Secondary constructors in Kotlin - Step-by-Step Execution

Concept Flow - Secondary constructors

Class Declaration

↓

Primary Constructor?

Yes→Initialize Primary

↓

Secondary Constructor

↓

Call Primary or Another Secondary

↓

Initialize Properties

↓

Object Created

Shows how Kotlin class creation flows through primary and secondary constructors, with secondary constructors calling primary or other secondary ones before initializing.

Execution Sample

Kotlin

class Person(val name: String) {
    var age: Int = 0
    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }
}

val p = Person("Anna", 25)

Creates a Person object using a secondary constructor that sets name and age.

Execution Table

Step	Action	Constructor Called	Property Values	Notes
1	Create Person("Anna", 25)	Secondary constructor (name, age)	name=uninitialized, age=0	Start secondary constructor
2	Call primary constructor with name="Anna"	Primary constructor (name)	name="Anna", age=0	Primary constructor initializes name
3	Execute secondary constructor body	Secondary constructor (name, age)	name="Anna", age=25	Secondary constructor sets age
4	Object creation complete	Person instance	name="Anna", age=25	Person object ready

💡 Secondary constructor calls primary constructor first, then sets additional properties, finishing object creation.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
name	uninitialized	uninitialized	"Anna"	"Anna"	"Anna"
age	0	0	0	25	25

Key Moments - 3 Insights

Why does the secondary constructor call the primary constructor first?

Because in Kotlin, every secondary constructor must delegate to the primary constructor (directly or indirectly) to ensure the base properties are initialized properly, as shown in execution_table step 2.

What happens if the secondary constructor does not call the primary constructor?

The code will not compile. Kotlin requires secondary constructors to delegate to the primary constructor to maintain consistent initialization, as seen in the flow and step 2 of the execution_table.

When is the property 'age' set in the example?

The 'age' property is set in the body of the secondary constructor after the primary constructor has initialized 'name', as shown in execution_table step 3.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'name' after step 2?

A "Anna"

B uninitialized

C null

D 25

💡 Hint

Check the 'Property Values' column in execution_table row for step 2.

At which step does the secondary constructor set the 'age' property?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Notes' column in execution_table for when 'age' changes.

If the secondary constructor did not call the primary constructor, what would happen?

A The object is created without 'name'

B Compilation error occurs

C The program runs normally

D The secondary constructor is ignored

💡 Hint

Refer to key_moments about constructor delegation requirement.

Concept Snapshot

Secondary constructors in Kotlin allow multiple ways to create objects.
Each secondary constructor must call the primary constructor.
Primary constructor initializes main properties.
Secondary constructors add or override initialization.
This ensures consistent object setup.

Full Transcript

In Kotlin, a class can have a primary constructor and one or more secondary constructors. When you create an object using a secondary constructor, Kotlin requires that this constructor calls the primary constructor first. This ensures that the main properties defined in the primary constructor are initialized properly. After calling the primary constructor, the secondary constructor can set additional properties or perform extra setup. For example, in the Person class, the secondary constructor takes both name and age. It calls the primary constructor with name, then sets the age property. This flow guarantees that the object is fully and correctly initialized before it is used.