Java Identifiers

Identifiers in Java

Basics in using Identifiers in Java Programming

Identifiers are programmer-designed tokens. They are used for naming classes, methods, variables, objects, labels, packages and interfaces in a program. Java identifier’s basic rules are as follows:

1. They can have alphabets, digits, and the underscore and dollar sign characters.
2. They must not begin with a digit.
3. Uppercase and lowercase letters are distinct.
4. They can be of any length.

Identifier must be meaningful, short enough to be quickly and easily typed and long enough to be descriptive and easily read. Java developers have followed some naming conventions.

• Names of all public methods and instance variables start with a leading lowercase letter. 

              Example: 

                       average

                       sum

• When more than one word are used in a name, the second and subsequent words are marked with a leading uppercase letters. 

              Example: 

                       dayTemperature

                       firstDayOfMonth

                       totalMarks

• All private and local variables use only lowercase letters combined with underscores.

              Example: 

                       length

                       Batch_strength

• All classes and interfaces start with a leading uppercase letter(and each subsequent word with a leading uppercase letter).

Example: 

                       Student

                       HelloJava

                       Vehicle

                       MotorCycle

• Variables that represent constant values use all uppercase letters and underscores between words.

Example: 

                       TOTAL

                       F_MAX

                       PRINCIPAL_AMOUNT

We may follow our own conventions as long as we do not break the basic rules of naming identifiers. 

The following table shows some valid and invalid identifiers:

Valid	Invalid
HelloWorld	Hello World (uses a space)
Hi_JAVA	Hi JAVA! (uses a space and punctuation mark)
value3	3value(begins with a number)
Tall	short (this is a Java keyword)
$age	#age (does not begin with any other symbol except _ $ )

It is standard Java practice to name multiple-word identifiers in lowercase except for the beginning letter of words in the middle of the name.

Java Keywords

Keywords in Java

Java language has reserved 49 words as keywords.

Java Keywords also called a reserved word. Keywords are identifiers that Java reserves for its own use. These identifiers have built-in meanings that cannot change. Thus, programmers cannot use these identifiers for anything other than their built-in meanings. Technically, Java classifies identifiers and keywords as separate categories of tokens. Keywords are an essential part of a language definition. There are 49 reserved keywords currently defined in the Java language and they are shown in the below table.

abstract	double	int	switch
assert	else	interface	synchronized
boolean	extends	long	this
break	false	native	throw
byte	final	new	transient
case	finally	package	true
catch	float	private	try
char	for	protected	void
class	goto	public	volatile
const	if	return	while
continue	implements	short
default	import	static
do	instanceof	super

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. 

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Java Tokens

Tokens used in Java Programs

Reserved Keywords, Identifiers, Literals, Operators, Separators

A Java program is basically a collection of classes. A class is defined by a set of declaration statements and methods containing executable statements. Most statements contain expressions, which describe the actions carried out on data. Smallest individual unit in a program are known as tokens. The compiler recognizes them for building up expressions and statements. 

Java Character Set

The smallest units of Java language are the characters used to write Java tokens. These characters are defined by the Unicode character set, and emerging standard that tries to create characters for a large number of scripts world wide.
The Unicode is a 16-bit character coding system and currently supports more than 34,000 defined characters derived from 24 languages from America, Europe, Middle East, Africa and Asia. However, most of us use only  the basic ASCII characters, which include letters, digits and punctuation marks, used in normal English. We therefore, have used only ASCII character character set (a subset of UNICODE character set) in developing the programs. 

Java language includes five types of tokens and they are:

• Reserved Keywords

• Identifiers

• Literals

• Operators

• Separators

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Data Types in Java

Different Data Types used in Java

Data type defines a set of permitted values on which the legal operations can be performed.

There are two data types available in Java:

• Primitive Data Types
• Reference/Object Data Types

Primitive Data Types

     Primitive Data Types defines 8 simple types of data: byte, short, int, long, char, float, double, and boolean. These can be put in four groups.

Integer:

This groups include byte, short, int, and long which are for whole valued signed number. 

Floating:

This group includes float and double, which represents numbers with fractional precision. 

Characters:

This group includes char, which respresents symbls in a character set like letters and numbers. 

Boolean:

This group includes boolean, which is a special type for representing true/false.

Data Type	Default Value (for fields)	Size (in bits)	Minimum Range	Maximum Range
byte	0	8 bits	-128	+127
short	0	16 bits	-32768	+32767
int	0	32 bits	-2147483648	+2147483647
long	0L	64 bits	-9223372036854775808	+9223372036854775807
float	0.0f	32-bit	1.40129846432481707e-45	3.40282346638528860e+38
double	0.0d	64-bit	4.94065645841246544e-324d	1.79769313486231570e+308d
char	'\u0000'	16-bit		0 to 65,535
boolean	false	1- bit	NA	NA

Reference Data Types

     Reference variables are created using defined constructors of the classes. They are used to access objects. Class objects, and various type of array variables come under reference data type. Default value of any reference variable is null. These non-primitive types are often called "reference types" because they are handled "by reference"--in other words, the address of the object or array is stored in a variable, passed to methods, and so on. By comparison, primitive types are handled "by value"--the actual primitive values are stored in variables and passed to methods. The reference data types are arrays, classes and interfaces that are made and handle according to a programmer in a java program  which can hold the three kind of values as:

• Array Type
• class type
• Interface Type