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

```
```
var str1: String? = ""

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

var str3: String? = str2  

```

> 
> Try This, Here ?: stands for checking if a value is null or not, if
> not null its fine, but if it gets null you can set a default value in
> it, that why i used a default value "" in it.
> 
> 
>