Primary constructor and init blocks in Kotlin - Step-by-Step Execution

Concept Flow - Primary constructor and init blocks

Class Declaration with Primary Constructor

↓

Object Creation

↓

Primary Constructor Parameters Assigned

↓

Init Blocks Run

↓

Object Ready

When creating an object, Kotlin assigns primary constructor parameters to properties first, then runs init blocks in order, preparing the object.

Execution Sample

Kotlin

class Person(val name: String) {
    init {
        println("Init block: Name is $name")
    }
}

fun main() {
    val p = Person("Alice")
}

This code creates a Person object with a primary constructor and an init block that prints the name.

Execution Table

Step	Action	Evaluation	Output
1	Call Person("Alice")	Primary constructor parameter name = "Alice"
2	Enter init block	Access name = "Alice"	Init block: Name is Alice
3	Finish init block	No further action
4	Object creation complete	Person object with name="Alice" ready

💡 Init block runs after primary constructor parameters are set; object is ready after init blocks finish.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
name	undefined	"Alice"	"Alice"	"Alice"
Person object	none	creating	init block running	created

Key Moments - 2 Insights

Why can the init block access the primary constructor parameter 'name'?

Because the primary constructor parameters are available as properties during init block execution, as shown in step 2 of the execution_table.

Does the init block run before or after the primary constructor parameters are assigned?

The parameters are assigned first, then init blocks run, so init blocks can use those parameters (see step 1 and 2 in execution_table).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'name' during the init block (step 2)?

A "Alice"

B undefined

C null

D "Bob"

💡 Hint

Check the 'Evaluation' column at step 2 in execution_table.

At which step does the init block finish running?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Action' column describing init block progress in execution_table.

If we add another init block after the first, when will it run?

A Before primary constructor parameters are assigned

B Before the first init block

C After the first init block

D Never

💡 Hint

Init blocks run in the order they appear, after parameters are assigned (see concept_flow).

Concept Snapshot

class ClassName(val param: Type) {
  init {
    // code runs after primary constructor parameters are set
  }
}

- Primary constructor parameters are available in init blocks.
- Init blocks run in order during object creation.
- Useful for initialization logic needing constructor data.

Full Transcript

In Kotlin, when you create a class with a primary constructor, the parameters are set first. Then, any init blocks run in the order they appear. These init blocks can use the constructor parameters to run code during object creation. For example, if you have a class Person with a parameter name, the init block can print the name. The object is fully ready after all init blocks finish running.

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