Choosing List vs Set vs Map

Ask these questions in order:

1. Do I need to look things up by a key? → Map.
2. Do duplicates need to be automatically prevented, and order doesn't matter? → Set.
3. Otherwise — order matters, and duplicates are fine (or irrelevant)? → List.

Most real scenarios answer clearly with the first matching question — you rarely need to weigh all three carefully.

• "Store every Order for a customer, in the order they were placed." → List (order matters, duplicates like two identical orders are fine).
• "Track which product categories appear at least once across all orders." → Set (only uniqueness matters, order doesn't).
• "Look up a Customer's discount tier by their Customer Id." → Map (a direct key-based lookup).
• "Group every Order by which Customer placed it." → Map of Lists, from the previous lesson (a key — Customer Id — mapping to a collection of values).

// Works, but obscures intent
List<String> processedOrderIds = new List<String>();
if (! processedOrderIds.contains(orderId)) {
    processedOrderIds.add(orderId);
}

// Clearer: the Set itself communicates "these must be unique"
Set<String> processedOrderIds = new Set<String>();
processedOrderIds.add(orderId); // duplicates handled automatically

Both versions technically prevent duplicates, but the Set version is shorter, faster, and — importantly — the type itself documents the intent ("this must never contain duplicates") without needing a comment to explain it.