Best Kotlin Programming Guides to Buy in October 2025
Head First Kotlin: A Brain-Friendly Guide
Android Programming with Kotlin for Beginners: Build Android apps starting from zero programming experience with the new Kotlin programming language
Head First Android Development: A Learner's Guide to Building Android Apps with Kotlin
Programming Android with Kotlin: Achieving Structured Concurrency with Coroutines
Kotlin from Scratch: A Project-Based Introduction for the Intrepid Programmer
Kotlin In-Depth: A Guide to a Multipurpose Programming Language for Server-Side, Front-End, Android, and Multiplatform Mobile (English Edition)
Kotlin: An Illustrated Guide
Functional Programming in Kotlin
Kotlin Design Patterns and Best Practices: Elevate your Kotlin skills with classical and modern design patterns, coroutines, and microservices
Kotlin in Action
In Kotlin, you can set a custom return type as nullable by appending a question mark (?) after the type declaration. This indicates that the return type can either be of the specified type or null.
For example, if you have a function that returns a String and you want to make the return type nullable, you can declare the function like this:
fun exampleFunction(): String? { return "Hello" }
In this case, the return type of the function exampleFunction is a nullable String. This means that the function can return a String value or null.
By using nullable return types, you can make your code more flexible and handle cases where a function may not always return a valid value. Just be sure to handle null values appropriately when calling functions with nullable return types.
How to declare a nullable return type in Kotlin?
In Kotlin, you can declare a nullable return type by appending a question mark (?) after the type in the function signature. This indicates that the function may return a value of the specified type or null.
For example, a function that returns a nullable String would be declared like this:
fun getString(): String? { // function implementation return "nullable string" }
In this case, the function getString can return a String or null.
How to set a custom return type as nullable in Kotlin interfaces?
In Kotlin, you can set a custom return type as nullable in interfaces by using the nullable type modifier "?" after the type. Here is an example:
interface CustomInterface { fun customFunction(): CustomType? }
class CustomType{ // class definition here }
In the above example, the return type of the customFunction() in the CustomInterface is set as nullable by adding the "?" after CustomType. This means that the function can return either an instance of CustomType or null.
You can then implement the CustomInterface in a class and return null from the function when needed.
How to check for null return types in Kotlin?
In Kotlin, you can check for null return types by using the safe call operator "?." combined with the Elvis operator "?:", or by using the safe cast operator "as?".
- Using the safe call operator and Elvis operator:
val result: String? = getString() val output = result?.length ?: 0
In this example, the safe call operator "?." is used to safely access the length property of the result variable. If the result is null, the Elvis operator "?: 0" is used to provide a default value of 0.
- Using the safe cast operator:
val result: Any? = getResult() val output = (result as? String)?.length ?: 0
In this example, the safe cast operator "as?" is used to safely cast the result variable to a String type. If the result is not a String, it will return null and the Elvis operator "?: 0" is used to provide a default value of 0.
These are two ways to check for null return types in Kotlin. Choose the one that fits your specific use-case and coding style.
What is the default return type in Kotlin?
The default return type in Kotlin is Unit. This is similar to void in other programming languages, indicating that the function does not return any meaningful value.
How to specify a custom return type as nullable in Kotlin lambdas?
In Kotlin, you can specify a custom return type as nullable in lambdas by using a nullable type annotation (?) after the return type declaration. For example, if you have a lambda that returns a custom class MyClass, you can specify it as nullable like this:
val myLambda: () -> MyClass? = { // lambda implementation return null // or return an instance of MyClass }
By adding the ? after MyClass, you are indicating that the lambda can return either an instance of MyClass or null.
How to document a function with a nullable return type in Kotlin?
In Kotlin, you can document a function with a nullable return type by using the @return tag in the documentation comment. Here's an example:
/** * This function takes a number as input and returns a string representation of the number. * * @param number the number to convert to a string * @return the string representation of the number, or null if the input is null */ fun convertNumberToString(number: Int?): String? { return number?.toString() }
In the above code snippet, the convertNumberToString function takes an Int? (nullable integer) as input and returns a String? (nullable string). The @param tag is used to document the function parameter, and the @return tag is used to document the return type and specify that it can be null.