Kotlin From Scratch: Ranges and Collections

Kotlin, a modern, open-source language compiling to Java bytecode, enhances Android development. Building on a previous tutorial covering nullability, loops, and conditions, this guide explores Kotlin's ranges and collections API.

1. Ranges

A Kotlin range defines a closed interval between a start and end value (inclusive). Here's how to create ranges:

• until() and downTo(): These infix functions create number ranges with a specified step. Infix functions allow function calls without dot or parenthesis notation.

  For example:

   // 1 3 5 7
   for (i in 1 until 8 step 2) {
       print("$i ")
   }

2. Collections

Collections store groups of related objects. Kotlin's collections API, built upon Java's, offers various interfaces linked to their implementations at compile time. Examples include:

• ArrayList: The add() function inserts elements; elements at specific indices can be directly modified.

   val stringList: ArrayList<string> = arrayListOf("Hello", "You", "There")
   stringList[2] = "Here"
   stringList.add(1, "Are")
   // Prints: [Hello, Are, You, Here]
   println(stringList)</string>

• LinkedHashSet: This mutable set maintains insertion order.

   val intsLinkedHashSet: LinkedHashSet<int> = linkedSetOf(5, 2, 7, 2, 5)
   intsLinkedHashSet.add(4)
   intsLinkedHashSet.remove(2)
   // [5, 7, 4]
   println(intsLinkedHashSet)</int>

• LinkedHashMap: A mutable map maintaining entry iteration order using a doubly linked list.

   val postalCodesHashMap: LinkedHashMap<string string> =
       linkedMapOf("NG" to "Nigeria", "AU" to "Australia", "CA" to "Canada")
   postalCodesHashMap.put("NA", "Namibia")
   postalCodesHashMap.remove("AU")
   // {NG=Nigeria, CA=Canada, NA=Namibia}
   println(postalCodesHashMap)</string>

• maxOrNull(): Returns the maximum element (note: max() is deprecated).

   val intList: List<int> = listOf(1, 3, 4)
   print(intList.maxOrNull()) // will print 4</int>

• drop(): Returns a new collection excluding the first n elements.

   print(stringList.drop(2)) // will print [You, Here]

• plus(): Adds an element to the collection, returning a new collection.

   print(intList.plus(6)) // will print [1, 3, 4, 6]

• minus(): Removes an element, returning a new collection.

   print(intList.minus(3)) // will print [1, 4]

• average(): Calculates the average of elements.

   print(intList.average()) // will print 2.6666666666666665

Many more extension functions are available in Kotlin's standard library. Consult the documentation for a comprehensive list.

Conclusion

This tutorial covered Kotlin's ranges and collections API. The next tutorial in this series will introduce Kotlin functions. For further Kotlin learning, refer to the official Kotlin documentation.

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