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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.
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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.
Java
Optional with Streams
`Optional::stream` (Java 9+) bridges optionals into the Stream API — collect all the `Some` values from a list of optionals, drop the empties, in one pipeline.
Java
Collectors.partitioningBy — Split by Predicate
Special case of groupingBy when the key is boolean — returns a `Map<Boolean, List<T>>` for the "true" and "false" buckets. Slightly more efficient than groupingBy.
Java
Stream.generate / iterate — Infinite Streams
Build streams from a seed + a function. `generate(supplier)` calls the supplier each time; `iterate(seed, next)` applies a unary op. Always pair with `limit` to bound them.
Java
Stream Basics — filter / map / collect
The fluent pipeline for transforming collections. `filter` keeps matching elements, `map` transforms each one, `collect(toList())` materializes the result.
Java
Stream.reduce — Aggregate to a Single Value
`reduce` collapses a stream to one value: sum, product, max, min, custom accumulation. Two-arg form needs an identity (zero/seed); three-arg form supports parallel streams via a combiner.