Java Reference
In-Depth Information
example, consider the statement
System.out.println(2);
. Static variable
out
of class
System
is an instance of class
PrintStream
, which belongs to package
java.io
, and
println
is a method of this class.
Package
java.text
Contains classes for formatting numbers.
Package
java.util
Contains a number of useful utilities, e.g. classes that repre-
sent dates, provide a Gregorian calendar, give methods for processing
Strings
into “tokens”, provide dynamic arrays, and provide random numbers.
The Swing package, package
javax.swing
Provides replacements for some of
the GUI classes in package
java.awt
and introduces some new GUI classes.
For example, class
javax.swing.JFrame
is now used instead of class
java.-
awt.Frame
. See Chap. 17 for a discussion of writing GUIs in Java.
11.4
Key concepts
• Package.
A package is a collection of files for Java classes —perhaps organ-
ized as subpackages— that reside in one directory on a hard drive.
• Package name.
A package name is a suffix of the path-name on your comput-
er of the directory of the package. However, periods are used to separate com-
ponents on the path, not / or \.
• Class path.
You can set variable
CLASSPATH
in your operating system to con-
tain some paths to directories that contain packages.
• Using a class in a package.
To reference a class in a package, use the form
<
package-name
>
.
<
class-name
>. Some path in variable
CLASSPATH
catenated
with <
package-name
> must yield the actual path on your computer to the direc-
tory the package.
• Import statement.
To remove the need to use the form <
package-
name
>
.
<
class-name
> everywhere in your program, use an import statement
import
<
package.name
>
.*;
. Thereafter, you can just use <
class-name
>.
• Java packages.
Java comes with hundreds of prewritten classes organized into
tens of packages. Package
java.lang
is automatically imported in every Java
program.
• Your packages.
Your classes are placed in a default package. If you want to
place a class in another package, put a package statement on the first line of the
file for the class —then store the file in the appropriate directory.
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