Packages

Good things come in small packages.
—Traditional proverb

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

For small programs and casual development, a package can be unnamed (§7.4.2) or have a simple name, but if Java code is to be widely distributed, unique package names should be chosen (§7.7). 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.