|
Whitespace
|
|
Java
is a free-form language. This means that you do not need to follow any
special indentation
|
|
rules. For instance,
the
|
|
Example
|
|
program could have been
written all on one line or in any
|
|
other
strange way you felt like typing it, as long as there was at least one
whitespace character
|
|
between
each token that was not already delineated by an operator or separator. In
Java,
|
|
whitespace is a space,
tab, or newline.
|
|
Identifiers
|
|
Identifiers are used
for class names, method names, and variable names. An identifier may
|
|
be any descriptive
sequence of uppercase and lowercase letters, numbers, or the underscore
|
|
and dollar-sign
characters. They must not begin with a number, lest they be confused with a
|
|
numeric literal. Again,
Java is case-sensitive, so
|
|
VALUE
|
|
is a different
identifier than
|
|
Value
|
|
.
|
|
Some examples of valid
identifiers are
|
|
AvgTemp
|
|
count
|
|
a4
|
|
$test
|
|
this_is_ok
  |
|
Invalid identifier
names include these:
|
|
2count
|
|
high-temp
|
|
Not/ok
   |
|
Literals
|
|
A constant value in
Java is created by using a
literal representation of it. For
example, here
|
|
are some literals:
|
|
100
|
|
98.6
|
|
'X'
|
|
"This
is a test"
   |
|
Left to right, the
first literal specifies an integer, the next is a floating-point value, the
third is
|
|
a
character constant, and the last is a string. A literal can be used anywhere
a value of its type
|
|
is allowed.
|
|
Comments
|
|
As
mentioned, there are three types of comments defined by Java. You have
already seen two:
|
|
single-line
and multiline. The third type is called a
documentation comment. This
type of comment
|
|
is
used to produce an HTML file that documents your program. The documentation
comment
|
|
begins
with a
|
|
/**
|
|
and
ends with a
|
|
*/
|
|
.
|
|
Documentation
comments are explained in Appendix A.
|
|
Separators
|
|
In
Java, there are a few characters that are used as separators. The most
commonly used
|
|
separator
in Java is the semicolon. As you have seen, it is used to terminate
statements.
|
|
The
separators are shown in the following table:
 |
|
Symbol
|
|
Name
|
|
Purpose
|
|
(
)
|
|
Parentheses
|
|
Used to contain lists of parameters in method
definition and invocation.
|
|
Also used for defining precedence in expressions,
containing expressions
|
|
in control statements, and surrounding cast types.
|
|
{
}
|
|
Braces
|
|
Used
to contain the values of automatically initialized arrays. Also used
|
|
to
define a block of code, for classes, methods, and local scopes.
|
|
[
]
|
|
Brackets
|
|
Used
to declare array types. Also used when dereferencing array values.
|
|
;
|
|
Semicolon
|
|
Terminates
statements.
|
|
,
|
|
Comma
|
|
Separates
consecutive identifiers in a variable declaration. Also used to
|
|
chain
statements together inside a
|
|
for
|
|
statement.
|
|
.
|
|
Period
|
|
Used
to separate package names from subpackages and classes. Also
|
|
used
to separate a variable or method from a reference variable.
       |
|
The Java Keywords
|
|
There
are 50 keywords currently defined in the Java language (see Table 2-1). These
keywords,
|
|
combined
with the syntax of the operators and separators, form the foundation of the
Java
|
|
language.
These keywords cannot be used as names for a variable, class, or method.
|
|
The
keywords
|
|
const
|
|
and
|
|
goto
|
|
are
reserved but not used. In the early days of Java, several
|
|
other
keywords were reserved for possible future use. However, the current
specification for
|
|
Java
only defines the keywords shown in Table 2-1.
|
|
In addition to the
keywords, Java reserves the following:
|
|
true false
|
|
,
|
|
, and
|
|
null
|
|
. These are
|
|
values
defined by Java. You may not use these words for the names of variables,
classes,
|