Section 17.9 "Sleep and Yield" in the Java Language Specification Third Edition, has the following statement:

  "Neither a sleep for a period of zero time nor a yield operation need have observable effects."

This precludes java.lang.Thread.yield from having any strong specification as it could be challenged as a contradiction to this statement. Effectively this statement in the JLS allows yield to be a noop and prevents it from gaining any strongly specified behavior.

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