Multi-stream: Collectors.toMap rejects a null value that HashMap.put accepts

A left join that produces nulls cannot be collected with toMap - it throws a NullPointerException with no hint about which row. The three-argument collect keeps the nulls if the nulls are the point.

• 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: UNORDERED is a promise you make, not one you receive

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Published 2026-09-20

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