Java Reference
In-Depth Information
Single-Type-Import vs. Type-Import-On-Demand Declarations
Lines 3-4 of Fig. 21.5 are
single-type-import declarations
—they each specify
one
class to
import. When a source-code file uses
multiple
classes from a package, you can import those
classes with a a
type-import-on-demand declaration
of the form
import
packagename
.*;
which uses an asterisk (
*
) at its end to inform the compiler that
all
public
classes from the
packagename
package can be used in the file containing the
import
. Only those classes that
are
used
are loaded at execution time. The preceding
import
allows you to use the simple
name of any type from the
packagename
package. Throughout this topic, we provide
single-type-import declarations as a form of documentation to show you specifically which
types are used in each program.
Common Programming Error 21.1
Using the
import
declaration
import
java.*;
causes a compilation error. You must spec-
ify the full package name from which you want to import classes.
Error-Prevention Tip 21.1
Using single-type-import declarations helps avoid naming conflicts by importing only the
types you actually use in your code.
Specifying the Classpath When Compiling a Program
When compiling
ListTest
,
javac
must locate the
.class
files for classes
List
and
Emp-
tyListException
to ensure that class
ListTest
uses them correctly. The compiler uses a
special object called a
class loader
to locate the classes it needs. The class loader begins by
searching the standard Java classes that are bundled with the JDK. Then it searches for
op-
tional packages
. Java provides an
extension mechanism
that enables new (optional) pack-
ages to be added to Java for development and execution purposes. If the class is not found
in the standard Java classes or in the extension classes, the class loader searches the
class-
path
—a list of directories or
archive files
containing reusable types. Each directory or ar-
chive file is separated from the next by a
directory separator
—a semicolon (
;
) on Windows
or a colon (
:
) on UNIX/Linux/Mac OS X. Archive files are individual files that contain di-
rectories of other files, typically in a compressed format. For example, the standard classes
used by your programs are contained in the archive file
rt.jar
, which is installed with the
JDK. Archive files normally end with the
.jar
or
.zip
file-name extensions.
By default, the classpath consists only of the current directory. However, the classpath
can be modified by
1.
providing the
-classpath
listOfDirectories
option to the
javac
compiler or
2.
setting the
CLASSPATH
environment variable
(a special variable that you define
and the operating system maintains so that programs can search for classes in the
specified locations).
If you compile
ListTest.java
without specifying the
-classpath
option, as in
javac ListTest.java
the class loader assumes that the additional package(s) used by the
ListTest
program are
in the
current directory
. As we mentioned, we placed our package in the
parent
directory so
