7

C H A P T E R

Packages

Good things come in small packages.

 Traditional proverb

J

AVA 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 organiza 

tion 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 automat 

ically 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

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