OBJECT ORIENTED PROGRAMMING THROUGH JAVA : Unit - 1 : Variables in Java

 

What is a Variable in Java?

A variable in Java is a named memory location that stores data values of a specific type. It acts as a container for values that can change during program execution.

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Variable Declaration and Initialization

Syntax:

type variableName;               // Declaration
type variableName = value;       // Declaration + Initialization

Example:

int age;             // Declaration
age = 25;            // Initialization

String name = "Loki"; // Declaration and Initialization

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Types of Variables in Java

Java variables are classified based on their scope and position in the program:

Type	Declared Inside	Scope	Memory Location
Local Variable	Method or block	Within method/block only	Stack
Instance Variable	Inside class (outside methods)	Whole object (per instance)	Heap
Static Variable	Inside class using static	Shared across all instances	Method Area (static memory)

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1. Local Variable

• Exists only within methods, constructors, or blocks.

• Must be initialized before use.

public class Test {
    public void display() {
        int localVar = 10;  // Local variable
        System.out.println(localVar);
    }
}

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2.  Instance Variable (Non-static Field)

• Declared in a class but outside any method.

• Each object gets its own copy.

public class Student {
    String name;   // Instance variable
    int rollNo;

    void showDetails() {
        System.out.println(name + " " + rollNo);
    }
}

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3.  Static Variable (Class Variable)

• Declared with static keyword.

• Shared among all instances of the class.

public class Student {
    int rollNo;
    static String college = "ABC College"; // Static variable

    Student(int r) {
        rollNo = r;
    }

    void display() {
        System.out.println(rollNo + " " + college);
    }
}

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Variable Naming Rules and Conventions

Rules (enforced by compiler):

• Must begin with a letter, $, or _.

• Cannot start with a digit.

• No spaces or special characters (except $ and _).

• Can't use Java keywords (like int, class).

Conventions (good practices):

Element Type	Naming Style	Example
Variable	camelCase	studentName
Constant	UPPER_CASE	MAX_SPEED

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Data Types for Variables

Java is strongly typed, so every variable must have a type.

➤ Primitive Types:

• Integer types: byte, short, int, long

• Floating types: float, double

• Character: char

• Boolean: boolean

int age = 25;
float marks = 95.5f;
char grade = 'A';
boolean isPassed = true;

➤ Reference Types:

• Arrays

• Objects

• Strings

• Interfaces

String name = "Loki";
int[] scores = {90, 85, 95};

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Variable Scope and Lifetime

Type	Scope	Lifetime
Local	Method/block only	During method execution
Instance	Whole class (per object)	As long as object exists
Static	Class-wide	Till end of program

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Example Program Showing All Types

public class Demo {

    // Static variable
    static int staticCount = 0;

    // Instance variable
    int instanceCount = 0;

    public void showCounts() {
        // Local variable
        int localCount = 10;

        System.out.println("Local: " + localCount);
        System.out.println("Instance: " + instanceCount);
        System.out.println("Static: " + staticCount);
    }

    public static void main(String[] args) {
        Demo obj1 = new Demo();
        obj1.instanceCount = 1;
        staticCount = 5;

        obj1.showCounts();
    }
}

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 Best Practices for Variable Usage

• Use meaningful names (balance, not b).

• Initialize variables before using them.

• Keep variable scope as small as possible.

• Use constants (final) for fixed values.

final int MAX_USERS = 100;