I was playing around with Locale extensions on JDK 7/8. There was some strange behavior with toString() and extension input validation.
Summarizing I was able to create Locale instances that were not valid as defined in the specs (with duplicate singletons or tags).
More details are described in my blog entry here:
For our use cases according formatting/parsing the mechanism implemented by the JDK looks for me far too restrictive, to be able to replace, what we have defined as of now int the format package.
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