Constructors 

Constructors are used to initialize class properties. As in Java, a constructor is a special member function that is invoked when an object is instantiated. However, they work slightly different in Kotlin.

In Kotlin, there are two constructors:

• Primary constructor: This is a concise way to initialize the properties of a class
• Secondary constructor: This is where additional initialization logic goes

The primary constructor is part of the class header. Here's an example:

class User() {

}

The block of code surrounded by parentheses is the primary constructor. Consider the following code for 14a_PrimaryConstructor.kts:

class User(var firstName: String, var lastName: String) {

}

val user = User("Norman", "Lewis")
println("First Name= ${user.firstName}")
println("Last Name= ${user.lastName}")

The output is as follows:

The constructor goes here as part of the class header. The constructor declares two properties—firstName and lastName. Let's look at another example for 14b_PrimaryConstructor.kts:

class User(var firstName: String, val id: String) {

}
val user = User("Norman", "myId")
println("First Name = ${user.firstName}")
println("User Id = ${user.id}")

The output of the preceding code is as follows:

The secondary constructor is created using the constructor keyword. The class can declare a secondary constructor to initialize its properties. This is shown in the following code:

class AuditData {
    constructor(message: String) {
        //init logic
    }
    constructor(message: String, locale: String) {
        // init logic
    }
}

In the preceding code snippet, we wrote two constructors. One had a message as an argument and one had two arguments—message and locale. Consider the following code for 14c_SecondaryConstructor.kts:

var audit = AuditData("record added")
println("Message ${audit.message}")

audit = AuditData("record updated", "en-US")
println("Message: ${audit.message}")
println("Locale: ${audit.locale}")

When we call AuditData with only a message, the constructor with one argument will be invoked. Similarly, when we pass two arguments, a message and a locale, the constructor with two arguments will be invoked. 

The output is as follows: