Understanding classes and objects is at the heart of Java and object-oriented programming (OOP).

Imagine you are building a model of a car. You wouldn't just create a random assortment of attributes; instead, you'd define a blueprint for what a car is, what it can do, and how it behaves.

That's essentially what classes and objects allow you to do in Java—they let you define blueprints (classes) and create actual instances of those blueprints (objects).

What Are Classes and Objects?

At its core, a class in Java is a blueprint for creating objects. It encapsulates data for the object and methods to manipulate that data. When you define a class, you're essentially defining a new data type. An object, on the other hand, is an instance of a class.

Think of a class as a blueprint for a house. The house itself, built according to that blueprint, is the object.

Basic Structure of a Class

Here's a simple example:

In this example, Car is a class with three attributes: color, model, and year. The method displayDetails() allows us to print out the details of the car.

Creating Objects

Once you have a class defined, you can create objects of that class:

In this Main class, we create an instance of Car called myCar. We then set its properties and call the displayDetails() method. This illustrates how you can use classes to create structured, organized code.

Attributes and Methods

When defining a class, you will often have both attributes (also known as fields) and methods. Attributes represent the state or properties of an object, while methods define its behavior.

Instance Variables vs. Class Variables

In Java, attributes can be instance variables or class variables (static). Instance variables are tied to a specific instance of a class, while class variables are shared across all instances.

Here, numberOfVehicles keeps track of how many Vehicle instances have been created, while type is specific to each instance.

Methods

Methods can also have parameters and return types. For example, let’s modify our Vehicle class to include a method that allows us to change the vehicle type:

Now, you can create a Vehicle object and change its type:

This modular approach keeps your code organized and makes it easier to maintain.

The Importance of Encapsulation

Encapsulation is one of the fundamental principles of OOP. It refers to the bundling of data (attributes) and methods that operate on that data within one unit (the class). Encapsulation helps protect the integrity of the data by restricting direct access to it.

Using Access Modifiers

Java provides access modifiers like private, public, and protected to control access to class members. Here's how we can encapsulate our Car class:

By making the attributes private, we ensure that they cannot be accessed directly from outside the class. Instead, we provide public methods (getters/setters) to interact with them.

Real-World Application of Encapsulation

Consider an online banking application. You wouldn’t want customers to access their account balance directly. Instead, you would provide methods to check the balance, deposit money, or withdraw funds, enforcing rules around how those actions can be performed.

Inheritance and Polymorphism

Inheritance allows a class to inherit properties and methods from another class. This promotes code reuse and establishes a hierarchical relationship between classes.

Creating a Subclass

Let’s extend our Car class to create a SportsCar subclass:

In this case, SportsCar inherits color, model, and year from Car while adding its own unique attribute, topSpeed.

Polymorphism

Polymorphism allows you to call methods on objects of different classes that share a common interface or superclass. For example, if you have a list of Car objects, you can call a method on each object regardless of their specific types:

Abstract Classes and Interfaces

Sometimes, you want to define a class that cannot be instantiated directly. This is where abstract classes come into play. Abstract classes can contain abstract methods (without a body) that must be implemented by subclasses.

Abstract Class Example

In this example, Vehicle is an abstract class. It cannot be instantiated, but Bike provides an implementation for the start() method.

Interfaces

Interfaces are similar to abstract classes but can only contain method signatures (no body) and static final variables. A class can implement multiple interfaces, allowing for a more flexible design.

With this interface, you can ensure that any class that implements Drivable must provide a drive() method.

Conclusion

Classes and objects are foundational concepts in Java that enable you to model real-world entities in your applications. They allow for encapsulation, inheritance, and polymorphism, promoting cleaner and more maintainable code.

By understanding how to define classes, create objects, and leverage OOP principles, you equip yourself with powerful tools for software development.

In the next chapter, we will look at how constructors help initialize objects, ensuring they are set up correctly before use and diving into some common patterns and practices that can make your code cleaner and more efficient.