is used by more than one unnamed package. It is specifically not required-indeed, it is strongly discouraged-for an implementation to support use of a named package by more than one unnamed package by maintaining multiple compiled versions of the named package.

7.4.3 Scope and Hiding of a Package Name

Package names never hide other names.

7.4.4 Access to Members of a Package

It is strongly recommended that the protections of a file system or database used to store Java programs be set to make all compilation units of a package available whenever any of the compilation units is available.

7.5 Import Declarations

ImportDeclaration:

	SingleTypeImportDeclaration

	TypeImportOnDemandDeclaration

An import declaration makes types available by their simple names only within the compilation unit that actually contains the import declaration. The scope of the name(s) it introduces specifically does not include the package statement, other import statements in the current compilation unit, or other compilation units in the same package. Please see §7.5.4 for an illustrative example.

7.5.1 Single-Type-Import Declaration

SingleTypeImportDeclaration:

	import TypeName ;

The example:

import java.util.Vector;

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 another type with the same name is otherwise declared in the current compilation unit except by a type-import-on-demand declaration (§7.5.2), then a compile-time error occurs.

So the sample program:

import java.util.Vector;

class Vector { Object[] vec; }

import java.util.Vector;

import myVector.Vector;

package myVector;

public class Vector { Object[] vec; }

Note that an import statement cannot import a subpackage, only a type. For example, it does not work to try to import java.util and then use the name util.Random to refer to the type java.util.Random:

import java.util;										// incorrect: compile-time error

class Test { util.Random generator; }

7.5.2 Type-Import-on-Demand Declaration

TypeImportOnDemandDeclaration:

	import PackageName . * ;

The example:

import java.util.*;

7.5.3 Automatic Imports

import java.lang.*;

The full specification of java.lang is given in Chapter 20. The following public types are defined in java.lang:

AbstractMethodError										LinkageError
ArithmeticException										Long
ArrayStoreException										Math
Boolean										NegativeArraySizeException
Character										NoClassDefFoundError
Class										NoSuchFieldError
ClassCastException										NoSuchMethodError
ClassCircularityError										NullPointerException
ClassFormatError										Number
ClassLoader										NumberFormatException
ClassNotFoundException										Object
CloneNotSupportedException										OutOfMemoryError
Cloneable										Process
Compiler										Runnable
Double										Runtime
Error										RuntimeException
Exception										SecurityException
ExceptionInInitializerError										SecurityManager
Float										StackOverflowError
IllegalAccessError										String
IllegalAccessException										StringBuffer
IllegalArgumentException										System
IllegalMonitorStateException										Thread
IllegalThreadStateException										ThreadDeath
IncompatibleClassChangeError										ThreadGroup
IndexOutOfBoundsException										Throwable
InstantiationError										UnknownError
InstantiationException										UnsatisfiedLinkError
Integer										VerifyError
InternalError										VirtualMachineError
InterruptedException

7.5.4 A Strange Example

package Vector;

public class Mosquito { int capacity; }

package strange.example;


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());
	}
}

class java.util.Vector
class Vector.Mosquito

7.6 Type Declarations

TypeDeclaration:

	ClassDeclaration

	InterfaceDeclaration

	;

class date { int month, day, year; };

By default, the types declared in a package are accessible only within the compilation units of that package, but a type may be declared to be public to grant access to the type from code in other packages (§6.6, §8.1.2, §9.1.2).

A Java implementation must keep track of types within packages by their fully qualified names (§6.7). Multiple ways of naming a type must be expanded to fully qualified 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;

When Java packages are stored in a file system (§7.2.1), 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:

• The type is referred to by code in other compilation units of the package in which the type is declared.
• The type is declared public (and therefore is potentially accessible from code in other packages).

When Java packages are stored in a database (§7.2.2), the host system need not enforce such restrictions.

In practice, many Java programmers choose to put each class or interface type in its own compilation unit, whether or not it is public or is referred to by code in other compilation units.

7.7 Unique Package Names

If unique package names are not used, then package name conflicts may arise far from the point of creation of either of the conflicting packages. This may create a situation that is difficult or impossible for the user or programmer to resolve. The class ClassLoader (§20.14) of the standard Java Virtual Machine environment can be used to isolate packages with the same name from each other in those cases where the packages will have constrained interactions, but not in a way that is transparent to a naïve Java program.

You form a unique package name by first having (or belonging to an organization that has) an Internet domain name, such as Sun.COM. You then reverse this name, component by component, to obtain, in this example, COM.Sun, and use this as a prefix for your package names, using a convention developed within your organization to further administer package names.

Such a convention might specify that certain directory name components be division, department, project, machine, or login names. Some possible examples:

COM.Sun.sunsoft.DOE
COM.Sun.java.jag.scrabble
COM.Apple.quicktime.v2
EDU.cmu.cs.bovik.cheese
GOV.whitehouse.socks.mousefinder

The name of a package is not meant to imply anything about where the package is stored within the Internet; for example, a package named EDU.cmu.cs.bovik.cheese is not necessarily obtainable from Internet address cmu.EDU or from cs.cmu.EDU or from bovik.cs.cmu.EDU. The Java convention for generating unique package names is merely a way to piggyback a package naming convention on top of an existing, widely known unique name registry instead of having to create a separate registry for Java package names.

If you need to get a new Internet domain name, you can get an application form from ftp://ftp.internic.net and submit the complete forms by E-mail to [email protected]. To find out what the currently registered domain names are, you can telnet to rs.internic.net and use the whois facility.

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Java Language Specification (HTML generated by Suzette Pelouch on February 24, 1998)
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