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kotlinlang

Hi, I have a silly question. Can a list of non-null type ever be possible to contain null value somehow?

You will get a descriptive exception thrown when accessing the list from Kotlin though

Java code:
```
class Evil {
  public static void beEvil(List&lt;String&gt; list) {
    list.add(null);
  }
}
```
In Kotlin:
```
fun main() {
    val list = mutableListOf&lt;String&gt;()
    Evil.beEvil(list)
    println(list[0]) // throws an exception
}
```

yeah, I’m indeed using Kotlin + Java. The example definitely make senes but the NPE from a non-null type list is certainly from Kotlin code only.

Yes, Java has no non-null types, hence there is no way for Java code to throw

I think my question should be . In Kotlin environment, I’m seeing NPE from iterating non-null list.

Then some Java code put null into the list.

From the Kotlin side you must ensure that you don't pass a non-null list where Java might add a null.

umm…it’s unlikely Java code. Could it ever be Kotlin?

I checked, no Java code is modifying that list, very weird

my guess is that the NPE is thrown from Java code that is probably related to the implementation that you use

NPE is actually from Kotlin code. And how can concurrency inject null value into non-null list?

it depends on how well the abstraction holds

<https://stackoverflow.com/questions/53015361/non-null-value-in-kotlin-produces-nullpointerexception>

@SinceKotlin("1.1") public actual typealias ArrayList&lt;E&gt; = java.util.ArrayList&lt;E&gt;

ok, so the built in ArrayList is just an alias for the jvm one

can you try to use a thread safe array list

check if that fixes it. if it does it's a concurrency issue

yeah, guess this is the only thing  I can do

my guess is some jvm code is modifying your list in whatever way

or it's really an android/jvm bug which I doubt

yeah, I’m more leaning towards concurrency issue too. Very hard to reproduce it.

One more dirty workaround consider list values as nullable if this possible for your code, but it may hide real bug in some cases