LuTiao Programming
Nov 9, 2026 · Backend Development
Stream.toMap() Duplicate Key Crash: Why the Common Fix Silently Loses Data
A duplicate phone number in a 5000-row Excel import caused Stream.toMap() to throw IllegalStateException; the typical merge-function fix silently discards data, so the article presents a robust validation pipeline using groupingBy, key normalization, batch database checks, and explicit error reporting to handle duplicates correctly.
Collectors.toMapJava Stream APIbatch import
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