Multi-stream: UNORDERED is a promise you make, not one you receive

Collector.Characteristics.UNORDERED tells the framework it is free to reorder your input. It costs nothing on a sequential stream, which is exactly why a wrong one survives every test and only shows up in parallel.

• Multi-stream: a record with a mutable component is a broken Map key
• Multi-stream: an unmodifiable view is not a snapshot of the feed
• Multi-stream: toMap's merge function runs only on a collision
• Multi-stream: nothing runs until the terminal operation
• Multi-stream: indexing the other side costs memory a sorted merge does not
• Multi-stream: two sorts equal one thenComparing only because sorted() is stable

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Published 2026-09-20

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