I'm converting a Java application to Kotlin. In one area it's using apache IO's FileUtils listFiles functions.
These return collections and I'm having problems converting/casting the collections into ArrayList
val myFiles = FileUtils.listFiles(mediaStorageDir, extensions, true) as ArrayList<File>
Whilst this compiles I get a runtime error as follows:
java.util.LinkedList cannot be cast to java.util.ArrayList
What's the correct way to convert a collection object into an ArrayList?
ArrayList is a class with implementation details. FileUtils is returning a LinkedList and that is why you are having issues. You should do a cast to List<File> instead since it is an interface that both ArrayList and LinkedList implement.
If you would need to modify the returned list you can use MutableList<File> instead the immutable one.
But this is not the best approach because if the creators of FileUtils later change the implementation details of their collections your code may start crashing.
A better soulition if explicitly need it to be an arrayList instead of a generic list could be:
val myFiles = FileUtils.listFiles(mediaStorageDir, extensions, true)
val array = arrayListOf<File>()
array.addAll(myFiles)