JAVA TOKENS

Smallest individual units in a program are known as Tokens. The complier recognized them

for building up expressions and statements. Java languages include 5 types of Tokens. They

are:

# Reserved Keywords

# Identifiers

# Literals

# Operators

# Separators

Keywords:

A keyword is a word that has special meaning defined by the Java programming language.

The program shown earlier in Listing 1-1 uses four keywords:public, class, static, and void.

In all, Java has 51 keywords. They’re listed in alphabetical order in Table 1-1. abstract do if

package synchronized boolean double implements private this break else import protected

throw byte extends instanceof public throws case false int return transient catch final

interface short true char finally long static try class float native strictfp void

Identifiers:

An identifier is a word that you make up to refer to a Java programming element by

name. Although you can assign identifiers to many different types of Java elements, they’re

most commonly used for the following elements:

Classes, such as the HelloApp class in Listing 1-1

Methods, such as the main method in Listing 1-1

Variables and fields, which hold data used by your program

Parameters, which pass data values to methods

Comments:

A comment is a bit of text that provides explanations of your code. Comments are completely

ignored by the compiler, so you can place any text you wish in a comment. Using plenty of

comments in your programs is a good idea to explain what your program does and how it

works.

Java has three basic types of comments:

 end-of-line comments

 traditional comment

 Multi comments

 JavaDoc comments.

End-of-line comments (single Line comments):

An end-of-line comment begins with the sequence // and ends at the end of the line. You can

place an end-of-line comment at the end of any line.

Everything you type after the // is ignored by the compiler. For example:

total = total * discountPercent; // calculate the discounted total

If you want, you can also place end-of-line comments on separate lines,

like this:

// calculate the discounted total

total = total * discountPercent;

You can place end-of-line comments in the middle of statements that span

two or more lines. For example:

total = (total * discountPercent) // apply the discount first

+ salesTax; // then add the sales tax

Traditional comments (Multi Line comments):

A traditional comment begins with the sequence /* and ends with the sequence */ and can

span multiple lines. For example:

/* HelloApp sample program.

This program demonstrates the basic structure

that all Java programs must follow. */

A traditional comment can begin and end anywhere on a line. If you want, you can even

sandwich a comment between other Java programming elements, like this:

x = (y + /* a strange place for a comment */ 5) / z;

Usually, traditional comments appear on separate lines. One common use for traditional

comments is to place a block of comment lines at the beginning of a class to indicate

information about the class such as what the class does, who wrote it, and so on. However,

that type of comment is usually better coded as a JavaDoc comment, as described in the next

section.

You may be tempted to temporarily comment out a range of lines by placing

/* in front of the first line in the range and */ after the last line in the range.

However, that can get you in trouble if the range of lines you try to comment

out includes a traditional comment. That’s because traditional comments

can’t be nested. For example, the following code won’t compile:

/*

int x, y, z;

y = 10;

z = 5;

x = (y + /* a strange place for a comment */ 5) / z;

*/

Here, I tried to comment out a range of lines that already included a traditional comment.

Unfortunately, the */ sequence near the end of the fifth line is interpreted as the end of the

traditional comment that begins in the first line. Then, when the compiler encounters the */

sequence in line 6, it generates an error message.

Multi Comments (Comments with comments or Nested comments):

Here, comments with in comments, nested comments. It can be represented with

/** Multi comments or Comments with in comments or Nested comments */

JavaDoc comments:

JavaDoc comments are actually a special type of traditional comment that you can use to

automatically create Web-based documentation for your programs. Because you’ll have a

better appreciation of JavaDoc comments when you know more about object-oriented

programming, I devoted a section in Book III, Chapter 8 to creating and using JavaDoc

comments.