- The smallest individual unit of the program is called a token.
- Every statement is an organized collection of Java tokens. Every java statement is terminated by; a sign.
- Every java program is an organized collection of statements/instructions.
Types of Tokens in Java
We have the following types of tokens in Java:
- Constant
- Identifier
- Keywords
- Operator
- Separator
Constant Tokens
A value is called a constant. They are also called literals. A constant can never be changed.
Integer
Any number without a decimal point is called an integer. They may be positive, negative, or zero.
They can be represented in the following number system in java:
- Binary Integer: A binary integer is a combination of 1 and 0. The binary literals are preceded with 0b. Eg: 0b10110
- Octal Integer: A octal integer is a combination of digits 0 to 7. The octal literals are preceded with 0. Eg: 0123
- Decimal Integer: A decimal integer is a combination of digits 0 to 9. Eg: 12345
- Hexadecimal Integer: A Hexadecimal integer is a combination of decimal digits 0 to 15.
The decimal 10 to 15 is represented by letters A to Z(small a to z can also be used) in hexadecimal.
The hexadecimal literals begin with 0x. Eg: 0x123A
Real
Any number with a decimal point is called a real value.
223.5, -123.4, 0.5, .121, 0.0 etc.
Single Character
A single alphabet, digit, or symbol which is enclosed inside single quotes is called a single character constant. Java supports Unicode characters.
'x', '?', '8' etc.
String
Anything enclosed in double quotes is called string. A string can contain a combination of alphabets, digits, or symbols.
"vermatarun", "777123", "[email protected]", "[email protected]#$#<"
Note: An empty pair of double quotes is also a valid string in java, Eg: ""
Boolean
Boolean values are true and false.
Identifier Tokens
Any name used in our program is called an identifier. Eg: variable name, interface name, class name, function name, package name, etc.
We must follow certain rules while creating identifier:
- Identifier can contain alphabets, digits, dollar ($), and underscore (_) symbol.
- Except for dollar and underscore we cannot use any special symbol in identifier.
- Identifier can be started with underscore or dollar sign.
- Identifier cannot be started with a digit.
- Keywords cannot be used as identifiers.
Note: We can use both small as well as capital letters in identifiers.
Keyword Tokens
- Keywords are special words that have their predefined work in java.
- Keywords cannot be used as identifiers.
- We have 48 keywords in java.
- We also have two reserved words const and goto in java, which do not have any work but cannot be used as an identifier.
Note: Most of the keywords are in small letters only.
Operator Tokens
Operators are special symbols or keywords which are used to operate on operands and generate the result.
Eg. 501+200, here + is an operator which performs the addition of 501 and 200.
Separator Tokens
The separator is a symbol that is used to separate two tokens.
Consider the following example:
int a;
In the above statement int is a keyword and a is an identifier, and they are separated by space, so space is a separator.