This section describes the process of Java program creation, compilation and execution. JDK 'javac' and 'java' commands are also described.
A Java application program is made of one or more classes and zero or more interfaces.
One of the class must have the main() method as the execution starting point of the program.
There are 3 steps involved in creating the classes and interfaces of a Java program,
and running the program:
Creating Classes and Interfaces: Creating a text file for each class or
interface that contains the definition of the class or interface written in Java
language statements.
Such a file is called source code file, which must have a file name identical to the
name of the class or interface defined in the file, and an extension of ".java".
Compiling Classes and Interfaces: Converting a source code file into a bytecode
file that contains the same definition of the class or interface written Java Virtual
Machine (JVM) instructions. A bytecode file must have a file name identical to the
name of the class or interface defined in the file, and an extension of ".class".
Executing a Java Program: Loading bytecode files into JVM, and executing
its JVM instructions starting from the entry point of the specified class.
Creating a source code file can be done by a simple text editor, notepad, or a sophisticated
Java development environment tool, like Eclipse, or Visual J++.
Compiling a source code file needs a Java compiler. The J2SDK package from Sun
Microsystems contains a Java compiler, which can be invoked by command "javac".
The "javac" command takes source code file names as command line arguments. For
example, entering the following command in command window will invoke the "javac"
compiler to compile the "Hello.java" source code file:
c:\jdk\bin\javac Hello.java
The "javac" command also has several command options available:
-d: Specify the directory where the bytecode files generated by the compiler should
be placed, if you don't want to place them in the current directory.
-classpath: Specify the directories where to search for classes or interfaces
definitions in bytecode format, if the classes or interfaces are used in source code file.
-sourcepatch: Specify the directories where to search for classes or interfaces
definitions in source code format, if the classes or interfaces are used in source code file.
Compile the "Test.java" source code file in the ".\src" directory.
Place the bytecode file, "Test.class", in the ".\cls" directory.
Search ".\lib" and ".\cls" directories for the definitions in bytecode format
of classes or interfaces that are used in "Test.java".
Search ".\src" directory for the definitions in source code format of classes
or interfaces that are used in "Test.java".
During the compilation process, if the definition of a class or interface is used
in the source code file, the compiler will use the following rules to search for
the definition:
If the compiler could not find the class or interface in the class path and
the source path, the compiler will produce a compilation error saying
that "cannot resolve symbol" where the symbol is the name of the class or interface.
If the compiler find the class or interface only in the class path,
the compiler will be happy to use it as it is.
If the compiler find the class or interface only in the source path,
the compiler will compile it before using it.
If the compiler find the class or interface in the source path and in the class path,
the compiler will compare those two files. If the source code file is newer than
the bytecode file, the compiler will compile the source code file.
Executing a Java program needs a JVM. The J2SDK also contains
a JVM, called HotSpot, which can be invoked by command "java".
The "java" command takes the name of the starting class as the command
argument. For example, the following command:
c:\jdk\bin\java -cp .\lib;.\cls;. Test
invokes HotSpot to:
Load "Test" class into HotSpot.
Execute the main() method of the "Test" class.
The "-cp" option specifies the class path, which contains directories where
the JVM will search for the bytecode files of the starting class and
other classes when needed.
