OOP: Classes & Objects

☕ Java Lesson 10 Intermediate

Java is built entirely around Object-Oriented Programming (OOP). OOP organizes software around data models called objects, which are instances of structures called classes.

1 Classes, Instances, and Constructors

A **Class** is a blueprint/template. An **Object** is the concrete instance created in heap memory from that template using the `new` keyword. Classes contain:

Fields (Instance Variables): The data states of the object.
Constructors: Special initialization methods block invoked when creating objects. Constructors match the class name exactly and have no return type.
Methods: Behaviors the object can execute.

2 Constructor Chaining using this()

You can define multiple constructors (overloading). Using the keyword `this()` as the first line inside a constructor allows you to call another constructor inside the same class. This is called constructor chaining and reduces duplicate code. Let's inspect object construction:

Java — OOP Classes & Objects ▶ Run Code

class Car {
    String model;
    int year;

    // Parameterized Constructor
    Car(String model, int year) {
        this.model = model;
        this.year = year;
    }

    // Overloaded Constructor calling the main one (Chaining)
    Car(String model) {
        this(model, 2026); // Default year to 2026
    }

    void displayInfo() {
        System.out.println("Car model: " + model + ", Year: " + year);
    }
}

public class Main {
    public static void main(String[] args) {
        // Instantiate using parameterized constructor
        Car myCar1 = new Car("Toyota Supra", 2022);
        
        // Instantiate using chained constructor
        Car myCar2 = new Car("Tesla Model S");

        myCar1.displayInfo();
        myCar2.displayInfo();
    }
}

3 Code Challenge

Challenge: Write a class representing a `Student`. Give it two fields: `String name` and `int gradeLevel`. Create a main constructor that accepts both, and a default chained constructor that takes only a name and passes a default grade level of 1. Write an instance method showing student details, instantiate both students, and print details.