PACKAGES

Unique Package Names

7.7

then within that compilation unit the simple name

Vector

 and the fully qualified

name

java.util.Vector

 refer to the same type.

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).

This restriction implies that there must be at most one such type per compilation

unit. This restriction makes it easy for a Java compiler and Java Virtual Machine

to find a named class within a package; for example, the source code for a

public

type

wet.sprocket.Toad

 would be found in a file

Toad.java

 in the directory

wet/sprocket

, and the corresponding object code would be found in the file

Toad.class

 in the same directory.

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

Did I ever tell you that Mrs. McCave

Had twenty three sons and she named them all  Dave ?

Well, she did. And that wasn't a smart thing to do. . . .

 Dr. Seuss (Theodore Geisel),

Too Many Daves

 (1961)

Developers should take steps to avoid the possibility of two published packages

having the same name by choosing

unique package names

 for packages that are

widely distributed. This allows packages to be easily and automatically installed

and catalogued. This section specifies a standard convention, not enforced by a

Java compiler, for generating such unique package names. Java systems are

encouraged to provide automatic support for converting a set of packages from

local and casual package names to the unique name format described here.

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

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