Argument indexes enable you to reuse an argument without having to specify it twice : Formatter Argument Index « Development « Java Tutorial

import java.util.Formatter;

public class MainClass {
  public static void main(String args[]) {
    Formatter fmt = new Formatter();

    fmt.format("%d in hex is %1$x", 255);
    System.out.println(fmt);

  }
}

255 in hex is ff

6.11.Formatter Argument Index
	6.11.1.	Using an Argument Index
	6.11.2.	Argument indexes enable you to reuse an argument without having to specify it twice
	6.11.3.	Relative index enables you to reuse the argument matched by the preceding format specifier
	6.11.4.	Relative indexes are especially useful when creating custom time and date formats