Null Pointer Exception working with Map (Kotlin) - kotlin

Question

I have the following class:
class SymbolTable(){
var map = mutableMapOf<String, Entry>()
var kindCounter = mutableMapOf<String, Int>()
fun define(name:String, kind:String, type:String){
if(kindCounter[kind]==0){
kindCounter[kind]=0
}
var index = 1
map[name]= Entry(type, kind, index)
kindCounter[kind]=kindCounter[kind]!!.plus(1)
}
class Entry looks like this:
class Entry(var type:String, var kind:String, var index:Int)
Main:
fun main(args:Array<String>){
var example = SymbolTable()
example.define("ex1", "ex1", "ex1")
example.define("ex2", "ex2", "ex2")
}
When I run the program and try using the "define" function I get the following error:
Exception in thread "main" kotlin.KotlinNullPointerException
at SymbolTable.define(SymbolTable.kt:21)
at SymbolTableKt.main(SymbolTable.kt:31)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
I assume the problem has something to do with how I create a new symbolTable class, but seeing as Kotlin doesn't have "new" I don't know how to avoid the null pointer exception.

Answer 1

if(kindCounter[kind]==0){
kindCounter[kind]=0
}
This doesn't make much sense: you test if the value is 0, and if it is, you set it to 0. So it's basically a noop.
What you want is to test if the value is null:
if (kindCounter[kind] == null) {
kindCounter[kind] = 0
}
You could also avoid using the dangerous !! operator by saving the value into a variable.
And you should really use val rather than var: all of your fields shouldn't be mutable:
class SymbolTable() {
val map = mutableMapOf<String, Entry>()
val kindCounter = mutableMapOf<String, Int>()
fun define(name: String, kind: String, type: String) {
val count = kindCounter[kind] ?: 0
map[name] = Entry(type, kind, 1)
kindCounter[kind] = count + 1
}
}
class Entry(val type: String, val kind: String, val index: Int)
fun main(args:Array<String>) {
val example = SymbolTable()
example.define("ex1", "ex1", "ex1")
example.define("ex2", "ex2", "ex2")
}