A token is the smallest element of a program that is meaningful to the compiler. Tokens can be classified as follows:
- Keywords
- Identifiers
- Constants
- Special Symbols
- Operators
- Keyword: Keywords are pre-defined or reserved words in a programming language. Each keyword is meant to perform a specific function in a program. Since keywords are referred names for a compiler, they can’t be used as variable names because by doing so, we are trying to assign a new meaning to the keyword which is not allowed. Java language supports following keywords:
abstract assert boolean break byte case catch char class const continue default do double else enum exports extends final finally float for goto if implements import instanceof int interface long module native new open opens package private protected provides public requires return short static strictfp super switch synchronized this throw throws to transient transitive try uses void volatile while with
- Identifiers: Identifiers are used as the general terminology for naming of variables, functions and arrays. These are user-defined names consisting of an arbitrarily long sequence of letters and digits with either a letter or the underscore(_) as a first character. Identifier names must differ in spelling and case from any keywords. You cannot use keywords as identifiers; they are reserved for special use. Once declared, you can use the identifier in later program statements to refer to the associated value. A special kind of identifier, called a statement label, can be used in goto statements.
Examples of valid identifiers :
MyVariable MYVARIABLE myvariable x i x1 i1 _myvariable $myvariable sum_of_array geeks123
Examples of invalid identifiers :
My Variable // contains a space 123geeks // Begins with a digit a+c // plus sign is not an alphanumeric character variable-2 // hyphen is not an alphanumeric character sum_&_difference // ampersand is not an alphanumeric character
- Constants/Literals: Constants are also like normal variables. But, the only difference is, their values can not be modified by the program once they are defined. Constants refer to fixed values. They are also called as literals.
Constants may belong to any of the data type.
Syntax:final data_type variable_name;
- Special Symbols: The following special symbols are used in Java having some special meaning and thus, cannot be used for some other purpose.
[] () {}, ; * =- Brackets[]: Opening and closing brackets are used as array element reference. These indicate single and multidimensional subscripts.
- Parentheses(): These special symbols are used to indicate function calls and function parameters.
- Braces{}: These opening and ending curly braces marks the start and end of a block of code containing more than one executable statement.
- comma (, ): It is used to separate more than one statements like for separating parameters in function calls.
- semi colon : It is an operator that essentially invokes something called an initialization list.
- asterick (*): It is used to create pointer variable.
- assignment operator: It is used to assign values.
- Operators: Java provides many types of operators which can be used according to the need. They are classified based on the functionality they provide. Some of the types are-
