My Code

fun changeIntToDouble(number: Int) {
    return number.toDouble()
}

but my ide said Type mismatch: inferred type is Int but Unit was expected How can I fix it?

If you don't explicitly specify a return type it defaults to

Unit

, i.e. "nothing". You need to explicitly define it as:

fun changeIntToDouble(number: Int): Double {

Or write it as expresssion body like

fun changeIntToDouble(number: Int) = number.toDouble()

^ That works too, although my understanding is that too many inferred return types can make IntelliJ slow.