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Java
Collectors.groupingBy — Group by Key
Like SQL GROUP BY for streams. Produces a `Map<K, List<V>>` (or any downstream collector you specify). Indispensable for analytics-style aggregations.
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Java
Collectors.toMap with Merge Function
`toMap` builds a `Map<K, V>` from a stream. The 3-arg form takes a merge function for handling duplicate keys — without it, duplicates throw IllegalStateException.