Basics of Java

Variables

“ Variable is a name of memory location “ 

int data=123; 

Variable is a symbolic name refer to a memory location used to store values that can change during the execution of a program. 

Example: 

   int a = 10; 

Here, int = data type 

          a = identifier 

         10 = literal 

● There are three types of variables in java: local, instance and static.

1) Local Variable 

A variable which is declared inside the method is called local variable.

2) Instance Variable

A variable which is declared inside the class but outside the method, is called instance variable . It is not declared as static.

3) Static variable 

A variable that is declared as static is called static variable. It cannot be local.

Demo:-

  class A    

{      

int data=50;//instance variable      

static int m=100;//static variable      

void method() 

{      

int n=90;//local variable      

}      

}

Data Types:-

● There are two types of data types in java: primitive and non-primitive. 

● Primitive data types are the basic blocks of any programming language. 

● It can have only one value at a time.

Data Types:- 

● Non primitive data types are also referred to as derived types. 

● Java supports  non primitive data types classes, interfaces, and arrays.

Identifiers:-

● Are names assigned to variables,  methods,constant,classes,packages and interfaces. 

● No limit has been specified for length. 

● Rules: 

    1. first letter must be a letter,underscore,dollar sign 

    2. subsequent can be letter,underscore,dollar,digit 

    3. white space is not allowed 

    4. identifiers are case sensitive 

    5. don't use keywords. 

Naming Convention:-

● Class or Interface Identifiers 

– Must begin with a capital letter. 

– First alphabet of internal word should also be capitalized. 

– Example: 

    ● public class MyClass 

    ● public class Employee 

● Variable or Method Identifiers 

– Must begin with a small case letter. 

– First alphabet of internal word should also be capitalized. 

– Example: 

   ● int myNumber

● Constant Identifiers 

– Must be specified in upper case. 

– _ is used to separate internal words. 

– Example: 

      ● final double RATE = 2.6 

● Package Identifiers 

– Must be specified in small case letters. 

– Example: 

     ● Package mypackage.subpackage.x;

Keywords

● Are predefined identifiers meant for specific purpose. 

● Reserve words 

● All keywords are in lower case. 

   Ex: case,catch,if,float,int,package,while,do,long

Literals

● Is a value that  can be passed to a variable or constant in a program. 

● Numeric : binary,octal(0-7,017),hexa 

● Boolean: 0’s and 1’s 

● Character : enclosed with quotes 

● String : zero or more characters 

● Null : assigned to object reference variable

Operators

● Using one or more operands 

● Unary operator(+) 

● Binary operator(/) 

● Ternary operator(?:)

Assignment/Arithmetic operator

● Sets the value of a variable to some new value. 

● Right to left associativity 

● a=b=0 

● Ex: +=,-=,|= 

● a+=b

Relational operator

● Java return either true or false

● Equal to == 

● Not equal to != 

● Less than < 

● Greater than > 

● Less than or equal to <= 

● Greater than or equal to >=

Boolean logical operators

● Conditional OR || 

● Conditional AND && 

● Logical OR |(TRUE: one is true) 

● Logical AND &(FALSE: one is false) 

● Logical XOR ^(FALSE: T & T) 

● Unary logical NOT ! Ex: bolean a=true; bolean b=false;

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