Java Reference
In-Depth Information
Welcome to Java Programming!
Fig. 2.1
|
Text-printing program. (Part 2 of 2.)
The program includes line numbers. We've added these for instructional purposes—
they're
not
part of a Java program. This example illustrates several important Java features.
We'll see that line 9 does the work—displaying the phrase
Welcome
to
Java
Programming!
on the screen.
Commenting Your Programs
We insert
comments
to
document programs
and improve their readability. The Java com-
piler
ignores
comments, so they do
not
cause the computer to perform any action when the
program is run.
By convention, we begin every program with a comment indicating the figure number
and filename. The comment in line 1
// Fig. 2.1: Welcome1.java
begins with
//
, indicating that it's an
end-of-line
comment
—it terminates at the end of
the line on which the
//
appears. An end-of-line comment need not begin a line; it also
can begin in the middle of a line and continue until the end (as in lines 6, 10 and 11). Line
2
// Text-printing program.
by our convention, is a comment that describes the purpose of the program.
Java also has
traditional comments
, which can be spread over several lines as in
/* This is a traditional comment. It
can be split over multiple lines */
These begin and end with delimiters,
/*
and
*/
. The compiler ignores all text between the
delimiters. Java incorporated traditional comments and end-of-line comments from the C
and C++ programming languages, respectively. We prefer using
//
comments.
Java provides comments of a third type
—Javadoc comments
. These are delimited by
/**
and
*/
. The compiler ignores all text between the delimiters. Javadoc comments
enable you to embed program documentation directly in your programs. Such comments
are the preferred Java documenting format in industry. The
javadoc
utility program
(part
of the JDK) reads Javadoc comments and uses them to prepare program documentation
in HTML format. We demonstrate Javadoc comments and the
javadoc
utility in online
Appendix G, Creating Documentation with
javadoc
.
Common Programming Error 2.1
Forgetting one of the delimiters of a traditional or Javadoc comment is a syntax error. A
syntax error
occurs when the compiler encounters code that violates Java's language rules
(i.e., its syntax). These rules are similar to a natural language's grammar rules specifying
sentence structure. Syntax errors are also called
compiler errors
,
compile-time errors
or
compilation errors
,
because the compiler detects them when compiling the program.
When a syntax error is encountered, the compiler issues an error message. You must elim-
inate all compilation errors before your program will compile properly.
