When vararg parameter is not the last one in list, like this: fun format(format: String, vararg params: String, encoding: String) params) Declaring params after the vararg vararg param as the last one in listĭeclaring vararg param as the last one in list, in a Kotlin function, looks like this: fun format(format: String, vararg params: String)Īnd will be compiled into the corresponding Java code: void String format, String.

Now let’s see how the decompiled corresponding Java source code looks like when declaring vararg parameters. While in Kotlin, the vararg parameter doesn’t have to be the last one in the list, multiple vararg parameters are still prohibited. In Java, the vararg parameter has to be the last one in the parameters list - so you can only have one vararg parameter. You can do that by prefixing parameter name with the vararg modifier: fun format(format: String, vararg args: Any) vararg rules Kotlin also supports declaring a function that can have a variable number of arguments. In this article we will start by looking at the basics about vararg and spread operator, and then explore some more elaborated scenarios - while analysing what happens behind the scenes. If you, on the other hand, want to rely on ArrayList (via arrayListOf), you probably have a concrete scenario were only this implementation is acceptable.In some cases, you might need to define a function that should have a variable number of parameters a couple of examples that you probably have encountered in Android are the printf(String format, Obj. For instance, if Kotlin decides to add a more powerful backward-compatible version of an ArrayList, they could change the implementation and your code would keep working (in an improved way). As of now this happens to be an ArrayList which could change in future releases.

The point of not using arrayListOf is that you want to rely on the library to choose the actual implementation. On the other hand, if you don't fully rely on type inference, you can make the variable for the ArrayList read-only as well: val l3: List = arrayListOf(2) It's definitely preferred to use read-only data structures whenever possible. One notable and important difference is that listOf gives you a read-only List whereas arrayListOf gives you a MutableList.

Kotlin arraylist