Why does Collectors.toMap report value instead of key on Duplicate Key error?

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This is reported as a bug, see JDK-8040892, and it is fixed in Java 9. Reading the commit fixing this, the new exception message will be

String.format("Duplicate key %s (attempted merging values %s and %s)", k, u, v)

where k is the duplicate key and u and v are the two conflicting values mapped to the same key.

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