Class declarations in Kotlin

Question

I was creating a sample Android project using Kotlin programming language using this blog. I'm new to Kotlin programming. I came across this line,

data class Cats(var data: Data? = null)

which I believe that it is, creating a class named Cats that has a variable named data. What does this data: Data? = null means? My whole class model is:

data class Cats(var data: Data? = null)

data class Data(var images: ArrayList<Image>? = null)

data class Image(var url: String? = "", var id: String? = "", var source_url: String? = "")

Answer 1

You are right that Cats class will have a mutable property named data, since it is declared inside a primary constructor of a class with var keyword (see the docs pages about classes and properties).

Next, question mark at type name in Kotlin means that a variable has nullable type, that is, it can store null value. Variables with not-null types, on the other hand, cannot hold nulls in Kotlin.

data: Data? = null is syntax for default parameter value. It allows not to pass data parameter to the constructor invocation, and in this case default value null will be used. This also works for functions.

And finally, data modifier at class declaration means that equals, hashCode, toString, copy and destructuring will be generated for the class, based on the properties declared in primary constructor.