Named companion objects in Kotlin - Mini Project: Build & Apply

Using Named Companion Objects in Kotlin

📖 Scenario: Imagine you are creating a simple app that manages books. Each book has a title and an author. You want to add a special helper inside the Book class that can create a book from a single string with the format "title - author".

🎯 Goal: You will create a Kotlin class Book with a named companion object called Factory. This companion object will have a function to create a Book from a string. Finally, you will print the book details.

📋 What You'll Learn

Create a Kotlin class named Book with two properties: title and author.

Add a named companion object called Factory inside the Book class.

Inside Factory, create a function fromString that takes a single String parameter and returns a Book.

Use the fromString function to create a Book from a string formatted as "title - author".

Print the book's title and author.

💡 Why This Matters

🌍 Real World

Named companion objects help organize helper functions related to a class, like factory methods, making code cleaner and easier to understand.

💼 Career

Understanding companion objects is important for Kotlin developers, especially when working on Android apps or backend services where factory methods and static-like helpers are common.

1

Create the Book class with properties

Create a Kotlin class called Book with two val properties: title and author, both of type String. Use a primary constructor to set these properties.

Kotlin

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

Need a hint?

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

2

Add a named companion object called Factory

Inside the Book class, add a named companion object called Factory.

Kotlin

class Book(val title: String, val author: String) {
    companion object Factory {
        // Your code here
    }
}

Need a hint?

Write companion object Factory { } inside the class.

3

Add a fromString function inside Factory

Inside the named companion object Factory, add a function called fromString that takes a String parameter named data and returns a Book. The function should split data by " - " and use the parts to create and return a new Book.

Kotlin

class Book(val title: String, val author: String) {
    companion object Factory {
        // Your code here
    }
}

Need a hint?

Use data.split(" - ") to separate title and author, then return a new Book.

4

Create a Book using Factory and print details

Use the named companion object Factory to create a Book from the string "The Hobbit - J.R.R. Tolkien". Store it in a variable called book. Then print the book's title and author in the format: Title: The Hobbit, Author: J.R.R. Tolkien.

Kotlin

class Book(val title: String, val author: String) {
    companion object Factory {
        fun fromString(data: String): Book {
            val parts = data.split(" - ")
            return Book(parts[0], parts[1])
        }
    }
}

// Your code here

Need a hint?

Call Book.Factory.fromString("The Hobbit - J.R.R. Tolkien") and print the properties.