Classes and Objects

Classes and objects are the foundational concepts of Object-Oriented Programming (OOP). A class can be thought of as a blueprint for creating objects. It defines a set of properties, known as fields, and behaviors, known as methods, that will be common to all objects of that type. An object, in turn, is a specific instance of a class.

Defining a Class

To define a class, you use the class keyword. For example, here is a simple class that serves as a template for creating Dog objects. It includes fields for the dog's breed, age, and color, as well as a method for the behavior of barking.

public class Dog {
    // Fields (properties) of the Dog class
    String breed;
    int age;
    String color;

    // Method (behavior) of the Dog class
    void bark() {
        System.out.println("Woof!");
    }
}

Creating an Object

Once a class is defined, you can create objects, or instances, of that class using the new keyword. After creating an object, you can access its fields and methods. For example, you can create a Dog object and then set its breed, age, and color, and then call its bark method.

public class Main {
    public static void main(String[] args) {
        // Create a new Dog object
        Dog myDog = new Dog();

        // Access and set the fields of the myDog object
        myDog.breed = "Golden Retriever";
        myDog.age = 3;
        myDog.color = "Golden";

        // Call the bark method on the myDog object
        myDog.bark(); // This will print "Woof!" to the console
    }
}

Constructors

A constructor is a special type of method that is automatically called when an object is created. Its primary purpose is to initialize the object's fields. A constructor must have the same name as the class and cannot have a return type.

public class Dog {
    String breed;
    int age;

    // This is the constructor for the Dog class
    public Dog(String dogBreed, int dogAge) {
        breed = dogBreed;
        age = dogAge;
    }

    public static void main(String[] args) {
        // Create a new Dog object using the constructor
        Dog anotherDog = new Dog("Labrador", 5);

        // Access the fields of the anotherDog object
        System.out.println(anotherDog.breed); // Prints "Labrador"
        System.out.println(anotherDog.age);   // Prints "5"
    }
}