Class X (ICSE) Character sets and Tokens

CLASS - X (ICSE)

COMPUTER  APPLICATION

Character Sets

®     Set of valid characters that a language can recognize.

®     Letters:  A-Z , a-z 

®     Digits:  0-9

®     Special Symbols: Space + - * / ^ \ ( ) [ ] { } = != <> . „ “ $ , ; : % ! & ? _ # <= >= @ 

®     White Spaces:  Blank spaces, horizontal tab, vertical tab 

®     Other Characters : Any of the 256 ASCII character  or Unicode characters

Tokens

®     The smallest individual unit in a program is known as token.

®     Tokens used in java are: Keywords, Identifiers, Literals, Punctuators and Operators.

Identifiers:-

®     Identifiers are programmer defined names given to the various program elements such as variables, functions, arrays, objects, classes, etc.

®     It may contain digits, letters and underscore

®     It must begin with a letter or underscore or dollar($) but not a digit.

®     Java is case sensitive.

®     Examples of legal identifiers: age, $salary, _value, __1_value.

®     Examples of illegal identifiers: 123abc, -salary.

Literals or Constants:-

®     The data items which never change their value throughout the program run.

®     There are several kinds of literals:

·         Integer literals

·         Character literals

·         Floating  or Real literals

·         String literals

·         Boolean literals

·         Null literal

1. Integer literals:

®     Integer literals are whole numbers without any fractional part.

®     An integer literal must have at least one digit and must not contain any decimal point.

®     It may contain either + or - sign. A number with no sign is assumed as positive.

®     There are three types of integer literals

F  Decimal (base 10):- starts with a digits other than 0. E.g. 24

F  Octal (base 8):- starts with a digit 0 but can’t contain digits 8,9. E.g. 030

F  Hexadecimal (base 16):- starts with a 0X or 0x but can’t contain letter G-Z. E.g. 0x18

2. Character literals: 

®     Any single character enclosed within single quotes is a character literal.

®     e.g   ‘ A’  ,  ‘3’  

3. Floating or Real literals: 

®     Numbers which are having the fractional part are referred as floating literals or real literals.

®     It may be a positive or negative number.

®     A number with no sign is assumed to be a positive number. 

®     e.g     2.0, 17.5, -0.00256

®     A real literal in exponent form consists of two parts: mantissa and exponent

®     E.g  0.147 x 10⁸ = 0.147E08

®     Part appearing before E is mantissa and after E is exponent.

4. String Literals: 

®     It is a sequence of character surrounded by double quotes. 

®     e.g.,  “abc” , “23”.

5. Boolean Literal: 

®     It is true or false. 

6. Null Literal: 

®     It is represented as ‘\0’.

®     It is applied as a string terminator to mark the end of a string.

Keywords:-

®     Keywords are reserved words that convey special meaning to the compiler.

   Eg:- while, do, for, if, else, public, void, switch, break, continue etc.

Punctuators:-

The following characters are used as punctuators:

®     [ ]    Brackets         These indicates single and multidimensional array subscripts

®     ()     Parenthesis     These indicate function calls and function parameters.

®     { }   Braces             Indicate the start and end of compound statements.

®     ;       Semicolon      This is a statement terminator.

®     ,       Comma           It is used as a separator.

®     :       Colon              It indicates a labeled  statement

®     =       Equal to          It is used as an assigning operator.

Operators:-

®     An operator is a symbol or character which trigger some operation (computation) on its operands.

®     Types of operators:-

1.      Arithmetic Operators

2.      Relational Operators

3.      Assignment Operators

4.      Logical Operators

5.      Conditional Operators

6.      Bitwise Operators