Collections Framework in Kotlin

# CHAPTER 21

Collections Framework in Kotlin #

1. Chapter Introduction #

2. Learning Objectives #

• Sort collections (Ascending, Descending, By specific properties).

• Group collections into Maps.

• Check conditions using any, all, and none.

• Find specific elements safely.

• Build a Mini Project: Inventory Manager.

3. Sorting Collections #

• sorted(): Sorts numbers or strings in natural ascending order.

• sortedDescending(): Sorts in descending order.

fun main() {
    val numbers = listOf(5, 2, 8, 1)
    
    println(numbers.sorted())           // [1, 2, 5, 8]
    println(numbers.sortedDescending()) // [8, 5, 2, 1]
}

If you have a list of Objects (like Product data classes), you must tell Kotlin *which property* to sort by using sortedBy.

data class Product(val name: String, val price: Double)

fun main() {
    val cart = listOf(
        Product("Laptop", 1200.0),
        Product("Mouse", 25.0),
        Product("Keyboard", 75.0)
    )

    // Sort by price! (Using trailing lambda)
    val sortedCart = cart.sortedBy { it.price }
    
    println(sortedCart) 
    // Output: [Product(name=Mouse, price=25.0), Product(name=Keyboard, price=75.0), Product(name=Laptop, price=1200.0)]
}

4. Grouping Data (groupBy) #

data class Person(val name: String, val city: String)

fun main() {
    val people = listOf(
        Person("Alice", "New York"),
        Person("Bob", "London"),
        Person("Charlie", "New York")
    )

    // Group the list into a Map where the Key is the City!
    val groupedByCity = people.groupBy { it.city }

    // Result is a Map<String, List<Person>>
    println(groupedByCity["New York"]) // Prints Alice and Charlie
}

5. Checking Conditions (any, all, none) #

• any: Is there at least one item that matches?

• all: Do ALL items match?

• none: Do ZERO items match?

val numbers = listOf(1, 2, 3, 4, 5)

println(numbers.any { it > 4 })  // true (5 is greater than 4)
println(numbers.all { it > 0 })  // true (all are greater than 0)
println(numbers.none { it < 0 }) // true (none are less than 0)

6. Finding Elements Safely #

val words = listOf("Apple", "Banana", "Cherry")

// Returns "Banana"
val result = words.find { it.startsWith("B") } 

// Returns null
val missing = words.find { it.startsWith("Z") } 

7. Mini Project: Inventory Manager #

data class Item(val name: String, val category: String, val price: Double, val inStock: Boolean)

fun main() {
    val inventory = listOf(
        Item("Laptop", "Electronics", 999.0, true),
        Item("Phone", "Electronics", 500.0, false),
        Item("Desk", "Furniture", 150.0, true),
        Item("Chair", "Furniture", 50.0, true)
    )

    // 1. Check if ANY item is out of stock
    val hasOutofStock = inventory.any { !it.inStock }
    println("Any out of stock? $hasOutofStock")

    // 2. Get only Electronics that are In Stock, sorted by price
    val availableElectronics = inventory
        .filter { it.category == "Electronics" && it.inStock }
        .sortedBy { it.price }
        
    println("Available Electronics: $availableElectronics")

    // 3. Group by Category
    val catalog = inventory.groupBy { it.category }
    println("Total Categories: ${catalog.keys.size}")
}

8. Common Mistakes #

• Using Mutable Lists needlessly: It is tempting to write val sortedList = mutableListOf() and use a for loop to sort items manually. This is "Java thinking". Trust the functional operators! Let .sortedBy create the new immutable list for you.

9. Best Practices #

• Chain operations logically: When chaining filter and map, always filter first. It reduces the size of the list, so map has to do less work.

10. Exercises #

1. 1. Create a list of integers: 10, 5, 20, 15.

1. 2. Use .sortedDescending() to order them highest to lowest.

1. 3. Use .find { } to search for a number greater than 15. Print the result.

11. MCQs with Answers #

12. Interview Questions #

• Q: Explain the difference between .map { } and .filter { }. (Answer: Map transforms every element, creating a new list of the same size. Filter evaluates a condition, keeping only items that return true, creating a smaller list).

• Q: How would you group a list of Employee objects by their department using Kotlin Collections?

13. Summary #

14. Next Chapter Recommendation #