Lectures

We break down how to “cut” strings safely in Kotlin: how to correctly interpret the result of indexOf (including -1), how substring boundaries work (the end is not included), why split on spaces produces empty tokens, and how the discipline of trim + filtering + returning null helps you avoid crashes on “messy” user input.

We’ll walk through practical console output formatting in Kotlin: string interpolation ( $ x and $ { expr }), separating “compute” from “display” (functions return String), aligning columns with padEnd()/ padStart(), building a multi-line report with joinToString() and """...""" templates + trimIndent()—using a mini project “expense tracking” as an example.

Let’s break down why string concatenation with + and += inside loops leads to extra copying, how to efficiently assemble multi-line text using StringBuilder (methods append(), appendLine(), finalizing via toString()), when it’s easier to use joinToString(), and which mistakes most often happen when formatting reports and line breaks.

We cover a minimally sufficient set of regular expressions in Kotlin: how to create a Regex object via Regex("..."), how to apply replace to normalize “dirty” input, why raw strings """...""" are convenient, and which patterns are most often needed in practice: \s+ for collapsing whitespace, [-_\s]+ for unifying separators, and \d for masking digits. At the end — typical mistakes when using Regex.replace.

We break down how takeIf and takeUnless turn “bad” values into null, so you can then handle them uniformly via the safe call ?. and the Elvis operator ?:. Using practical examples (validating indexOf, normalizing CLI input, parsing via toIntOrNull), we learn to write compact guard expressions without extra branching—and understand where a plain if is more honest and readable.