buildString

object Raindrops {
    fun convert(num: Int) = buildString {
        if (num % 3 == 0) append("Pling")
        if (num % 5 == 0) append("Plang")
        if (num % 7 == 0) append("Plong")
        if (isEmpty()) append(num)
    }
}

An object declaration is used to define Raindrops as essentially a singleton object instantiation of the class. This is sufficient, since there is no object state that needs to change with each call of the convert method.

The convert function is implemented with a call to the buildString function. Although the convert function has multiple lines, it consists only of the one buildString expression, so it is defined using single-expression function syntax, with the curly braces omitted from the convert function call and the return type inferred.

The builderAction argument is a block of statements, each of which calls one or more methods on an implicit StringBuilder object. The modulus operator (%) is used to check if the input number is evenly divisible by 3, 5, or 7. If so, then the append method is called on theStringBuilder to add the associated sound.

After all of the numbers have been checked, the isEmpty method of StringBuilder is used to see if no sound was added. If no sound was added, then the input number number is automatically converted to a String when appended to the StringBuilder.

The convert function returns the result of calling buildString, which is the result of the implicit call to the toString method of the StringBuilder after the block of statements in the buildAction argument.