What are the main differences between `ArrayList` and `List` in Kotlin?

The primary distinction between ArrayList and List in Kotlin is that ArrayList is mutable, meaning that it allows for modifications such as adding, removing, or updating elements, while List represents an immutable collection by default when used in its original form. This immutable aspect means that once a List is created, its size and contents cannot be changed.

This differentiation is essential in Kotlin as it promotes functional programming practices, encouraging developers to leverage immutability for safer and more predictable code. Thus, when working with a List, you can be assured that its elements remain constant throughout its usage, which can help reduce bugs associated with unintended changes. Meanwhile, ArrayList, as a subtype of List, allows for the flexibility needed in scenarios where the collection’s size or content needs to be dynamic.

This understanding is critical for making informed choices about which collection type to use based on the specific requirements of a task, such as whether data needs to remain unalterable or if frequent modifications are expected.