Altering format string output by changing a format specifier's argument_index

A format string is a string which can include one or more format specifiers.

String hungry = "hungry";
String hippo = "hippo";
String s = String.format(
  "%s %s",
  hungry, 
  hippo);

Here, “%s %s” is a format string, and %s is a format specifier. The value of s is “hungry hippo”.

Modify the order that the arguments appear in the format string by specifying an argument index in the format specifiers.

Argument indexes take the form of a non-negative integer followed by $, where the integer specifies the position of the argument in the argument list.

String s = String.format(
  "%2$s %1$s",
  hungry, 
  hippo);

The output of the example above will be “hippo hungry” because we have specified that argument 2 (%2$s) will come before argument 1 (%1$s).