For people just migrating from java, In Kotlin List is by default immutable and mutable version of Lists is called MutableList.
Hence if you have something like :
val list: List<String> = ArrayList()
In this case you will not get an add() method as list is immutable. Hence you will have to declare a MutableList as shown below :
val list: MutableList<String> = ArrayList()
Now you will see an add() method and you can add elements to any list.