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
}

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
    }
}

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
    }
}

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

Hint

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

Practice

1. What is the main purpose of naming a companion object in Kotlin?

easy

A. To give a clear and specific name to the singleton object inside a class

B. To create multiple instances of the companion object

C. To make the companion object inherit from another class

D. To prevent the companion object from being accessed outside the class

Solution

1. Step 1: Understand companion objects

   Companion objects are singletons inside a class used to hold members related to the class itself.

2. Step 2: Purpose of naming companion objects

   Giving a name to a companion object helps to clearly identify and access its members, improving code readability.

3. Final Answer:

   To give a clear and specific name to the singleton object inside a class -> Option A

4. Quick Check:

   Named companion object = clear singleton name [OK]

Hint: Named companion objects clarify singleton usage [OK]

Common Mistakes:

• Thinking companion objects create multiple instances
• Confusing naming with inheritance
• Assuming naming restricts access

2. Which of the following is the correct syntax to declare a named companion object in Kotlin?

easy

A. class MyClass { companionobject Named { fun greet() = "Hi" } }

B. class MyClass { companion Named object { fun greet() = "Hi" } }

C. class MyClass { companion object Named { fun greet() = "Hi" } }

D. class MyClass { object companion Named { fun greet() = "Hi" } }

Solution

1. Step 1: Recall correct companion object syntax

   The correct syntax is 'companion object Name { ... }' inside the class.

2. Step 2: Check each option

   class MyClass { companion object Named { fun greet() = "Hi" } } matches the correct syntax exactly. Others have misplaced keywords or wrong order.

3. Final Answer:

   class MyClass { companion object Named { fun greet() = "Hi" } } -> Option C

4. Quick Check:

   companion object Name { } syntax [OK]

Hint: Remember: 'companion object Name' order is fixed [OK]

Common Mistakes:

• Swapping 'object' and 'companion' keywords
• Writing 'companionobject' as one word
• Placing the name before 'companion'

3. What will be the output of this Kotlin code?

class Box {
    companion object Factory {
        fun create() = "Box created"
    }
}
fun main() {
    println(Box.Factory.create())
}

medium

A. Factory created

B. Box created

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand named companion object usage

   The companion object named 'Factory' holds the function 'create' which returns "Box created".

2. Step 2: Analyze the main function call

   Calling 'Box.Factory.create()' accesses the named companion object and calls 'create', printing "Box created".

3. Final Answer:

   Box created -> Option B

4. Quick Check:

   Box.Factory.create() prints "Box created" [OK]

Hint: Call named companion with ClassName.Name.method() [OK]

Common Mistakes:

• Trying to call create() without Factory prefix
• Expecting 'Factory created' output
• Confusing syntax causing compile errors

4. Identify the error in this Kotlin code snippet:

class Sample {
    companion object Companion {
        fun info() = "Info"
    }
}
fun main() {
    println(Sample.companion.info())
}

medium

A. Incorrect companion object name usage; should be 'Companion' not 'companion'

B. Missing 'object' keyword in companion declaration

C. Function 'info' must be static

D. No error; code runs fine

Solution

1. Step 1: Check companion object name and usage

   The companion object is named 'Companion' with uppercase C.

2. Step 2: Analyze the call in main()

   The code calls 'Sample.companion.info()' with lowercase 'companion', which does not match the name and causes a compile error.

3. Final Answer:

   Incorrect companion object name usage; should be 'Companion' not 'companion' -> Option A

4. Quick Check:

   Use exact companion object name when calling [OK]

Hint: Match companion object name exactly (case sensitive) [OK]

Common Mistakes:

• Using lowercase 'companion' instead of named object
• Forgetting 'object' keyword
• Assuming companion functions must be static

5. You want to create a Kotlin class Logger with a named companion object Factory that provides a method create() returning a new Logger instance. Which code correctly implements this?

hard

A. class Logger {
companion Factory object {
fun create() = Logger()
}
}

B. class Logger {
companion object {
fun create() = Logger()
}
}

C. class Logger {
object Factory companion {
fun create() = Logger()
}
}

D. class Logger {
companion object Factory {
fun create() = Logger()
}
}

Solution

1. Step 1: Recall correct syntax for named companion object

   The correct syntax is 'companion object Factory { ... }' inside the class.

2. Step 2: Check method implementation

   The method 'create()' returns a new instance of Logger using 'Logger()'. class Logger { companion object Factory { fun create() = Logger() } } matches both syntax and logic.

3. Final Answer:

   class Logger { companion object Factory { fun create() = Logger() } } -> Option D

4. Quick Check:

   Named companion object with factory method = class Logger { companion object Factory { fun create() = Logger() } } [OK]

Hint: Use 'companion object Name' and return new instance inside [OK]

Common Mistakes:

• Swapping 'object' and 'companion' keywords
• Omitting the companion object name
• Incorrect order of keywords