Java Classes and Objects - Complete Guide
Introduction to Classes and Objects
Classes and objects are the fundamental building blocks of Object-Oriented Programming (OOP) in Java. A class is a blueprint or template that defines the properties and behaviors of objects, while an object is an instance of a class that represents a real-world entity.
Understanding classes and objects is essential for creating modular, reusable, and maintainable Java programs that model real-world scenarios effectively.
Defining Classes in Java
A class in Java is defined using the class keyword followed by the class name and a code block containing class members such as fields, constructors, and methods.
Example
// Class definition
public class Car {
// Fields (attributes)
private String brand;
private String model;
private int year;
private double price;
// Constructor
public Car(String brand, String model, int year, double price) {
this.brand = brand;
this.model = model;
this.year = year;
this.price = price;
}
// Methods (behaviors)
public void startEngine() {
System.out.println("Engine started for " + brand + " " + model);
}
public void stopEngine() {
System.out.println("Engine stopped for " + brand + " " + model);
}
public void displayInfo() {
System.out.println("Brand: " + brand);
System.out.println("Model: " + model);
System.out.println("Year: " + year);
System.out.println("Price: $" + price);
}
// Getter example
public String getBrand() {
return brand;
}
}
// Main class to demonstrate class usage
public class Main {
public static void main(String[] args) {
// Creating objects from the Car class
Car car1 = new Car("Toyota", "Camry", 2022, 25000.0);
Car car2 = new Car("Honda", "Civic", 2023, 23000.0);
// Accessing object methods
car1.displayInfo();
car1.startEngine();
System.out.println();
car2.displayInfo();
car2.startEngine();
car2.stopEngine();
// Accessing object fields through getters (recommended)
System.out.println("\nCar 1 brand: " + car1.getBrand());
}
}Output
Brand: Toyota Model: Camry Year: 2022 Price: $25000.0 Engine started for Toyota Camry Brand: Honda Model: Civic Year: 2023 Price: $23000.0 Engine started for Honda Civic Engine stopped for Honda Civic Car 1 brand: Toyota
Class Components and Structure
A well-structured Java class typically contains the following components that work together to define the object's characteristics and behaviors.
Example
public class BankAccount {
// Fields
private String accountNumber;
private String accountHolder;
private double balance;
private static int totalAccounts = 0;
// Constructor
public BankAccount(String accountNumber, String accountHolder, double initialBalance) {
this.accountNumber = accountNumber;
this.accountHolder = accountHolder;
this.balance = initialBalance;
totalAccounts++;
}
// Instance methods
public void deposit(double amount) {
if (amount > 0) {
balance += amount;
System.out.println("Deposited: $" + amount);
}
}
public void withdraw(double amount) {
if (amount > 0 && amount <= balance) {
balance -= amount;
System.out.println("Withdrew: $" + amount);
} else {
System.out.println("Invalid withdrawal or insufficient funds");
}
}
public void displayBalance() {
System.out.println("Balance: $" + balance);
}
// Static method
public static int getTotalAccounts() {
return totalAccounts;
}
// Getters and setters
public String getAccountHolder() {
return accountHolder;
}
public void setAccountHolder(String accountHolder) {
if (accountHolder != null && !accountHolder.trim().isEmpty()) {
this.accountHolder = accountHolder;
}
}
}
public class BankDemo {
public static void main(String[] args) {
BankAccount account1 = new BankAccount("123456", "John Doe", 1000);
BankAccount account2 = new BankAccount("789012", "Jane Smith", 500);
account1.deposit(200);
account1.withdraw(150);
account1.displayBalance();
account2.deposit(1000);
account2.withdraw(2000);
account2.displayBalance();
System.out.println("Total accounts: " + BankAccount.getTotalAccounts());
System.out.println("Account 1 holder: " + account1.getAccountHolder());
account1.setAccountHolder("Johnathan Doe");
System.out.println("Account 1 holder after change: " + account1.getAccountHolder());
}
}Output
Deposited: $200.0 Withdrew: $150.0 Balance: $1050.0 Deposited: $1000.0 Invalid withdrawal or insufficient funds Balance: $1500.0 Total accounts: 2 Account 1 holder: John Doe Account 1 holder after change: Johnathan Doe
Object Creation and Memory Management
Objects in Java are created with the `new` keyword and stored in heap memory. Reference variables point to these objects. Java uses automatic garbage collection to reclaim memory from objects that are no longer referenced.
Example
public class ObjectMemoryDemo {
public static void main(String[] args) {
String str1 = new String("Hello");
String str2 = new String("Hello");
String str3 = "Hello";
String str4 = "Hello";
System.out.println("str1 == str2: " + (str1 == str2));
System.out.println("str1.equals(str2): " + str1.equals(str2));
System.out.println("str3 == str4: " + (str3 == str4));
Rectangle rect1 = new Rectangle(5, 10);
Rectangle rect2 = rect1;
System.out.println("rect1 area: " + rect1.getArea());
System.out.println("rect2 area: " + rect2.getArea());
rect1.setWidth(8);
System.out.println("After modification:");
System.out.println("rect1 area: " + rect1.getArea());
System.out.println("rect2 area: " + rect2.getArea());
Rectangle rect3 = null;
System.out.println("rect3 is null: " + (rect3 == null));
System.out.println("\nObject lifecycle:");
for (int i = 0; i < 3; i++) {
Rectangle temp = new Rectangle(i + 1, i + 2);
System.out.println("Created rectangle with area: " + temp.getArea());
}
System.gc();
}
}
class Rectangle {
private double width;
private double height;
public Rectangle(double width, double height) {
this.width = width;
this.height = height;
System.out.println("Rectangle created: " + width + "x" + height);
}
public double getArea() {
return width * height;
}
public void setWidth(double width) {
this.width = width;
}
public void setHeight(double height) {
this.height = height;
}
}Output
str1 == str2: false str1.equals(str2): true str3 == str4: true Rectangle created: 5.0x10.0 rect1 area: 50.0 rect2 area: 50.0 After modification: rect1 area: 80.0 rect2 area: 80.0 rect3 is null: true Object lifecycle: Rectangle created: 1.0x2.0 Created rectangle with area: 2.0 Rectangle created: 2.0x3.0 Created rectangle with area: 6.0 Rectangle created: 3.0x4.0 Created rectangle with area: 12.0
Best Practices for Classes and Objects
- ✅ Use meaningful and descriptive class names (PascalCase)
- ✅ Each class should have a single clear responsibility
- ✅ Keep fields private and expose them through getters/setters
- ✅ Override toString(), equals(), and hashCode() when appropriate
- ✅ Use constructors for correct initialization
- ✅ Avoid unnecessary object creation to optimize memory
- ✅ Use static members for class-level data and behavior
- ✅ Consider interfaces for defining contracts between classes