Questions tagged [kotlin-null-safety]
153 questions
323
votes
7 answers
What is the Kotlin double-bang (!!) operator?
I'm converting Java to Kotlin with Android Studio. I get double bang after the instance variable. What is the double bang and more importantly where is this documented?
mMap!!.addMarker(MarkerOptions().position(london).title("Marker in London"))
mbr_at_ml
- 3,585
- 2
- 13
- 12
158
votes
6 answers
Best way to null check in Kotlin?
Should I use double =, or triple =?
if(a === null) {
//do something
}
or
if(a == null) {
//do something
}
Similarly for 'not equals':
if(a !== null) {
//do something
}
or
if(a != null) {
//do something
}
pdeva
- 43,605
- 46
- 133
- 171
75
votes
8 answers
How do I run a block of code if a nullable type is null?
In Kotlin, I can run code if an object is not null like this:
data?.let {
// execute this block if not null
}
But how can I execute a block of code if the object is null?
Toni Joe
- 7,715
- 12
- 50
- 69
50
votes
9 answers
Idiomatic way of handling nullable or empty List in Kotlin
Say I have a variable activities of type List?. If the list is not null and not empty, I want to do something, otherwise I want to do something else. I came up with following solution:
when {
activities != null && !activities.empty ->…
Kirill Rakhman
- 42,195
- 18
- 124
- 148
43
votes
7 answers
how to use spring annotations like @Autowired or @Value in kotlin for primitive types?
Autowiring a non-primitive with spring annotations like bellow works:
@Autowired
lateinit var metaDataService: MetaDataService
But this doesn't work:
@Value("\${cacheTimeSeconds}")
lateinit var cacheTimeSeconds: Int
Error:
lateinit modifier is…
fkurth
- 868
- 1
- 8
- 11
39
votes
5 answers
How to idiomatically test for non-null, non-empty strings in Kotlin?
I am new to Kotlin, and I am looking for help in rewriting the following code to be more elegant.
var s: String? = "abc"
if (s != null && s.isNotEmpty()) {
// Do something
}
If I use the following code:
if (s?.isNotEmpty()) {
The compiler will…
iForests
- 6,757
- 10
- 42
- 75
31
votes
2 answers
Is `a?.let{} ?: run{}` idiomatic in Kotlin?
I saw the following comment in a S.O. post, and I'm intrigued:
why don't you use if for null checks? a?.let{} ?: run{} is only appropriate in rare cases, otherwise it is not idiomatic – voddan May 15 '16 at 7:29 best way to null check in…
Andy Marchewka
- 412
- 1
- 5
- 8
29
votes
2 answers
How is Dart "sound null-safety" different from Kotlin null safety?
This Dart official video states that Dart's so-called "sound null safety" is better than Kotlin's null safety design, because it can optimise the code based on whether a variable is declared nullable, and other languages (I assume this refers to…
Yihao Gao
- 535
- 7
- 11
27
votes
3 answers
Only safe or non null assserted calls are allowed on a nullable receiver type of arraylist
Just started using kotlin for android development.My arraylist is declared like this-
var day1: ArrayList? = null
Now I am trying to access an element by its position
val dietPlan= day1[position]
but i am getting below…
Android Developer
- 9,157
- 18
- 82
- 139
25
votes
3 answers
Kotlin - lateinit VS Any? = null
In Kotlin there appears to be two method of declaring a variable inside an object that can be null and instantiated after the object is created.
var myObject : Any? = null
or
var lateinit myObject : Any
I am confused about why the lateinit…
fergdev
- 963
- 1
- 13
- 27
21
votes
6 answers
Cannot set non-nullable LiveData value to null
The error from the title is returned for the following code, which makes no sense
private val _error = MutableLiveData()
val error: LiveData get() = _error
_error.postValue(null) //Error Cannot set non-nullable LiveData value to…
pedja
- 3,285
- 5
- 36
- 48
19
votes
1 answer
How to set lateinit Kotlin property to null
The below class has a very unique lifecycle, which requires me to temporarily null out lateinit properties
class SalesController : BaseController, SalesView {
@Inject lateinit var viewBinder: SalesController.ViewBinder
@Inject lateinit var…
ZakTaccardi
- 12,212
- 15
- 59
- 107
18
votes
6 answers
Kotlin call function only if all arguments are not null
Is there a way in kotlin to prevent function call if all (or some) arguments are null? For example Having function:
fun test(a: Int, b: Int) { /* function body here */ }
I would like to prevent null checks in case when arguments are null. For…
xinaiz
- 7,744
- 6
- 34
- 78
15
votes
3 answers
Combine null safety and assertNotNull
In test we usually have assertNotNull, but it does not perform smart cast from a nullable type to a non-nullable. I have to write something like this:
if (test == null) {
Assert.fail("")
return
}
Is it a workaround to perform smart cast…
Boris
- 4,944
- 7
- 36
- 69
14
votes
3 answers
Android Room returns Null as Non-Null type
I have Dao to return simple object. If object does not exist, Room return null, but Android app have no crashes. Also if I assign that value to non-null variable, no crashes are in app.
Dao:
@Query("SELECT * FROM users WHERE id LIKE :id LIMIT…
Francis
- 6,788
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