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A secondary constructor in Kotlin is an auxiliary constructor defined within the class body using the constructor keyword. It provides alternative ways to instantiate a class by accepting different parameter signatures and executing specific initialization logic.

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Idiomatic Kotlin: Default Arguments

In Kotlin, secondary constructors are rarely required. Constructor overloading is idiomatically handled by defining default arguments directly in the primary constructor. This eliminates the boilerplate of writing multiple secondary constructors just to handle omitted parameters. Secondary constructors are typically only introduced when default arguments cannot express the required initialization logic, or when interoperating with Java frameworks that require specific constructor signatures. 
// Idiomatic Kotlin: Using default arguments instead of secondary constructors
class User(val name: String, val age: Int = 0, val isEmployed: Boolean = false)

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Syntax

Secondary constructors are declared inside the class block. Unlike primary constructors, they cannot declare class properties; parameters passed to a secondary constructor cannot be prefixed with val or var and act strictly as local variables within the constructor’s scope. 
class ClassName {
    constructor(parameter: Type) {
        // Initialization logic
    }
}

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Delegation Rules

Kotlin enforces strict delegation rules for secondary constructors depending on the presence of a primary constructor. 1. With a Primary Constructor If a class declares a primary constructor, every secondary constructor must delegate to it, either directly or indirectly (via another secondary constructor). Delegation to another constructor in the same class is handled using the this keyword. 
class User(val name: String) { // Primary constructor
    var age: Int = 0
    var isEmployed: Boolean = false

    // Direct delegation to the primary constructor
    constructor(name: String, age: Int) : this(name) {
        this.age = age
    }

    // Indirect delegation (calls the secondary constructor above)
    constructor(name: String, age: Int, isEmployed: Boolean) : this(name, age) {
        this.isEmployed = isEmployed
    }
}
2. Without a Primary Constructor If a class does not have a primary constructor, secondary constructors must delegate to the superclass constructor using the super keyword, or delegate to another secondary constructor that eventually calls super. If the superclass has a default no-argument constructor, the super() call is implicit. 
open class Entity(val id: String)

class Document : Entity {
    // Delegating to the superclass constructor
    constructor(id: String) : super(id) {
        // Initialization logic
    }
}

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Execution Order

When a class is instantiated via a secondary constructor, the initialization sequence follows a strict hierarchy. In Kotlin, the primary constructor does not have a separate body; its execution consists of delegating to the superclass and then running the property initializers and init blocks. The execution order is:
  1. Primary Constructor Delegation: The secondary constructor delegates to the primary constructor (or directly to the superclass if no primary constructor exists). This delegation executes the superclass constructor, followed immediately by the class-level property initializers and init blocks in the exact top-to-bottom order they are defined in the class body.
  2. Secondary Constructor Body: Once the delegation phase completes, the body of the secondary constructor executes last.
class InitializationSequence(val name: String) {
    
    init {
        // Executes first as part of the primary constructor delegation
        println("1. Init block executes")
    }

    constructor(name: String, age: Int) : this(name) {
        // Executes second, after the primary constructor delegation finishes
        println("2. Secondary constructor body executes")
    }
}
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