Java Basics

File Structure

The class MyClass should be defined in a file called MyClass.java.

Compiling

To compile a project, use the command line

javac MainClass.java

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

java MainClass

will run the program.

java -Xmx1000M MainClass

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:

javac -classpath "." cluster/*.java

Example script:

#!/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

Packages

To include class MyClass in package betty.sue, the first line of MyClass.java should be

package betty.sue;

and MyClass.java, MyClass.class should be stored in directory sue, which is in directory betty.

To use a class from a package:

import <package_name>.<class_name>;

or

import <package_name>.*;

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

jar xf jarfile.jar

Strings

String text = "abcde";
text.substring(2) == "cde"
text.substring(0,1) == "a"
text.substring(1,5) == "bcde"