Kotlin: Foundations

What to look for:

• val = immutable reference (prefer); var = mutable reference
• Type inference: val name = "Alice" — no type annotation needed
• String templates: "Hello, $name!" and "Length: ${name.length}"
• when is Kotlin's switch — but it's an expression that returns a value

What to look for:

• String cannot hold null — String? can; the difference is at compile time
• ?. short-circuits the chain: a?.b?.c returns null if any step is null
• ?: provides a fallback: name ?: "Unknown" returns "Unknown" if name is null
• !! is an explicit crash request — avoid in production code

What to look for:

• data class generates: equals, hashCode, toString, copy, componentN — one line vs 50 in Java
• copy() creates a modified version: user.copy(age = 31) — original unchanged
• object MyObject — a singleton: one instance, created lazily on first access
• companion object — a class's static members equivalent in Kotlin

What to look for:

• listOf() returns read-only; mutableListOf() returns MutableList
• Collections API is lazy with sequences: .asSequence().filter{...}.map{...}.toList()
• it: the implicit lambda parameter name for single-argument lambdas
• groupBy returns Map<K, List<V>> — useful for categorising data

What to look for:

• Trailing lambda: if last arg is a lambda, move it outside: list.filter { it > 0 }
• Function type: (Int, Int) -> Int — a function that takes two Ints and returns Int
• Extension functions resolve statically — they can't override member functions
• inline keyword on extension functions avoids lambda object creation overhead