Kotlin Basics II — Collections, Functions, Lambdas, Null Safety

Key concepts

• Array vs ArrayList vs List
• Function declarations & default arguments
• Higher-order functions
• Lambdas & trailing lambda syntax
• map / filter / forEach
• Nullable types (?), safe call (?.), Elvis (?:)

Lectures in this day

1. 01

   Array and ArrayList

   An Array is a row of boxes with a size you decide at the start — you can't add more boxes later. An ArrayList is a stretchy row that grows and shrinks as you add or remove things. Use `listOf(...)` when you only need to read the list.

   Real life · An egg tray with 30 slots — that's an Array (fixed size). A shopping bag you keep adding items to — that's an ArrayList (grows).
   kotlin

   Copy

   0
   val eggs = IntArray(30) { it + 1 }   1
   
   2
   val cart = arrayListOf("Milk", "Bread")
   cart.add("Eggs")
   cart.remove("Bread")
   println(cart)   3

   Practice · 3 quick questions

   1. Q1.Which is fixed size?

      A.ArrayB.ArrayListC.Both

   2. Q2.Which one supports `add()` to grow?

      A.ArrayB.ArrayList

   3. Q3.First index in an array is…

      A.0B.1C.-1

   Check answersPick one answer per question.
2. 02

   Functions in Kotlin

   A function is a named piece of code you can run any time. You give it inputs (parameters) and it usually gives back a result. Parameters can have default values, and you can pass them by name so the order does not matter.

   Real life · A coffee machine: press one button and it makes a latte. Change a setting (size = large) to get a bigger cup — same machine, different input.
   kotlin

   Copy

   0
   fun latte(size: String = "M", shots: Int = 1): String =
       "$size latte with $shots shot(s)"
   
   println(latte())                       1
   println(latte(shots = 2))              2
   println(latte(size = "L", shots = 3))  3

   Practice · 3 quick questions

   1. Q1.Which keyword defines a function?

      A.defB.funC.func

   2. Q2.In `fun add(a: Int, b: Int): Int`, what is `Int` after the `)`?

      A.A parameterB.The return typeC.A variable

   3. Q3.A function with no return uses…

      A.UnitB.NullC.Void()

   Check answersPick one answer per question.
3. 03

   Higher Order Functions and Lambda

   A function can take another function as an input. That inner function is often written inline in curly braces — that short inline function is called a lambda. Inside a lambda, `it` is the name for the single input.

   Real life · A recipe that says 'season to taste'. You pass in the seasoning behavior. Different cook, different lambda, same recipe.
   kotlin

   Copy

   0
   1
   fun doTimes(times: Int, action: (Int) -> Unit) {
       for (i in 1..times) {
           action(i)   2
       }
   }
   
   3
   doTimes(3) { i -> println("Ring $i") }
   4
   
   5
   val numbers = listOf(1, 2, 3)
   val doubled = numbers.map { it * 2 }   6
   println(doubled)   7

   Practice · 3 quick questions

   1. Q1.A lambda is…

      A.A tiny function you can pass aroundB.A classC.A file

   2. Q2.In `list.map { it * 2 }`, what is `it`?

      A.The listB.The current itemC.An index

   3. Q3.A higher-order function is one that…

      A.Runs fasterB.Takes or returns a functionC.Uses classes

   Check answersPick one answer per question.
4. 04

   Null Safety in Kotlin

   'Null' means 'no value at all'. Kotlin splits types into two groups: normal (`String`, always has a value) and nullable (`String?`, may be null). Use `?.` to say 'only call this if it's not null', and `?:` to give a backup value when it IS null.

   Real life · Before you leave home, you check your umbrella. If you have one, you can open it. If not, you use a backup plan (like wearing a hoodie).
   kotlin

   Copy

   fun greet(name: String?) {   0
       1
       2
       val length = name?.length ?: 0
       println("Hi ${name ?: "stranger"} ($length chars)")
   }
   greet("Ayesha")   3
   greet(null)       4

   Practice · 3 quick questions

   1. Q1.What does `?.` do on `user?.name`?

      A.Throws if nullB.Returns null if user is nullC.Deletes user

   2. Q2.What does `?:` (Elvis) do?

      A.DividesB.Gives a default when left is nullC.Compares

   3. Q3.What does `!!` do?

      A.Adds 2B.Crashes if the value is nullC.Prints

   Check answersPick one answer per question.
5. 05

   Module 1 Wrap-up / Summary

   You now know the basic pieces of Kotlin: boxes for values, `if`/`when` for choices, loops for repeating, lists for groups of things, functions for reusable steps, and nullable types so empty boxes don't crash your app.

   Real life · You've learned the alphabet and grammar. Starting tomorrow, you write full sentences — with classes and objects.

   Practice · 3 quick questions

   1. Q1.Module 1 was mainly about…

      A.Kotlin language basicsB.FirebaseC.Google Maps

   2. Q2.Which topic was NOT in Module 1?

      A.LoopsB.Null safetyC.Room database

   3. Q3.You should be comfortable with…

      A.Val/var, if, when, loops, functions, lambdas, null safetyB.Firebase AuthC.Geofencing

   Check answersPick one answer per question.

Real-life analogies

Code examples

data class Task(val title: String, var done: Boolean = false)

val tasks = arrayListOf(
    Task("Buy milk"),
    Task("Study Kotlin", done = true),
    Task("Call mom")
)

val open = tasks.filter { !it.done }
                .map { it.title.uppercase() }

open.forEach(::println)

fun greet(name: String?) {
    0
    val length = name?.length ?: 0
    println("Hi ${name ?: "stranger"} ($length chars)")
}

greet("Ayesha")
greet(null)

Hands-on checklist

• Build a console task manager (add, remove, list, toggle done)
• Use at least one map, one filter, one forEach
• Support optional description with a nullable String
• No !! anywhere in your code

Milestone

Common pitfalls

• Reaching for !! to silence the compiler — use ?. or ?: instead.
• Using Array<Int> when you meant IntArray (they're different types).