Unless explicitly declared, a reference in kotlin cannot hold the value null
When a reference is nullable, a check on the reference must be done before accessing the object, otherwise we will get a compiler error:
var s : String = "a" s = null //<strong>does not compile </strong> var p: String? = "a" p = null //<strong>ok</strong> p.lenght //<strong>does not compile </strong> p?.length //<strong>ok</strong>
miniguide 