Primary constructor and init blocks in Kotlin - Mini Project: Build & Apply

Primary constructor and init blocks

📖 Scenario: Imagine you are creating a simple program to represent a book in a library system. Each book has a title and an author. You want to use Kotlin's primary constructor and init blocks to set up the book details and print a welcome message when a book object is created.

🎯 Goal: Build a Kotlin class called Book that uses a primary constructor to accept title and author. Use an init block to print a message confirming the book creation with its title and author.

📋 What You'll Learn

Create a class named Book with a primary constructor that takes title and author as parameters.

Use an init block inside the class to print a message: "Book created: [title] by [author]".

Create an instance of Book with the title "1984" and author "George Orwell".

Print the message from the init block automatically when the object is created.

💡 Why This Matters

🌍 Real World

Classes with primary constructors and init blocks are used to create objects with initial setup in many Kotlin applications like Android apps or backend services.

💼 Career

Understanding primary constructors and init blocks is essential for Kotlin developers to write clean and efficient code that initializes objects properly.

1

Create the Book class with primary constructor

Create a Kotlin class called Book with a primary constructor that takes two parameters: title of type String and author of type String.

Kotlin

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

Hint

Use the syntax: class ClassName(val param1: Type, val param2: Type) { }

2

Add an init block to print a message

Inside the Book class, add an init block that prints the message: "Book created: [title] by [author]" using the constructor parameters.

Kotlin

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

Hint

Use an init block and println with string interpolation to show the message.

3

Create an instance of Book

Create a variable called myBook and assign it a new Book object with the title "1984" and author "George Orwell".

Kotlin

class Book(val title: String, val author: String) {
    init {
        println("Book created: $title by $author")
    }
}

// Your code here

Hint

Use val myBook = Book("1984", "George Orwell") to create the object.

4

Run the program to see the output

Run the program and observe the output printed by the init block when myBook is created.

Kotlin

class Book(val title: String, val author: String) {
    init {
        println("Book created: $title by $author")
    }
}

val myBook = Book("1984", "George Orwell")

// Run the program to see the output

Hint

The message should print automatically when the object is created.

Practice

1. What is the main purpose of the init block in a Kotlin class with a primary constructor?

easy

A. To declare secondary constructors

B. To run additional code right after the primary constructor initializes properties

C. To define class-level constants

D. To override functions from a superclass

Solution

1. Step 1: Understand the role of primary constructor

   The primary constructor initializes the properties of the class when an object is created.

2. Step 2: Identify the purpose of the init block

   The init block runs immediately after the primary constructor to execute extra setup code or validations.

3. Final Answer:

   To run additional code right after the primary constructor initializes properties -> Option B

4. Quick Check:

   Init block = extra setup after constructor [OK]

Hint: Init blocks run right after primary constructor [OK]

Common Mistakes:

• Confusing init block with secondary constructor
• Thinking init block declares constants
• Assuming init block overrides superclass methods

2. Which of the following is the correct syntax to declare a primary constructor with two parameters name and age in Kotlin?

easy

A. class Person(name: String, age: Int) { }

B. class Person { constructor(name: String, age: Int) }

C. class Person(name String, age Int) { }

D. class Person(name: String; age: Int) { }

Solution

1. Step 1: Recall Kotlin primary constructor syntax

   Primary constructor parameters are declared in parentheses after the class name, separated by commas, with types after colons.

2. Step 2: Check each option

   class Person(name: String, age: Int) { } uses correct syntax: parentheses, commas, colons for types. Options A, B, and D have syntax errors.

3. Final Answer:

   class Person(name: String, age: Int) { } -> Option A

4. Quick Check:

   Primary constructor = class Name(params) [OK]

Hint: Primary constructor parameters go inside parentheses after class name [OK]

Common Mistakes:

• Using semicolons instead of commas
• Omitting colons before types
• Confusing primary with secondary constructor syntax

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

class User(val name: String) {
    init {
        println("Welcome, $name!")
    }
}

fun main() {
    val user = User("Anna")
}

medium

A. Compilation error

B. Welcome, name!

C. No output

D. Welcome, Anna!

Solution

1. Step 1: Understand object creation and init block

   When creating User("Anna"), the primary constructor sets name to "Anna" and then the init block runs.

2. Step 2: Analyze the init block output

   The init block prints "Welcome, $name!" where $name is replaced by "Anna".

3. Final Answer:

   Welcome, Anna! -> Option D

4. Quick Check:

   Init block prints welcome message with name [OK]

Hint: Init block runs on object creation, prints with property values [OK]

Common Mistakes:

• Expecting no output because println is inside class
• Confusing variable interpolation syntax
• Thinking init block runs only on demand

4. Identify the error in this Kotlin class:

class Product(val id: Int) {
    init {
        println("Product ID: $productId")
    }
}

medium

A. Init block cannot use println

B. Primary constructor must be empty

C. Variable productId is not declared

D. Class must have a secondary constructor

Solution

1. Step 1: Check variable usage in init block

   The init block tries to print productId, but the class has only id declared.

2. Step 2: Confirm variable declaration

   Since productId is not declared anywhere, this causes a compilation error.

3. Final Answer:

   Variable productId is not declared -> Option C

4. Quick Check:

   Use declared property names in init block [OK]

Hint: Use exact property names inside init blocks [OK]

Common Mistakes:

• Assuming init block can't use println
• Thinking primary constructor must be empty
• Believing secondary constructor is mandatory

5. You want to create a Kotlin class Employee with a primary constructor that takes name and salary. You also want to print a welcome message and calculate a 10% bonus right after object creation. Which is the best way to do this?

hard

A. Use primary constructor for parameters and an init block to print message and calculate bonus

B. Use only secondary constructors for all initialization

C. Put all code inside the primary constructor parentheses

D. Use companion object to run code after object creation

Solution

1. Step 1: Understand primary constructor role

   Primary constructor is for receiving parameters and initializing properties simply.

2. Step 2: Use init block for extra setup

   Init block runs after constructor to print messages and calculate bonus, keeping code organized.

3. Step 3: Evaluate other options

   Secondary constructors are unnecessary here; code can't go inside constructor parentheses; companion object is unrelated.

4. Final Answer:

   Use primary constructor for parameters and an init block to print message and calculate bonus -> Option A

5. Quick Check:

   Primary constructor + init block = clean setup [OK]

Hint: Use init block for extra setup after primary constructor [OK]

Common Mistakes:

• Trying to put logic inside constructor parentheses
• Using secondary constructors unnecessarily
• Misusing companion object for instance initialization