The reason for this is that Java is case-sensitive. At this point, the convention that filenames
correspond to class names may seem arbitrary. However, this convention makes it easier to
maintain and organize your programs.
Compiling the Program
To compile the Example program, execute the compiler, javac, specifying the name of the
source file on the command line, as shown here:
C:\>javac Example.java
The javac compiler creates a file called Example.class that contains the bytecode version
of the program. As discussed earlier, the Java bytecode is the intermediate representation of
your program that contains instructions the Java Virtual Machine will execute. Thus, the
output of javac is not code that can be directly executed.
To actually run the program, you must use the Java application launcher, called java.
To do so, pass the class name Example as a command-line argument, as shown here:
C:\>java Example
When the program is run, the following output is displayed:
This is a simple Java program.
When Java source code is compiled, each individual class is put into its own output file
named after the class and using the .class extension. This is why it is a good idea to give
your Java source files the same name as the class they contain--the name of the source file
will match the name of the .class file. When you execute java as just shown, you are actually
specifying the name of the class that you want to execute. It will automatically search for
a file by that name that has the .class extension. If it finds the file, it will execute the code
contained in the specified class.
A Closer Look at the First Sample Program
Although Example.java is quite short, it includes several key features that are common to
all Java programs. Let's closely examine each part of the program.
The program begins with the following lines:
/*
This is a simple Java program.
Call this file "Example.java".
*/
This is a comment. Like most other programming languages, Java lets you enter a remark into
a program's source file. The contents of a comment are ignored by the compiler. Instead, a
comment describes or explains the operation of the program to anyone who is reading its
source code. In this case, the comment describes the program and reminds you that the source
file should be called Example.java. Of course, in real applications, comments generally explain
how some part of the program works or what a specific feature does.
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