Variables and Objects in Java

What is a Variable?

A variable is a named memory location used to store data in a program.

Syntax:

dataType variableName = value;

Example:

int age = 25;
String name = "John";
double salary = 25000.50;

Rules for Creating Variables

• Names can contain letters, digits, _ and $.
• A variable name cannot start with a digit.
• Spaces are not allowed.
• Java keywords cannot be used as variable names.
• Variable names are case-sensitive (age and Age are different).
• Use meaningful names and follow camelCase.

Types of Variables in Java

Java has three types of variables:

1. Local Variables
2. Instance Variables (Non-static)
3. Static Variables

1. Local Variables

Declared inside a method, constructor, or block.

• Accessible only within that method or block.
• Must be initialized before use.
• Do not receive default values.

void display() {
    int age = 25;
    System.out.println(age);
}

Here, age is a local variable.

2. Instance Variables (Non-static)

Declared inside a class but outside methods and constructors.

• Each object gets its own copy.
• Can have different values for different objects.
• Receive default values if not initialized.

class Student {
    String name;
    int age;
}

Here, name and age are instance variables.

3. Static Variables

Declared inside a class using the static keyword.

• Only one shared copy exists for the class.
• All objects share the same variable.
• Can be accessed using the class name.

class Student {
    static String college = "ABC College";
}

Here, college is a static variable shared by all Student objects.

Remember: Instance variables belong to objects, whereas static variables belong to the class.

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What is an Object?

An object is an instance of a class.

A class is a blueprint used to create objects.

Every object has three characteristics:

• State: Properties or data (model, price).
• Behavior: Actions (drive, brake).
• Identity: Distinguishes one object from another.

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Creating a Class and Object

Let's understand this with a simple Car example.

Example:

public class Cars {

    String model;  // Instance variable
    int price;     // Instance variable

    void drive() {  // Instance method
        System.out.println("Car is driving...");
    }

    public static void main(String[] args) {

        Cars myCar = new Cars();  // Object creation

        myCar.model = "XUV";
        myCar.price = 600000;

        myCar.drive();  // Method calling
    }
}

Output:

Car is driving...

Understanding the code:

• Cars – Class name.
• model and price – Instance variables.
• drive() – Instance method (non-static).
• new Cars() – Creates an object.
• myCar – Reference variable that refers to the object.
• myCar.drive() – Calls the object's method.