Summary
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In order to accommodate the future addition of JapaneseEra without spec changes, loosening the current JapaneseEra spec would be desired.

Problem
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The current spec does not mention available eras returned from of()/valueOf()/values() methods, especially it is unclear if a new era is declared by the Japanese government. This would be more problematic for the maintenance releases.

Solution
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Specify the possible values returned from those static methods, and their parameters.

Specification
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Add the following sentence in the class description:

    * The number of the valid eras may increase, as new eras may be
    * defined by the Japanese government. Once an era is defined,
    * subsequent versions of this class will add a singleton instance
    * for it. The defined era is expected to have a consecutive integer
    * associated with it.

Add the following paragraph to of(int) method description:

     * <p>
     * In addition to the known era singletons, values for additional
     * eras may be defined. Those values are the {@link Era#getValue()}
     * of corresponding eras from the {@link #values()} method.

Add the following paragraph to valueOf(String) method description:

     * <p>
     * Valid era names are the names of eras returned from {@link #values()}.

Change the first paragraph of values() method from:

    * Returns an array of JapaneseEras.

to:

     * Returns an array of JapaneseEras. The array may contain eras defined
     * by the Japanese government beyond the known era singletons.