Differences between Unit and Nothing in Kotlin

Unit #

Equivalent to Java's void.

It is said that a function implicitly returns Unit if it doesn't return anything useful or has no value to return.

And such functions can perform operations with side effects. They might log/print something or perform manipulations without a return value.

fun printHelloUnit(name: String?): Unit {
    if (name != null)
        println("Hello $name")
    else
        println("Hi there!")
}

//The Unit return type declaration is also optional. The above code is equivalent to:

fun printHello(name: String?) {
    if (name != null)
        println("Hello $name")
    else
        println("Hi there!")
}

In the example above, if you explicitly mention Unit as the return type, the compiler suggests removing the explicit mention because the Unit type is redundant.

Nothing #

Literally, it means nothing, there is no return to life, and the game ends there.

That is, the function will not return from here; instead, an exception will be thrown or it will enter an infinite loop.

Furthermore, any code written after calling a function with a Nothing return type will be marked as unreachable by the compiler.

Conclusion #

Unit says, 'I will return, but there's nothing of value for you to care about.'
Nothing is like saying, 'I will never return.'

Thus, it helps to refer to the functionalities more clearly and individually.

class NothingClass {

    fun returnName(isSuccess: Boolean): String? {
        return if (isSuccess) {
            println("Sara")
            "Sara"
        } else null
    }

    fun reportError(): Nothing {
        println("no name found")
        throw RuntimeException()
        // var i = 1 // unreachable code
    }
// here if you don't specify Nothing explicitly, it shows compile time error
fun iWillAlwaysThrowException() : Nothing =  throwException("Unnecessary Exception")
}

fun main() {
val nothingClass = NothingClass()
val name: String = nothingClass.returnName(true) ?: nothingClass.reportError() // Compiles and the return type is String or Nothing 
val noName: String = nothingClass.returnName(false) ?: nothingClass.reportException()
}