What is the difference between properties and parameters in Kotlin?

Question

Here is a simple example of a class with some code (properties) inside the bracket

class Person(firstName: String) {
....
}

Now here is an example of a function with some code (parameters) inside the bracket

fun double(x: Int) {
...
}

I know this is a fundamental question but I am quite confused as a beginner.

Answer 1

You pass parameters to functions and constructors, and classes have properties.

The constructor of the Person class in your example has a single parameter, and so does the double function. In this case, the firstName parameter is not a property!

To make it a property, you have to declare it so:

class Person(firstName: String) {

  val firstName : String = firstName
}

Kotlin allows this to be shorter, which makes the firstName parameter serve as a property:

class Person(val firstName: String)

Answer 2

First, your firstName also is a parameter rather than a property in Person class.

//           v-- a parameter declared on primary constructor
class Person(firstName: String)

you can access a parameter declared on primary constructor in init block or property/field declaration, for example:

class Person(firstName: String){
   val first:String
   init{ first=firstName }
}

class Person(firstName: String){
   val first:String = firstName
}

class Person(firstName: String){
   @JvmField val first:String = firstName
}

to make the firstName to a property you can using keyword val or var, for example:

//            v--- immutable property
class Person(val firstName: String)

//            v--- mutable property
class Person(var firstName: String)

a Kotlin property will generate getter/setter(?) and a backing field(?) to java byte code. Take an example of a mutable property to java byte code as below:

public final class Person{
   private String firstName; // backing field

   //                   v--- parameter
   public Person(String firstName){ this.firstName = firstName; }

   //getter
   public final String getFirstName(){ return firstName; }

   //setter
   public final String setFirstName(String firstName){ this.firstName= firstName; }
}

a parameter only visible in function scope/constructor scope except parameter declared on primary constructor.

Note: a parameter is immutable like as java effective-final variables/parameters, so you can't reassign a parameter at all in Kotlin, for example:

fun foo(bar:String){
   bar = "baz"
// ^--- compile error  
}