Primary constructor and init blocks in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a primary constructor with a parameter name.

Kotlin

class Person [1](val name: String) {}

Drag options to blanks, or click blank then click option'

Avar

Bfun

Cconstructor

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Using fun instead of constructor.

Using var or init in the constructor declaration.

In Kotlin, the primary constructor is declared right after the class name using the constructor keyword or directly with parameters. Here, constructor keyword is used explicitly.

2fill in blank

medium

Complete the code to initialize the age property inside the init block.

Kotlin

class Person(val name: String, val age: Int) {
    init {
        println("Age is [1]")
    }
}

Drag options to blanks, or click blank then click option'

Aage

Bname

Cthis

Dconstructor

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use this without a property name.

Using name instead of age.

Inside the init block, you can access the primary constructor parameters directly by their names. Here, age is printed.

3fill in blank

hard

Fix the error in the primary constructor declaration by filling the blank.

Kotlin

class Car [1] val model: String, val year: Int) {}

Drag options to blanks, or click blank then click option'

A[

B{

C<

D(

Attempts:

3 left

💡 Hint

Common Mistakes

Using curly braces or square brackets instead of parentheses.

Forgetting the opening parenthesis.

The primary constructor parameters must be enclosed in parentheses after the class name.

4fill in blank

hard

Fill both blanks to create a property color with a default value and print it in the init block.

Kotlin

class Bike(val model: String, val year: Int, val color: String = [1]) {
    init {
        println("Bike color is [2]")
    }
}

Drag options to blanks, or click blank then click option'

A"red"

Bcolor

Cmodel

D"blue"

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong default value syntax.

Printing model instead of color.

The property color is given a default value "red". Inside the init block, the property color is printed.

5fill in blank

hard

Fill all three blanks to create a class with a primary constructor, an init block that prints a greeting, and a property greeting initialized from the constructor.

Kotlin

class Greeter [1](val name: String) {
    val greeting = "Hello, [2]!"
    init {
        println([3])
    }
}

Drag options to blanks, or click blank then click option'

Aconstructor

Bname

Cgreeting

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the constructor keyword.

Using init instead of greeting in the print statement.

The primary constructor is declared with constructor. The property greeting uses the name parameter. The init block prints the greeting property.

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