Primary and Secondary Constructor in Kotlin

Constructor is a special type of subroutine called to create an object. It is a special member function which can be invoked when an object of a class try to initialize variables or properties for that particular class.

Kotlin Constructor

Primary constructor

• A Kotlin class can only have one primary constructor.
• Primary constructor provide a simple way to initialize the member properties of a class.
• It takes a list of comma-separated parameters and declared just after the class name as a part of the header.

Primary Constructor

• Since primary constructor in Kotlin has a constrained syntax, it is defined only to declare class properties, it doesn’t accept any logic or code. So, to fill this gap Kotlin provides a flexible concept of init block in which we can add more custom code to perform some logic!

init block

• These init block is executed after the primary constructor is called and before any secondary constructors.

Secondary constructor

• A Kotlin class can have one or more and one or more secondary constructors.
• They must be prefixed by the keyword constructor.
• We can’t declare class properties inside secondary constructor the same way we do in primary constructor.
• Every secondary constructor must call explicitly the primary constructor. We can do that by using this keyword.

Reference

• https://kotlin-quick-reference.com/102b-R-secondary-constructors.html