Lectures

We break down two kinds of equality in Kotlin: structural comparison via == (by value, via equals, and safe for null) and referential comparison via === (object identity). We discuss aliasing with collections, why val does not make an object immutable, the === trap with nullable numbers ( Int?), and practical techniques like copying a list via toList() to create a data “snapshot”.

We break down basic list sorting in Kotlin without mutations: how sorted() and sortedDescending() work, what “natural order” means and what Comparable has to do with it, why it’s important not to break the original data, and how to apply sorting in the mini project ExpensePocket. At the end — a review of typical mistakes (lost result, confusing “sorting vs reversing”, unnecessary sorting just to get max/min).

We cover sorting collections in Kotlin “by key” using sortedBy and sortedByDescending, build the top‑N pattern as sortedByDescending(...) + take( n), and also learn how to sort by multiple criteria using sortedWith and compareBy. Along the way, we reinforce the difference between an “element” and a “sort key” and typical mistakes (for example, using take without sorting).

Let’s break down how to sort collections in Kotlin when the key can be null: why sorting needs an explicit contract (“ null first/last”), how to use the Elvis operator ?: with safe defaults ( sortedBy, sortedByDescending), why “magic numbers” like Int . MAX_VALUE can be dangerous, and when it’s better to split the data ( filterNotNull(), counting null, split+merge). Everything is shown using an expenses report example in a hypothetical BudgetBuddy app.

Let’s break down why arrays in Kotlin/JVM are not compared element-by-element via ==, how == and === differ, and which functions to use for correct data comparison: contentEquals() for flat arrays and contentDeepEquals() for nested arrays. We’ll also discuss the aliasing trap (“before/after” without a copy) and practical ways to diagnose issues with joinToString() and contentDeepToString().