Null Safety – one of the biggest features of Kotlin’s, this feature aims to eliminate the danger of null references. Kotlin’s type system is responsible for this by distinguishing references that can hold nullable references and non-null references.
For example, a regular variable of type String can not hold null:
var a: String = "abc"
a = null // compilation error
To allow nulls, we can declare a variable as nullable string, written String?:
var b: String? = "abc"
b = null // ok
Now, if you call a method or access a property on a, it’s guaranteed not to cause an NPE, so you can safely say
val l = a.length
But if you want to access the same property on b, that would not be safe, and the compiler reports an error:
val l = b.length // error: variable 'b' can be null
But what if we still need to access property of the variables which are declared as nullable? There are a several ways of doing that.
1. Checking for null in good old if conditions
First, you can explicitly check if b is null, and handle the two options separately:
val l = if (b != null) b.length else -1
The compiler tracks the information about the check you performed, and allows the call to length inside the if. More complex conditions are supported as well:
if (b != null && b.length > 0) {
print("String of length ${b.length}")
} else {
print("Empty string")
}
Note that this only works where b is immutable (i.e. a local variable which is not modified between the check and the usage or a member val which has a backing field and is not overridable), because otherwise it might happen that b changes to null after the check.
2. Safe Calls
Your second option is the safe call operator, written ?.:
b?.length
This returns b.length if b is not null, and null otherwise. The type of this expression is Int?.
Safe calls are useful in chains. For example, if Bob, an Employee, may be assigned to a Department (or not), that in turn may have another Employee as a department head, then to obtain the name of Bob’s department head (if any), we write the following:
bob?.department?.head?.name
Such a chain returns null if any of the properties in it is null.
To perform a certain operation only for non-null values, you can use the safe call operator together with let:
val listWithNulls: List<String?> = listOf("A", null)
for (item in listWithNulls) {
item?.let { println(it) } // prints A and ignores null
}
3. Elvis Operator
When we have a nullable reference r, we can say “if r is not null, use it, otherwise use some non-null value x“:
val l: Int = if (b != null) b.length else -1
Along with the complete if-expression, this can be expressed with the Elvis operator, written ?::
val l = b?.length ?: -1
If the expression to the left of ?: is not null, the elvis operator returns it, otherwise it returns the expression to the right. Note that the right-hand side expression is evaluated only if the left-hand side is null.
Note that, since throw and return are expressions in Kotlin, they can also be used on the right hand side of the elvis operator. This can be very handy, for example, for checking function arguments:
fun foo(node: Node): String? {
val parent = node.getParent() ?: return null
val name = node.getName() ?: throw IllegalArgumentException("name expected")
// ...
}
4. The !! Operator
The third option is for NPE-lovers. We can write b!!, and this will return a non-null value of b (e.g., a String in our example) or throw an NPE if b is null:
val l = b!!.length
Thus, if you want an NPE, you can have it, but you have to ask for it explicitly, and it does not appear out of the blue.
Safe Casts
Regular casts may result into a ClassCastException if the object is not of the target type. Another option is to use safe casts that return null if the attempt was not successful:
val aInt: Int? = a as? Int
Collections of Nullable Type
If you have a collection of elements of a nullable type and want to filter non-null elements, you can do so by using filterNotNull.
val nullableList: List<Int?> = listOf(1, 2, null, 4)
val intList: List<Int> = nullableList.filterNotNull()