Classes and Objects

Building blocks of OOP

Overview

Classes and objects are the building blocks of Object-Oriented Programming. A class is a blueprint that defines the structure and behavior, while an object is an instance of a class.

Key Concepts

Class: A template or blueprint for creating objects

Object: An instance of a class with actual values

Attributes: Data members that store the state

Methods: Functions that define behavior

Constructor: Special method to initialize objects

Code Example

java

1// Class definition
2public class Car {
3    // Attributes (instance variables)
4    private String brand;
5    private String model;
6    private int year;
7    private double price;
8    
9    // Constructor
10    public Car(String brand, String model, int year, double price) {
11        this.brand = brand;
12        this.model = model;
13        this.year = year;
14        this.price = price;
15    }
16    
17    // Methods
18    public void start() {
19        System.out.println(brand + " " + model + " is starting...");
20    }
21    
22    public void displayInfo() {
23        System.out.println(year + " " + brand + " " + model + " - $" + price);
24    }
25    
26    // Getters and Setters
27    public String getBrand() {
28        return brand;
29    }
30    
31    public void setPrice(double price) {
32        if (price > 0) {
33            this.price = price;
34        }
35    }
36}
37
38// Creating objects
39public class Main {
40    public static void main(String[] args) {
41        // Object creation
42        Car car1 = new Car("Toyota", "Camry", 2024, 28000);
43        Car car2 = new Car("Honda", "Accord", 2024, 30000);
44        
45        // Using objects
46        car1.start();
47        car1.displayInfo();
48        
49        car2.displayInfo();
50    }
51}

This example shows how to define a Car class with attributes and methods, then create and use car objects.

Real-World Example

Think of a class as a cookie cutter and objects as the actual cookies. The cookie cutter (class) defines the shape and properties, but the cookies (objects) are the actual instances you can eat.

In software: A "User" class defines what a user has (name, email, password) and can do (login, logout). Each actual user in your system is an object created from that class.

Best Practices

1.
Use meaningful class and variable names
2.
Follow single responsibility principle - one class, one purpose
3.
Make attributes private and provide public methods (encapsulation)
4.
Initialize objects properly using constructors
5.
Override toString() method for better debugging

💡 Interview Tips

Explain the difference between class and object clearly
Discuss memory allocation (class in method area, objects in heap)
Mention that multiple objects can be created from one class
Talk about object lifecycle: creation, use, and garbage collection