What is the Elvis Operator (`?:`) in Kotlin?

The Elvis operator ?: in Kotlin is a concise operator that is primarily used for handling null values in a more readable way. This operator is particularly beneficial in Kotlin, where nullability is a key concept.

Understanding Null Safety in Kotlin

Before we delve into the Elvis operator, it’s important to understand Kotlin’s approach to null safety. In Kotlin, types are non-null by default. This means if you have a variable of a certain type, it cannot be assigned null unless explicitly stated. You can declare a variable that can hold a null value by appending a question mark (e.g., String?), making it nullable.

val nullableString: String? = null

Basic Usage of the Elvis Operator

The Elvis operator is used to provide a default value when a nullable expression evaluates to null. It takes the form a ?: b and returns b if a is null, otherwise, it returns a.

var length: Int = nullableString?.length ?: 0

In the above code, nullableString?.length may yield null if nullableString is null, hence the Elvis operator ?: assigns 0 to length

Practical Examples

Example 1: Simple assignment with the Elvis operator

fun getNameLength(name: String?): Int {
    return name?.length ?: -1
}

val nameLength = getNameLength(null)  // Returns -1 because name is null

Example 2: Using Elvis Operator with Function Calls

fun maybeGetString(): String? {
    return null  // Simulating a situation where null might be returned
}

val result = maybeGetString() ?: "Default String"
println(result)  // Prints "Default String" because the function returned null

Example 3: Complex Objects

If you work with more complex objects, using the Elvis operator can save you from a lot of null checks:

data class Person(val name: String?, val age: Int?)

val person: Person? = Person(null, 25)

val displayName = person?.name ?: "Unknown"
println(displayName)  // Prints "Unknown" since person.name is null

Conclusion

The Elvis operator ?: provides a neat way to handle nulls in Kotlin. By reducing the verbosity associated with null checks, the operator allows developers to write more concise and readable code. Utilizing the Elvis operator is crucial in a language like Kotlin that takes null safety seriously.