While I am learning Kotlin, I notice some variables appended by a question mark. It is not common in other programming languages, so I do some search and got answer from Google.
In Kotlin, programmer could express nullability explicitly in the type of a variable. By appending a question mark to the type, we let the compiler (and other programmers) know that the variable is nullable.
When we try to assign null to a variable of non-nullable type:
var x: Int = null // Doesn't compile: // non-null.kt:1:14: error: null can not be a value of a non-null type Int // var x: Int = null
After appending the question mark, it is possible for the variable to hold null:
var y: Int? = null println(y) // null
Safe call operator
In order to call a member function on a variable of nullable type, the safe call operator (?.) is one of the possible ways. If we try to call a member function the usual way, we get an error:
y.plus(5) // Doesn't compile: // hello.kt:5:4: error: only safe (?.) or non-null asserted (!!.) calls are allowed on a nullable receiver of type Int? // y.plus(5)
Though if we use the safe call operator, we get a different result:
println(y?.plus(5)) // null