Lectures

We break down collection aggregations in Kotlin: how fold() and reduce() work, what the accumulator acc is, why the start value ( initial) is critical for correctness, why empty lists are dangerous for reduce(), and when to choose the safe variant reduceOrNull(). Plus: hands-on examples (sum, product, maximum) and typical mistakes.

We break down fold as a universal aggregation technique “many elements → one result”: how to choose a neutral initial, how to compute sums and counts, how to neatly find min/max while supporting empty collections via null, how to assemble a report with StringBuilder, and how to accumulate data into a Map. At the end: a list of common mistakes and how to avoid them.

We break down how Sequence works in Kotlin: why filter/ map chains “do nothing” without a terminal operation, how intermediate operations differ from terminal ones, how asSequence() switches collections into lazy mode, when you need materialization via toList(), and why early operations like firstOrNull() can stop the pipeline before it reaches the end.

We break down how to “read the cost” of chains of operations on collections: extra passes, intermediate lists, and temporary objects. We learn to understand when a List pipeline materializes results at every step, how a lazy Sequence differs, why take(n) enables early termination, and which mistakes most often happen when trying to “speed up” map/ filter/ fold chains.

We break down ArrayDeque in Kotlin as a “double-ended queue”: why it’s better (and more honest in meaning) than MutableList for FIFO/LIFO scenarios, what basic operations exist ( addFirst/ addLast, removeFirst/ removeLast, first()/ last()), how not to crash when removing from an empty structure using isEmpty()/ isNotEmpty(), and how to use a deque in a CLI for an add-history and an undo command. At the end: typical mistakes and mental checks so you don’t confuse a queue with a stack.