Java Reference
In-Depth Information
nestedtypesthatdescribeaclass'sinterfacemustbedeclared
public
tobeaccessible
from beyond the package.
Note
Throughout this topic, I typically don't declare top-level types and their ac-
cessible members
public
, unless I'm creating a package.
Everypackagehasaname,whichmustbeanonreservedidentifier.Thememberac-
cessoperatorseparatesapackagenamefromasubpackagename,andseparatesapack-
ageorsubpackagenamefromatypename.Forexample,thetwomemberaccessoper-
ators in
graphics.shapes.Circle
separate package name
graphics
from the
shapes
subpackagename,andseparatesubpackagename
shapes
fromthe
Circle
type name.
Note
The standard class library organizes its many classes and other top-level
typesintomultiplepackages.Manyofthesepackagesaresubpackagesofthestandard
java
package.Examplesinclude
java.io
(typesrelatedtoinput/outputoperations),
java.lang
(language-orientedtypes),
java.lang.reflect
(reflection-oriented
language types),
java.net
(network-oriented types), and
java.util
(utility
types).
Package Names Must Be Unique
Suppose you have two different
graphics.shapes
packages, and suppose that
each
shapes
subpackage contains a
Circle
class with a different interface. When
the compiler encounters
System.out.println(new Circle(10.0, 20.0,
30.0).area());
inthesourcecode,itneedstoverifythatthe
area()
methodex-
ists.
Thecompilerwillsearchallaccessiblepackagesuntilitfindsa
graphics.shapes
package that contains a
Circle
class. If the found package contains the appropriate
Circle
classwithan
area()
method,everythingisfine;otherwise,ifthe
Circle
class does not have an
area()
method, the compiler will report an error.
This scenario illustrates the importance of choosing unique package names. Spe-
cifically, the top-level package name must be unique. The convention in choosing
this name is to take your Internet domain name and reverse it. For example, I would
choose
ca.tutortutor
asmytop-level package name because
tutortutor.ca
is
my
domain
name.
I
would
then
specify
ca.tutortutor.graphics.shapes.Circle
to access
Circle
.
