What is a Constructor?

A constructor is a special method that’s called automatically when a new object is created. It initializes the object's state (fields/variables).

Key Points:

• Constructor name must match the class name.

• It has no return type (not even void).

• If you don’t define one, Java provides a default constructor.

public class Person {
    String name;

    // Constructor
    Person(String inputName) {
        name = inputName;
    }

    void greet() {
        System.out.println("Hello, my name is " + name);
    }
}

public class Main {
    public static void main(String[] args) {
        Person p = new Person("Emma");
        p.greet(); // Output: Hello, my name is Emma
    }
}

What is Constructor Overloading?

Constructor overloading means creating multiple constructors in the same class with different parameters.

This gives users multiple ways to create an object.

Example: Constructor Overloading

public class Car {
    String brand;
    int year;

    // No-arg constructor
    Car() {
        brand = "Unknown";
        year = 0;
    }

    // Parameterized constructor
    Car(String brandName, int modelYear) {
        brand = brandName;
        year = modelYear;
    }

    void printDetails() {
        System.out.println(brand + " - " + year);
    }
}

Usage:

public class Main {
    public static void main(String[] args) {
        Car c1 = new Car();
        Car c2 = new Car("Toyota", 2022);

        c1.printDetails(); // Unknown - 0
        c2.printDetails(); // Toyota - 2022
    }
}

Constructor Chaining

Use this() to call another constructor in the same class.

class Book {
    String title;
    int pages;

    Book() {
        this("Untitled", 100); // Calls the next constructor
    }

    Book(String t, int p) {
        title = t;
        pages = p;
    }
}

Summary

• A constructor initializes new objects.

• Constructor overloading allows creating objects in different ways.

• Use this() for constructor chaining.

Assignment: Lesson 7 – Constructors and Overloading

Objective:

Practice defining constructors, overloading them, and chaining using this().

Part 1: Conceptual Questions

Answer these:

1. What is the role of a constructor?

2. What happens if you don’t define any constructor in your class?

3. Can a constructor have a return type?

Part 2: Coding Task

Step 1:

Create a class Book with the following:

• Fields: title (String), author (String), pages (int)

• Constructor 1: No-argument constructor that sets default values.

• Constructor 2: Constructor with title and author.

• Constructor 3: Constructor with all three fields (use this() to chain).

Step 2:

In the main() method, create 3 Book objects using each constructor and call a method printInfo() to display the details.

Github: https://github.com/tuanbeovnn/java-bootcamp/tree/java-bootcamp-oop-chapter7