Secondary constructors in Kotlin - Mini Project: Build & Apply

Using Secondary Constructors in Kotlin

📖 Scenario: You are creating a simple Kotlin class to represent a Book in a library system. Sometimes you know all the details about the book, and sometimes you only know the title and author.

🎯 Goal: Build a Kotlin class Book with a primary constructor and a secondary constructor to handle different ways of creating book objects.

📋 What You'll Learn

Create a class called Book with a primary constructor that takes title, author, and year.

Add a secondary constructor that takes only title and author, and sets year to 0.

Create two Book objects using both constructors.

Print the details of both books.

💡 Why This Matters

🌍 Real World

Secondary constructors let you create objects in different ways depending on the information you have, like when some details are missing or optional.

💼 Career

Understanding constructors is essential for Kotlin developers to write flexible and readable code, especially when working with data models in apps.

1

Create the Book class with a primary constructor

Create a Kotlin class called Book with a primary constructor that has three parameters: title of type String, author of type String, and year of type Int. Store these parameters as properties in the class.

Kotlin

class Book(val title: String, val author: String, val year: Int) {
    // Your code here
}

Need a hint?

Use class Book(val title: String, val author: String, val year: Int) to create the class with properties.

2

Add a secondary constructor to the Book class

Inside the Book class, add a secondary constructor that takes only title and author as parameters. This constructor should call the primary constructor and set year to 0.

Kotlin

class Book(val title: String, val author: String, val year: Int) {
    constructor(title: String, author: String) : this(title, author, 0) {
        // Your code here
    }
}

Need a hint?

Use constructor(title: String, author: String) : this(title, author, 0) inside the class.

3

Create two Book objects using both constructors

Create a variable called book1 using the primary constructor with "1984", "George Orwell", and 1949. Then create a variable called book2 using the secondary constructor with "Animal Farm" and "George Orwell".

Kotlin

class Book(val title: String, val author: String, val year: Int) {
    constructor(title: String, author: String) : this(title, author, 0) {
    }
}

// Create book1 and book2 below
// Your code here

Need a hint?

Use val book1 = Book("1984", "George Orwell", 1949) and val book2 = Book("Animal Farm", "George Orwell").

4

Print the details of both books

Print the details of book1 and book2 in the format: Title: [title], Author: [author], Year: [year]. Use two separate println statements.

Kotlin

class Book(val title: String, val author: String, val year: Int) {
    constructor(title: String, author: String) : this(title, author, 0) {
    }
}

val book1 = Book("1984", "George Orwell", 1949)
val book2 = Book("Animal Farm", "George Orwell")

// Print book1 and book2 details below
// Your code here

Need a hint?

Use println("Title: ${book1.title}, Author: ${book1.author}, Year: ${book1.year}") and similarly for book2.