Kotlin: Why is defining a field as "var"/"val" needed in constructor? - syntax

Question

This is giving an error:
class Apple(weightInGrams: Float){
fun grow() {
weightInGrams+= 2.0f
}
}

Answer 1

First of all, the equivalent of void (in Java) is Unit (in Kotlin), and the type a function returns goes at the end, so you should use fun grow(): Unit { ... } instead of fun void grow() { ... }. Moreover, you can omit Unit and just write fun grow() { ... } because the compiler knows that your function doesn't return any meaningful value.
Now, I'll try to explain the basics to answer your question and give you some clarity. In Java, the parameters of a constructor are visible only inside that constructor. In Kotlin, the parameters are only visible in initializer blocks and in property initializers, unless you transform them into properties. Let's explain all this with examples.
In Java, we see constructors in classes like this many times:
public class Person {
public final String name;
public final Integer age;
public Person(String name, Integer age) {
this.name = name;
this.age = age;
}
}
The parameters are used to initialize the fields of the class Person.
In Kotlin, the equivalent could be:
a) Use the parameters in initializer blocks.
class Person(name: String, age: Int) {
val name: String
val age: Int
init {
this.name = name
this.age = age
}
}
b) Use the parameters in property initializers declared in the class body.
class Person(name: String, age: Int) {
val name = name
val age = age
}
c) Declaring properties and initializing them directly in the primary constructor.
class Person(val name: String, val age: Int)
Therefore, if you write var or val, the parameters of the constructor will be also properties and you will be able to use them in your class like you want to do inside your function grow.
So, your final code should be:
class Apple(var weightInGrams: Float) {
fun grow() {
weightInGrams += 2.0f
}
}
var because you are assigning a value to weightInGrams multiple times.

Answer 2

make your property a class member
class Apple(var weightInGrams: Float){
fun void grow() {
weightInGrams+= 2.0f
}
}

Answer 3

I understand the question was already answered.
If you want to initialize an apple with an initialWeight, you can do it as below. The init block can help initialize the value and the grow function can effectively work on the actual variable without a need to declare the constructor variable as var:
class Apple(initWeight: Float){
var weightInGrams = 0.0f
init {
var weightInGrams = initWeight
}
fun grow() {
weightInGrams+= 2.0f
}
}
fun main(args: Array<String>) {
val a = Apple(10.0f)
a.grow()
println(a.weightInGrams)
};