Primary constructor and init blocks in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a primary constructor in Kotlin?

A primary constructor is a concise way to declare and initialize class properties directly in the class header.

beginner

What is the purpose of an init block in Kotlin?

An init block contains initialization code that runs when an instance of the class is created, allowing extra setup beyond property declarations.

intermediate

How do primary constructor parameters relate to properties?

Parameters in the primary constructor can be used to initialize properties by prefixing them with val or var, making them class properties.

intermediate

Can a Kotlin class have multiple init blocks? What happens when it does?

Yes, a class can have multiple init blocks. They run in the order they appear in the class, each executing during object creation.

advanced

Explain the order of execution when creating an object with a primary constructor and init blocks.

First, the primary constructor parameters are passed. Then, property initializers run. After that, all init blocks execute in order. Finally, the object is fully created.

What keyword do you use to declare a property in the primary constructor?

A class

B fun

C val or var

D init

Properties in the primary constructor must be declared with val (read-only) or var (mutable).

When is the code inside an init block executed?

A When the program ends

B When the class is compiled

C When a method is called

D When an object of the class is created

Init blocks run during object creation, right after the primary constructor parameters are passed.

Can you have multiple init blocks in a Kotlin class?

A Yes, and they run in the order they appear

B No, only one init block is allowed

C Yes, but only the first one runs

D No, init blocks are not allowed

Multiple init blocks are allowed and execute sequentially in the order they are written.

What happens if you declare a primary constructor parameter without val or var?

A It is just a parameter and not a property

B It becomes a global variable

C It causes a compile error

D It becomes a property automatically

Parameters without val or var are just constructor parameters and do not become properties.

Which of these is the correct order during object creation?

A Property initializers → primary constructor → init blocks

B Primary constructor → property initializers → init blocks

C Init blocks → primary constructor → property initializers

D Primary constructor → init blocks → property initializers

First, primary constructor parameters are passed, then property initializers run, followed by init blocks.

Describe how you use a primary constructor and init blocks together to initialize a Kotlin class.

Think about how constructor parameters and init blocks complement each other.

Explain why you might use multiple init blocks in a Kotlin class.

Consider how splitting initialization into parts can help keep code 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