Scott: /* The classpath */

2011-02-04T23:20:24Z

The classpath

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==File Structure==

The class <code>MyClass</code> should be defined in a file called <code>MyClass.java</code>.

==Compiling==
To compile a project, use the command line
<pre>
javac MainClass.java
</pre>
where MainClass.java contains the main method. If MainClass.java refers to other classes (through import statement or by belonging to a package), the compiler searches for the appropriate class files on the CLASSPATH. If it finds no .class file, or if it finds a corresponding .java file that has been modified more recently than the .class file, it will compile the .java file. Thus, the compiler has the functionality of make built in.

==Running==
<pre>
java MainClass
</pre>
will run the program.
<pre>
java -Xmx1000M MainClass
</pre>
sets the maximum heap size to 1000MB. 

==The classpath==
I have packages/directories "utility" and "cluster" in my home directory. cluster depends on utility, so I compile utility first. To compile cluster, I give the following command from my home directory:
<pre>
javac -classpath "." cluster/*.java
</pre>
Example script:
<source lang="bash">
#!/bin/bash
#!/bin/bash
cd /home/barney/work/mmm/code
CP=classes
CP=$CP:../../utility/classes
CP=$CP:lib/mysql-connector-java-3.0.11-stable-bin.jar
/opt/sun-jdk-1.4.2.05/bin/java -classpath $CP mmm.RunControl report
</source>

==Packages==
To include class MyClass in package betty.sue, the first line of MyClass.java should be
<source lang="java">
package betty.sue;
</source>
and MyClass.java, MyClass.class should be stored in directory sue, which is in directory betty.

To use a class from a package:
<source lang="java">
import <package_name>.<class_name>;
</source>
or
<source lang="java">
import <package_name>.*;
</source>
Each has the same performance, but the first form might serve as a mnemonic.

If the current class already belongs to a package (its first line uses the package statement), then the import is implied.

Classes have package visibility by default.

To execute something in a package, the directory containing the package directory structure must be in your classpath. In the example above, the directory ''containing'' betty/sue would be in your classpath.

==Jar Files==
Unzipping a jar file 
<pre>
jar xf jarfile.jar
</pre>

==Strings==
<source lang="java">
String text = "abcde";
text.substring(2) == "cde"
text.substring(0,1) == "a"
text.substring(1,5) == "bcde"
</source>

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Scott: /* The classpath */