Role of FieldPosition in Format class?

Question

I am learning about Format class, but I don't understand FieldPosition class which is used in format() method and ParsePosition() which is used in parseObject() method.
Can someone explain me their use?

Answer 1

import java.text.*;
import java.util.*;
public class BobMil{
  public static void main(String[] args){
    SimpleDateFormat sdf = new SimpleDateFormat("yyyy MM dd HH:mm:ss");
    Date d = new Date();
    System.out.println(d.toString());
    System.out.println(sdf.format(d));
    StringBuffer sb = new StringBuffer();
    FieldPosition fp = new FieldPosition(DateFormat.Field.MINUTE);
    sdf.format(d, sb, fp);
    System.out.println("minute = " + getField(sb, fp));
    System.out.println("-----");
    String text = "abcd2007 08 29 10:11:25xyz";
    ParsePosition pp = new ParsePosition(text.indexOf("2007"));
    Date dn = sdf.parse(text, pp);
    System.out.println(dn.toString());
    System.out.println("remaining text = " + text.substring(pp.getIndex()));
  static String getField(StringBuffer buf, FieldPosition pos){
    return buf.substring(pos.getBeginIndex(), pos.getEndIndex());
}