Lectures

We break down why Kotlin needs classes and how to “glue” data into meaningful entities: why a couple of variables ( title and amount) quickly turns into chaos, why Pair is convenient and why it is dangerous, how to create your own type ( class Expense), and how object references work: assignment copies a reference, the difference between val and var, and how to debug “weird changes” by checking ===.

We break down class syntax in Kotlin: how the primary constructor works, why parameters without val/ var do not become properties, how to create objects without new, set default values, use named arguments, and format a “readable” class header. At the end — migration from Pair to the Expense model and common mistakes.

We break down why, in OOP, it is beneficial to keep behavior next to data: we move “external” functions inside a class as methods, learn to call them with dot syntax, and understand why methods “see” an object’s properties. We also discuss mutable state via var, the nuance of a val reference, the role of this when names conflict, as well as invariants and validation via early return, require/ check—so that an object stays correct even with invalid input.

We break down why domain data is better stored not in Pair<String, Int>, but in a separate class Expense; we learn how to build and modify a MutableList<Expense>, print elements with indices, safely delete via removeAt(), build reports with filter()/ map()/ sortedByDescending()/ fold(), and also make a mini CLI plus a bridge for a “soft” migration from pairs to objects.

We go through the most common pitfalls when taking the first steps in OOP with Kotlin: why uncontrolled var turns a model into jelly, how val protects a reference but not an object, how shared references break expectations in MutableList<Expense>, why the Expense entity should not know about readln()/ println(), and how good names make code shorter and clearer. At the end: a mini-refactoring of BudgetBuddy and a block of typical mistakes.