How should Collectors.toMap handle duplicate keys?

Makes duplicate-key behavior explicit through validation, merge policies, grouping, and deterministic map selection.

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TL;DR

The simplest Collectors.toMap(keyMapper, valueMapper) expects unique keys and throws when two elements produce the same key. If duplicates are valid, supply a merge function that defines which value wins or how values combine. If every value must be retained, groupingBy to a list may represent the data more honestly. The right choice depends on what duplicate keys mean in the domain.

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Key uniqueness assumptions, merge-function correctness, data-loss avoidance, and diagnosis of collector failures.

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Core answer
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Overview
A duplicate key is a domain decision, not just an exception to silence with an arbitrary merge function.
The simplest Collectors.toMap(keyMapper, valueMapper) expects unique keys and throws when two elements produce the same key. If duplicates are valid, supply a merge function that defines which value wins or how values combine. If every value must be retained, groupingBy to a list may represent the data more honestly. The right choice depends on what duplicate keys mean in the domain.
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Stronger answer
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For a latest-reading-by-device map, a merge function can compare timestamps and keep the newer reading. For users keyed by supposedly unique email, silently keeping the first value may hide bad data; failing with a clear validation message is safer. The merge function must be associative enough for the intended execution mode, especially if the stream may be parallel. A map supplier can choose an ordered or sorted result when that guarantee matters.

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Details

Using (first, second) -> first without a domain reason.
Assuming the source key is unique without validation.
Dropping values that should have been grouped.
Depending on encounter order without documenting it.

How should Collectors.toMap handle duplicate keys?

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