Why Should You Prefer Collections Over Arrays in Kotlin?

The Core Difference: Arrays vs. Collections

At a high level, both arrays and collections are used to hold multiple items. However, they have fundamental differences in their structure, capabilities, and how you interact with them.

Arrays (Array<T>)

• Fixed Size: This is the most critical characteristic of an array. Once you create an array of a certain size, you cannot change that size. If you need to add more elements than its capacity, you have to create a new, larger array and copy the elements over.
• Mutable Elements: The elements within an array are mutable. You can change the value at any index (e.g., myArray[2] = "new value").
• Performance: Arrays can have a slight performance advantage in some specific, low-level scenarios because they map directly to a contiguous block of memory. This can lead to faster access, especially with primitive types (IntArray, CharArray, etc.), as it avoids the “boxing” of primitives into objects.
• Limited API: The built-in functions available for an array are relatively basic. You have size, get(), and set(), but not the rich set of manipulation functions that collections offer.

Example of an Array:

val numbers = arrayOf(1, 2, 3) // Creates an array of size 3
numbers[0] = 10 // This is fine, you are changing an element
// numbers.add(4) // This will not compile. You cannot change the size.

Collections (List, Set, Map)

• Flexible Size (for mutable collections): This is the primary reason for their preference. Collections come in two main flavors:
  • Read-only: List, Set, Map. Their size and elements are fixed after creation.
  • Mutable: MutableList, MutableSet, MutableMap. You can easily add, remove, or reorder elements. This dynamic nature is what you usually need in application development.
• Rich API: The Kotlin standard library provides a vast and powerful set of extension functions for all collections. These functions allow you to perform complex operations in a concise and readable way, such as map, filter, forEach, groupBy, sorted, and many more. This encourages a more functional and declarative style of programming.
• Abstraction: Collections are interfaces (List, Set, Map). This means you code against the interface, and you can easily swap out the underlying implementation (e.g., from an ArrayList to a LinkedList) without changing your code, depending on your specific performance needs.

Example of a Collection (a Mutable List):

val names = mutableListOf("Alice", "Bob") // Creates a mutable list
names[0] = "Alicia" // This is fine
names.add("Charlie") // This is also fine! The list now has 3 elements.

// You can use the rich API:
val longNames = names.filter { it.length > 4 }
println(longNames) // Output: [Alicia, Charlie]

Why Prefer Collections by Default?

Here is a summary of the reasons behind the recommendation:

Feature	Arrays (Array<T>)	Collections (List<T>, Set<T>, etc.)	Why Collections are Preferred
Size	Fixed. Cannot be changed after creation.	Dynamic (for mutable collections). Can grow and shrink as needed.	Most application logic requires adding or removing items, which is cumbersome with arrays.
API	Limited. Basic get/set/size operations.	Rich & Powerful. Huge number of extension functions (filter, map, sorted, etc.).	The rich API makes code more expressive, concise, and less error-prone. You write what you want to achieve, not how to do it.
Immutability	Elements are always mutable. The array itself is a mutable container.	Clear separation. List is read-only. MutableList is mutable.	This design makes your code safer. You can expose a read-only List to prevent unintended modifications.
Abstraction	Concrete implementation.	Interface-based. You program against List, not ArrayList.	This provides flexibility to change the underlying data structure without affecting the rest of the code.

When Should You Use Arrays?

Despite the general recommendation, there are specific situations where arrays are the right choice:

1. Performance-Critical, Low-Level Code: When you are working with a large number of primitive types (like Int, Double, Boolean) and you need to squeeze out every last bit of performance, the specialized primitive arrays (IntArray, DoubleArray, etc.) are ideal. They avoid the overhead of boxing primitives into wrapper objects (Integer, Double).
2. Interoperability with Java: When you are calling a Java library or framework that requires an array as a parameter, you will need to provide one.
3. Varargs: When you define a function that accepts a variable number of arguments, Kotlin uses an array under the hood for the vararg parameter.

Summary

Think of it this way:

• Start with List or MutableList by default. For 95% of your programming tasks, a collection will be more convenient, readable, and flexible.
• Reach for an Array only when you have a specific, justifiable reason to do so, such as a clear performance bottleneck with primitives or a requirement from a Java API.

The advice encourages you to use the more modern, powerful, and safer tool (collections) for your general-purpose work, while still acknowledging that the older, more primitive tool (arrays) has its specific, necessary uses.