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Lecture
10B |
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| Java File Organization |
Java file organization follows these guidelines:
- CLASSPATH
environment variable, needed by javac
to find the java classes, points to the class library directory
that holds the class archive file.
set CLASSPATH c:\java\lib\CLASSES.ZIP - 1.0, 1.1
set CLASSPATH c:\java\jre\lib\rt.jar - 1.2 (default)
- Classes are
usually grouped into Packages, where
package names reflect the directory location of the class files:
myJava.functions -> myJava\functions\*.class
- File names
reflect the directory/package where they reside.
java.applet.Applet -> java\applet\Applet.class
myJava.functions.Bessel -> myJava\functions\Bessel.class
- File name
+ CLASSPATH
gives the path to the file:
set CLASSPATH c:\java\jre\lib\rt.jar;c:\programs;
which will locate Bessel.class in
c:\programs\myJava\functions\Bessel.class
- Classes can
be explicitly named in the code:
public class myNewBessel extends myJava.functions.Bessel{..
- Or
the import
statement indicates into which package/directory the compiler should
look for a file
import myJava.functions.*;
public class myNewBessel extends Bessel{..
Note: For 1.2 it is no longer necessary to set CLASSPATH for the runtime library. By default, it will be found relative to the directory of the binary files (e.g. c:\java\bin), which is set in the PATH environment variable.
Similarly, in 1.2 if you set the CLASSPATH, it does not need to include the runtime library, e.g. if your use
set CLASSPATH c:\programs
javac would still look first in c:\java\jre\lib\rt.jar for the runtime files before looking in c:\programs for files not found there.
The classpath can also be set in the command line:
> java -classpath c:\programs myBesselProgram
Note: the archive files, ZIP or JAR, maintains the file structure. One can think of a JAR file as a file directory within a file.
We can, for example, look at the contents of the rt.jar file that holds most of the core Java classes.
Here we use the
jar tool to display some of the files.
Note that the files are listed with their directory structure included. For example, the java/applet/* files correspond to the java.applet package.
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