I could think of a couple reasons to declare a class final: 1) it's supposed to be immutable or 2) if you have other code that depends on this class working exactly the way you've implemented it. In the case of immutability, you want to make sure that instances of the final class don't change in value once they've been created. This is important for objects that will be used as hash keys, for example. If your supposedly immutable class weren't final, someone could come along, subclass it, give your code something you expect to be immutable, then change it under your nose. The second reason is sort of a superset of the first problem. You can (hopefully) trust the code you write. If your final class implements something really important, you might not want to allow anyone to subclass it and (perhaps maliciously) override the behavior you depend on. Of course, you sacrifice flexibility for safety here. More on reddit.com

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I understand the reasoning provided here: "By default, all classes in Kotlin are final, which corresponds to http://www.oracle.com/technetwork/java/effectivejava-136174.html">Effective Java, Item 17: Design and document for inheritance or else prohibit it." In practice, I find that this introduces ... More on discuss.kotlinlang.org

It's because you're accessing the result variable from inside the EventHandler. To fix it you could: Make result a member variable of the class, which you are allowed to modify Make result a final AtomicReference to a string, whose contents you are allowed to modify More on reddit.com