FieldPosition is a simple class used by Format and its subclasses to identify fields in formatted output. Fields can be identified in two ways:

By an integer constant, whose names typically end with _FIELD. The constants are defined in the various subclasses of Format.
By a Format.Field constant, see ERA_FIELD and its friends in DateFormat for an example.

FieldPosition keeps track of the position of the field within the formatted output with two indices: the index of the first character of the field and the index of the last character of the field.

One version of the format method in the various Format classes requires a FieldPosition object as an argument. You use this format method to perform partial formatting or to get information about the formatted output (such as the position of a field).

If you are interested in the positions of all attributes in the formatted string use the Format method formatToCharacterIterator.