The i18n Cookbook - recipies for a global society

Problem:

You want to retrieve all of the era names for the Japanese calendar.

Solution:

Use the icu4j Japanese calendar and DateFormatSymbols to retrieve the era names.

//Get a ULocale that specifies a Japanese Calendar in Japanese
ULocale jp = new ULocale("ja_JP@calendar=japanese");
//Get a Japanese calendar instance
Calendar jc = Calendar.getInstance(jp);
//The era field contains the numeric identifier for the reigning emperor at the current time
System.out.println(jc.get(Calendar.ERA));
//Get an instance of the DateFormatSymbols for Japanese using the Japanese calendar
DateFormatSymbols dfs = DateFormatSymbols.getInstance(jp);
//getEras() returns a String array of era names
String[] eras = dfs.getEras();
//Loop through the eras and output
for(int x = 0; x < eras.length; x++){
    System.out.println(eras[x]);
}
//Now let's get the English Era names
//Get a ULocale that specifies a Japanese Calendar in English
ULocale en = new ULocale("en@calendar=japanese");
//Get the DateFormatSymbols for English using a Japanese Calendar
dfs = DateFormatSymbols.getInstance(en);
eras = dfs.getEras();
for(int x = 0; x < eras.length; x++){
    System.out.println(eras[x]);
}

Discussion:

The Japanese calendar counts years based upon the reign of the emperor.  Since the reign of an emperor varies in length it is impossible to calculate the year programmatically.  icu4j's DateFormatSymbols can provide us with a list of all Emperors.  To find the Japanese year for a Gregorian date use this recipe.