# Kotlin Nullability

###### tags: `Kotlin` `Null`
[TOC]

## ? Operator
By stating a variable is optional(?), this variable can be null.

var nonNullable: String = "Lily"
var isNullable: String? = null

nonNullable = null //Compilation Error
isNullable = null //OK because it is optional variable

For **optional variables**, we will have to check for its nullability when we try to access its information.

var nonNullable: String = "Lily"
var isNullable: String? = null

nonNullable.length
isNullable?.length

## Aritmetic Operators on null values
| Operation | Non Null Operation | Replacement Method |
| -------- | -------- | -------- |
| Addition | + | ?.plus() |
| Substraction | - | ?.minus() |
| Multiplication | * | ?.times() |
| Divisor | / | ?.div() |
| Remainder | % | ?.rem() |

## Elvis Operator (?:)
Guaratess a result return, if is null it returns the default value given, if not just give the original value.

var isNullable: String? = null
println(isNullable?:"This String is null")
println(isNullable?:"Null".length)

## Non null assertion (!!)
We use this when we believe that the value is definitely non null. Unless we are 100% sure, we will try to avoid using it, as it has high possibility to give us a crash if the value is null.

var isNullable: String? = null
println(isNullable!!) //CRASH cannot assert a null value

var isNullable = "non Null"
println(isNullable!!) // It's alright as there is a value inside

## lateinit var
**lateinit var** is used when you do not especify the variable from the beginning, and you expect to instance it later.

### Wrong Practice
> **BE CAREFUL** [color=red]
> **NEVER** use == null to check a lateinit var, they are expected to be instanced, therefore if you check it like this you will 100% crash by getting **UninitializedPropertyAccessException**.

private lateinit var apiService : RetrofitApi

fun getInstance(addHeader: Boolean) : RetrofitApi{
    if(apiService == null){
        synchronized(RetrofitApi::class.java){
            makeService()
        }
    }
    return apiService
}

 //Result: 
 //kotlin.UninitializedPropertyAccessException: lateinit property apiService has not been initialized

### Right Practice
>**PLEASE USE**
>**::variable.isInitialized** to check whether lateinit was initialized or not.[color=green]

private lateinit var apiService : RetrofitApi
private lateinit var apiServiceHeader : RetrofitApi

fun getInstance(addHeader: Boolean) : RetrofitApi{
    if(!::apiService.isInitialized || !::apiServiceHeader.isInitialized ){
        synchronized(RetrofitApi::class.java){
            makeService()
        }
    }
    return if(addHeader){
        apiServiceHeader
    } else {
        apiService
    }
}