Lexical Issues,Whitespace,Identifiers, Literals,Separators,The Java Keywords

--> Lexical Issues

Java programs are a collection of whitespace,identifiers, comments, literals, operators, separators, and keywords.

Whitespace

Java is a free-form language. This means that you do not need to follow any special
indentation rules. For example, 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 variable names include:

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 */.

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:


--> The Java Keywords

There are 49 reserved keywords currently defined in the Java language.
These keywords, combined with the syntax of the operators and separators, form the
definition of the Java language. These keywords cannot be used as names for a variable,
class, or method.