Nullable types

Kotlin's type system is advanced enough that it can track the difference between nullable types and non-nullable types. When we define a variable in Kotlin, as we have been doing so far, we cannot assign a null to it. This code, for instance, would not compile:

    val name: String = null // does not compile 

Assigning null to a var will not compile either:

    var name: String = "harry" 
    name = null // does not compile 

To inform the Kotlin compiler that we will allow a variable to contain a null, we must suffix the type with a ?:

    val name: String? = null 
    var name: String? = "harry" 
    name = null 

Both the preceding snippets will now compile.

Similarly, we can return nullable and non-nullable types from a function, use them as function parameters, and so on:

    fun name1(): String = ... 
 
    fun name2(): String? = ... 

The name1 function cannot return a null reference. The name2 function may or may not. If we were...