The block of code in the curly brackets is only executed if a is non-null. If you only want to perform an operation if something is non-null you can use the let method: a?.let In this case we use the plus infix function rather than an operator so that we can use the safe call operator. That is, the integer a is a full object with methods and properties. There are a few small things to notice about this example – by type inference b is a nullable type: var b:Int? = a?.plus(1)Īnd nullable simple types are 'boxed'. Here, if a isn't null then b is set to 2, but if a is null then b is set to null. This compiles and you can be sure that a isn't null when the addition is performed.Īlternatively, if you are accessing a property then you can use the safe call operator ?. To use a nullable variable you can test for null using an if statement: var a: Int? = 1 Generates a compiler error because you are adding one to a without checking that it isn't null. Works without any problems as a can never be null as it is a non-nullable, but: var a:Int? = 1 To avoid accidentally using a null reference the compiler will throw an error if you try to use a nullable without first checking that it is non-null. Works without a compiler error or warning. Now myVariable can be set to null, i.e.: myVariable = null If you need a nullable reference then you have to explicitly declare it using ? as in: var myVariable: sometype? = something This often occurs if you are trying to use a Java function which, of course, doesn’t support non-nullable types. For example, if a function could return a null, you cannot assign it to a non-nullable. The compiler tracks operations that could generate a null, and flags any operation that could possibly set a non-nullable variable to null. If you declare a variable in the usual way you get a non-nullable: var myVariable: sometype = somethingĪnd you cannot set the variable to null as: myVariable = null References can be either non-nullable or nullable.

The compiler option : defines annotation package and reporting level.One of the most important features of Kotlin is that it eliminates the null reference error and you can’t meet null safety early enough. Additionally, we can specify which annotations must be reported and how by the compiler. Moreover, the class JvmAnnotationNames contains a complete list of supported annotations.

Kotlin nullable types