Explain the difference between primary and secondary constructors in Kotlin. How can you call a secondary constructor from a primary constructor?

In Kotlin, a class can have one primary constructor and one or more secondary constructors. The primary constructor is defined as part of the class header and is responsible for initializing the class.

On the other hand, secondary constructors are defined inside the class body and provide an additional way to initialize class properties. Secondary constructors are optional, and if they are present, they must directly or indirectly delegate to the primary constructor.

A class with a primary constructor:

class Person(firstName: String, age: Int) {
    // Class properties
    val firstName: String
    val age: Int

    init {
        // Initialize class properties in the primary constructor
        this.firstName = firstName
        this.age = age
    }
}

A class with both primary and secondary constructors:

class Person {
    // Class properties
    val firstName: String
    val age: Int

    constructor(firstName: String) {
        // Initialize a few properties in the secondary constructor
        this.firstName = firstName
        this.age = 0
    }

    constructor(firstName: String, age: Int) {
        // Initialize all the properties in the secondary constructor
        this.firstName = firstName
        this.age = age
    }
}

To call a secondary constructor from a primary constructor in Kotlin, we can use the ‘this‘ keyword along with the required arguments. Here is an example:

class Person(firstName: String) {
    val lastName: String

    constructor(firstName: String, lastName: String) : this(firstName) {
        // call to primary constructor to initialize firstName
        this.lastName = lastName
    }
}

In the above example, we have defined a secondary constructor that takes two arguments, ‘firstName‘ and ‘lastName‘. This constructor calls the primary constructor using the ‘this‘ keyword, passing only the ‘firstName‘ argument. After that, we can initialize ‘lastName‘ in the body of the secondary constructor. By doing this, we avoid duplicating the initialization logic of ‘firstName‘, which has already been done in the primary constructor.