Formatter Argument Index

In this chapter you will learn:

Using an Argument Index
How to reuse the parameter
How to use relative index
How to use relative index to create customized date format

Using an Argument Index

Formatter can have the argument to indicate a format specifier index. By using an argument index, you can control which argument a format specifier matches.

An argument index immediately follows the % in a format specifier. It has the following format:

n$

where n is the index of the desired argument, beginning with 1. For example, consider this example:

import java.util.Formatter;
//  j  a  va  2  s. c om
public class Main {
  public static void main(String args[]) {
    Formatter fmt = new Formatter();
    fmt.format("%3$d %1$d %2$d", 10, 20, 30); 
    System.out.println(fmt);
  }
}

The output:

Reuse the parameter

The following code uses the argument index to reuse the parameter.

import java.util.Formatter;
/*from  j  av  a  2 s.co  m*/
public class Main {
  public static void main(String args[]) {
    Formatter fmt = new Formatter();
    fmt.format("%d in hex is %1$x", 255); 
    System.out.println(fmt);
  }
}

Relative index

A relative index, <, enables you to reuse the argument matched by the preceding format specifier.

import java.util.Formatter;
//  j  av a 2 s . c  om
public class Main {
  public static void main(String args[]) {
    Formatter fmt = new Formatter();
    fmt.format("%d in hex is %<x", 255); 
    System.out.println(fmt);
  }
}

The output:

Relative indexes and customized date format

Relative indexes are useful when creating custom time and date formats.

Because of relative indexing, the argument cal need only be passed once, rather than three times.

import java.util.Calendar;
import java.util.Formatter;
//from j a  v a 2  s  .co m
public class Main {
  public static void main(String args[]) {
    Formatter fmt = new Formatter();
    Calendar cal = Calendar.getInstance();
    fmt.format("Today is day %te of %<tB, %<tY", cal);
    System.out.println(fmt);
  }
}