You might expect this code would not even compile, since the add operation is not available on read-only collections and we just learned that listOf would create a read-only list.

I’ll demonstrate this using the code below import java.util.* This means that an immutable collection can provide stronger guarantees of safety and immutability than a read-only collection in Kotlin. Once an immutable collection is created, its contents cannot be changed, and any attempt to do so will result in a new immutable collection being created. in Scala) are truly immutable and cannot be modified by any means. For example, if a read-only list is backed by an ArrayList, the contents of the ArrayList can still be modified, but the reference to the read-only list cannot be used to add, remove, or modify elements. Read-only collections are collections that can still be modified by their underlying data source, but cannot be modified directly through the reference to the collection. However, there are some differences between the two concepts. Read-only collections are similar to immutable collections in that they both provide collections that cannot be modified once they are created. It is a common mistake in Kotlin to confuse read-only collections with immutable collections.

Using any of the mutableOf() functions will create a mutable collection of that type. listOf() or mapOf()) you’ll get a read-only instance of that collection type. If you use any of the Of() functions (e.g. ImmutabilityĪs already mentioned above, Kotlin has read-only collections. Don’t use withDefault, use getOrDefault when you need to rely on a fallback value. This program will actually output default as we intended (because the key '2' is missing). What do you expect the outcome of this program to be? fun main (args: Array). Let’s start off with a rather simple “hello world” example. In this post I’ll show you 7 code snippets containing common mistakes (I made) when programming in Kotlin. Or maybe even worse: I can’t explain why the code DOES work. And even now I have some experience, every once in a while, when conducting a Kotlin training or when on a Kotlin assignment, I encounter situations where I simply cannot explain why the code doesn’t work the way I intended.

Have you ever felt like this kid when programming? I sure did! When I got into Kotlin development I felt like this kid many many times. Hopefully one day the API version Lint checks will work on Kotlin code, since it would've caught this problem.This article was originally published at my personal blog on March 9, 2023. What's wrong with the following code (when used on Android)? val map = mapOf('hello' to 'goodbye')

Mapof kotlin