Multi-stream: an unmodifiable view is not a snapshot of the feed

Collections.unmodifiableList stops writes THROUGH the view; it does not stop the underlying list changing underneath it. A joined result handed out as a view keeps changing after you return it - List.copyOf is the one that freezes.

• Multi-stream: a record with a mutable component is a broken Map key
• 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: UNORDERED is a promise you make, not one you receive
• Multi-stream: two sorts equal one thenComparing only because sorted() is stable

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Published 2026-09-20

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