The key to understanding why this works is rather simple: if you can only take items from a collection, then using a collection of Strings and reading Objects from it is fine. In other words, the wildcard with an extends-bound ( upper bound) makes the type covariant. In return for this limitation, you get the desired behavior: Collection is a subtype of Collection. This means that you can safely read E's from items (elements of this collection are instances of a subclass of E), but cannot write to it as you don't know what objects comply with that unknown subtype of E. You should consider using a sequence whenever you’re doing more than one operation on a list of data.The wildcard type argument ? extends E indicates that this method accepts a collection of objects of E or a subtype of E, not just E itself. With this in mind, it’s easy to see that sequences have two different types of operations on them: Operations that produce an intermediate (a lazy evaluated sequence) or operations that are terminal (operations that process the lazy sequence). Sequences have the same functions available to them as lists do, but instead of producing an intermediate collection each time, they are lazily evaluated. Sequences provide a clear and performant alternative to multiple intermediate lists. This is usually intended, however, it can become problematic when chaining multiple operations on the same list - You’ll create a ton of intermediate lists! That means that when you call map on a list, you’ll return a new list. However, Kotlin uses extension functions on Java Collections to create these functions in their standard library. One of the most common functional paradigms involves using a combination of map, filter, and reduce on Iterable types. Kotlin, despite being an object oriented language, offers a ton of support for Functional paradigms.
This tutorial may work with newer versions and possibly older versions, but has only been tested on the versions mentioned above. This project uses these versions of languages, frameworks, and libraries.
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