Primary constructor and init blocks in Kotlin

Introduction

Primary constructors and init blocks help set up an object when you create it. They make sure the object starts with the right values.

When you want to give initial values to an object right when you create it.

When you need to run some setup code as soon as an object is made.

When you want to keep your code simple by combining value setup and extra initialization.

When you want to check or change values before the object is ready to use.

Syntax

Kotlin

class ClassName constructor(parameter1: Type, parameter2: Type) {
    init {
        // code to run when object is created
    }
}

The constructor keyword is optional if there are no annotations or visibility modifiers.

The init block runs every time an object is created, right after the primary constructor.

Examples

Simple class with primary constructor and init block that prints info when a Person is created.

Kotlin

class Person(val name: String, val age: Int) {
    init {
        println("Person created: $name, age $age")
    }
}

Init block calculates area using constructor values and prints it.

Kotlin

class Rectangle(val width: Int, val height: Int) {
    val area: Int
    init {
        area = width * height
        println("Area calculated: $area")
    }
}

Class with empty primary constructor and init block that runs on creation.

Kotlin

class EmptyExample {
    init {
        println("Object created with no parameters")
    }
}

Init block checks the value and prints a warning if it is negative.

Kotlin

class SingleParam(val value: Int) {
    init {
        if (value < 0) {
            println("Warning: value is negative")
        }
    }
}

Sample Program

This program creates two Car objects. The init block prints details and warns if the year is before 1886, when cars were invented.

Kotlin

class Car(val brand: String, val year: Int) {
    init {
        println("Creating a car: $brand from year $year")
        if (year < 1886) {
            println("Warning: Cars were not invented before 1886!")
        }
    }
}

fun main() {
    println("Start program")
    val car1 = Car("Toyota", 2020)
    val car2 = Car("Ford", 1800)
    println("End program")
}

Important Notes

Time complexity: The init block runs once per object creation, so O(1) time per object.

Space complexity: No extra space beyond the object itself is used by init blocks.

Common mistake: Trying to use uninitialized properties inside init blocks before they are set.

Use init blocks when you need to run extra setup code beyond just assigning constructor parameters.

Summary

Primary constructors set initial values when creating an object.

Init blocks run right after the primary constructor to do extra setup.

Use them together to keep object creation simple and clear.

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