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6.1 Collectors.groupingBy, partitioningBy & Reductions
Topic Summary
Group complex data into Maps and multi-level hierarchies using downstream collectors and joining.
What You Will Learn
• Use `Collectors.groupingBy` for classification into `Map<K, List<V>>`.
• Apply downstream collectors (`counting()`, `mapping()`, `summarizingInt()`).
• Partition data cleanly into two halves using `Collectors.partitioningBy`.
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