Question: A nullable type can be checked and cast to a non-nullable type, but the other way around is quite difficult.

For example:

```
str1: String? = ""
// if str1 not null
str2: String = str1

str3: String? = str2 // This works!

```

In my case I'm talking about a function with an Array parameter in which the contents could be null, but the contents of the Array I want to pass to the function is non-nullable. That does not work:

```
var var1: Array<String?> = arrayOfNulls(10)

var var2: Array<String> = var1.filterNotNull().toTypedArray()

var var3: Array<String?> = var2 // This does not work!

```

My question is how to make this work?

I am able to cast the array to a nullable array, but I'm not sure if this is the correct way of doing this.

Answer: 
In Kotlin, arrays are invariant, which means that an array of a given type can't be assigned to an array of its parent type. That's why you can't assign `Array<String>` to `Array<String?>`, even if `String` is a subclass of `String?`. For this to work, you should either

1. use lists, which are covariant:

```
var var1: Array<String?> = arrayOfNulls(10)
var var2: List<String> = var1.filterNotNull()
var var3: List<String?> = var2  // Now works

```

2. cast the array to a nullable array, as you're doing:

```
var var1: Array<String?> = arrayOfNulls(10)
var var2: Array<String> = var1.filterNotNull().toTypedArray()
var var3: Array<String?> = var2 as Array<String?>  // Now works

```
what I can say is,

```
//This value can hold null values 
str1: String? = ""

// if str1 not null, store values in str2
// so, now str2 holds NOT NULL VALUES
str2: String = str1

// Now by default since str2 is NOT NULL
// All the values being passed inside str2 will always be NOT NULL
// 
str3: String? = str2 // This does not work

```

str3 will only throw an error in case str2 is null but it is not possible since as you said, it goes through a null check.

the workaround looks something like this.

1. This is one possible solution, i.e. defining str2 as a Nullable string

```
str1 : String? = ""

str2 : String? = str1

str3 : String? = str2

```

2. Another solution would be, putting null check at str2, because since str2 is not null str3 will by default be Not Null.

```
str1 : String? = ""

str2 : String = str1 ?: ""

str3 : String = str2

```

**Update**

As per the updated question,

```
// A list of nulls
var var1: List<String?> = listOf(null, null, null, null, null, null, null, null, null, null)

//Mutable List to hold not null values or filter out null values
var var2: MutableList<String> = mutableListOf()
for (i in var1) var2.add(i ?: "")

//var3 will always be not null
var var3: List<String?> = var2

```