Creating, Compiling and Executing Java Programs

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∟Execution Process, Entry Point, Input and Output

∟Creating, Compiling and Executing Java Programs

This section describes the process of Java program creation, compilation and execution. JDK 'javac' and 'java' commands are also described.

There are 3 steps involved in building and running a Java program:

Creating Program Source Code: Creating one text file written Java language statements to define each Java class or other program unit. The text file is called source code file, which must have a file name identical to the name of the class defined in the file, and an extension of ".java". One of the source code files should define the main class which contains the "main" method as the execution starting point. Other source code files define additional classes and other program units to support the main class.
Compiling Source Code: Converting all source code files of the program into bytecode files written in Java Virtual Machine (JVM) instructions. The bytecode file must have the same file name as the source code file, and an extension of ".class".
Executing the Java Program: Loading the bytecode file of the main class into JVM, and executing its JVM instructions starting from the "main" method.

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 JDK package contains a Java compiler, which can be invoked by the 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:

herong> 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.

For example, the following command:

herong> javac -d .\cls -classpath .\lib;.\cls \
   -sourcepath .\src .\src\Hello.java

invokes the "javac" compiler, and tells it to:

Compile the "Hello.java" source code file in the ".\src" directory.
Place the bytecode file, "Hello.class", in the ".\cls" directory.
Search ".\lib" and ".\cls" directories for the definitions in bytecode format of classes or interfaces that are used in "Hello.java".
Search ".\src" directory for the definitions in source code format of classes or interfaces that are used in "Hello.java".

Executing a Java program needs a JVM. The JDK also contains a JVM, called HotSpot, which can be invoked by the command "java". The "java" command takes the name of the starting class as the command argument. For example, the following command:

herong> java -classpath .\lib;.\cls;. Hello

invokes HotSpot to:

Load "Hello" class into HotSpot.
Execute the main() method of the "Hello" class.

The "-classpath" option specifies the class path, which contains directories where the JVM will search for the bytecode files of the starting class and other program units when needed.