How is inheritance built and what is the relationship between types that allow and do not allow null values?

In programming languages that support nullable types (such as Kotlin or Swift), inheritance and relationships between types that can accept null and those that cannot are constructed as follows:

• A type that allows null is usually an extension (subtype) of a type that does not allow null. This means that if you have a type String, then String? (nullable) is a type that can contain either a string or null.

• This approach allows objects of type String to be used where String? is expected, but not vice versa, ensuring type safety and preventing NullPointerException errors.

Example in Kotlin:

val nonNullString: String = "Hello"
val nullableString: String? = null

fun printLength(str: String) {
    println(str.length)
}

printLength(nonNullString)      // Works
// printLength(nullableString)  // Compilation error, as nullableString can be null

Thus, the relationship between nullable and non-nullable types is built on the principle of subtyping, where the nullable type is an extension of the non-nullable, allowing explicit handling of null cases.