This interface should be implemented by classes that want to interact with the Formatter class more intimately than is possible with the toString method. When a Formattable object is the argument for a %s or %S conversion, its formatTo( ) method is invoked rather than its toString( ) method. formatTo( ) is responsible for formatting a textual representation of the object to the specified formatter, subject to the constraints imposed by the flags, width, and precision arguments.

The flags argument is a bitmask of zero or more FormattableFlags constants. Each flag provides information about the format specification that resulted in the invocation of formatTo( ). FormattableFlags.ALTERNATE indicates that the # flag was used and that the Formattable should format itself using some alternate form. The interpretation of the alternate form is entirely up to the Formattable implementation. LEFT_JUSTIFY means that the - flag was used and that the Formattable should pad its output on the right, instead of on the left. UPPERCASE indicates that the %S conversion was used instead of %s and the Formattable should output uppercase characters instead of lowercase.

The width and precision arguments specify the width and precision specified along with the %s format specifier, or -1 if no width and precision are specified. The Formattable object should treat these values the same way that Formatter does. The text to be output should first be truncated to fit within precision characters and then padded on the left (or right if the LEFT_JUSTIFY flag is set) with spaces for a total length of width characters. Note that a Formattable implementation may fulfill the obligations imposed by the LEFT_JUSTIFY and UPPERCASE flags and the width and precision arguments by constructing a suitable format string to pass back to the specified Formatter.

If a Formattable implementation wants to perform locale-specific formatting, it can query the Locale of the Formatter with the locale( ) method. Note, however, that the returned value is the locale specified when the Formatter was created, not the Locale, if any, passed to the format( ) method. There is no way for a Formattable object to access that Locale.

public interface Formattable {
// Public Instance Methods
     void formatTo(java.util.Formatter formatter, int flags, int width, int precision);  
}