The key to understanding why this works is rather simple: if you can only take items from a collection, then using a collection of Strings and reading Objects from it is fine. In other words, the wildcard with an extends-bound ( upper bound) makes the type covariant. In return for this limitation, you get the desired behavior: Collection is a subtype of Collection. This means that you can safely read E's from items (elements of this collection are instances of a subclass of E), but cannot write to it as you don't know what objects comply with that unknown subtype of E. You can do that by prefixing parameter name with the vararg modifier: fun format (format: String, vararg args: Any) In Java, the vararg parameter has to be the last one in the parameters list - so you can only have one vararg parameter.The wildcard type argument ? extends E indicates that this method accepts a collection of objects of E or a subtype of E, not just E itself. Kotlin also supports declaring a function that can have a variable number of arguments. How to declare variable number of arguments in Kotlin? If you try to call format function defined above like this: val params = arrayOf(“param1”, “param2”) It is needed when you want to pass an array as the vararg parameter. It’s Kotlin’s spread operator - the operator that unpacks an array into the list of values from the array. When to use the spread operator in Kotlin? To convert individual elements to an array you can use arrayOf (…), but you don’t need that in this case. Within the vararg -function the vararg parameter will be an array anyway.
How to convert an array to a Vararg in Kotlin?Ī vararg expects individual elements, and *array is the Kotlin way to say “treat this array as individual elements for that purpose”. listOf(1, 2, 3, 4, 5, 6) returns a read-only list of integers from 1 through 6. How do I use a list on Kotlin?Ĭreate a new List using the Kotlin standard library function listOf(), and pass in the elements of the list as arguments separated by commas. We can pass n number of parameters to a vararg variable of the defined datatype or even of a generic type. Kotlin provides us to achieve the same by defining a parameter of a function as vararg. asList(1, 2, 3), or, if we already have an array and want to pass its contents to the function, we use the spread operator (prefix the array with *): From the Kotlin Reference… When we call a vararg-function, we can pass arguments one-by-one, e.g. The * operator is known as the Spread Operator in Kotlin.
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