Companion vs top-level functions decision in Kotlin - Hands-On Comparison

Deciding Between Companion and Top-Level Functions in Kotlin

📖 Scenario: You are building a simple Kotlin program to manage a library of books. You want to organize your functions properly to keep your code clean and easy to understand.

🎯 Goal: Learn when to use companion object functions and when to use top-level functions by creating a Book class with a companion object and a top-level function to display book details.

📋 What You'll Learn

Create a Book class with a property title of type String

Add a companion object inside the Book class with a function createSampleBook() that returns a Book instance with the title "Sample Book"

Create a top-level function printBookTitle(book: Book) that prints the book's title

Call the companion object function and the top-level function to show their usage

💡 Why This Matters

🌍 Real World

Organizing code in Kotlin projects to keep related functions inside classes and general helpers outside helps maintain clean and readable code.

💼 Career

Understanding companion objects and top-level functions is important for Kotlin developers to write idiomatic and maintainable code in Android or backend projects.

1

Create the Book class

Create a class called Book with a property title of type String.

Kotlin

# Your code here

Need a hint?

Use class Book(val title: String) to create the class with a property.

2

Add a companion object with a factory function

Inside the Book class, add a companion object with a function called createSampleBook() that returns a Book instance with the title "Sample Book".

Kotlin

class Book(val title: String) {
    # Your code here
}

Need a hint?

Use companion object inside the class and define fun createSampleBook() that returns Book("Sample Book").

3

Create a top-level function to print the book title

Create a top-level function called printBookTitle that takes a Book parameter named book and prints its title property.

Kotlin

class Book(val title: String) {
    companion object {
        fun createSampleBook(): Book {
            return Book("Sample Book")
        }
    }
}

# Your code here

Need a hint?

Define a function printBookTitle outside the class that prints book.title.

4

Use the companion and top-level functions

Call the companion object function Book.createSampleBook() to create a book, then call the top-level function printBookTitle with that book. Print the output.

Kotlin

class Book(val title: String) {
    companion object {
        fun createSampleBook(): Book {
            return Book("Sample Book")
        }
    }
}

fun printBookTitle(book: Book) {
    println(book.title)
}

# Your code here

Need a hint?

Use val sampleBook = Book.createSampleBook() and then printBookTitle(sampleBook) inside main().