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.