Reuse a parameter in String.format?

Question

From the docs:

• The format specifiers for general, character, and numeric types have the following syntax:

      %[argument_index$][flags][width][.precision]conversion
  

  The optional argument_index is a decimal integer indicating the position of the argument in the argument list. The first argument is referenced by "1$", the second by "2$", etc.

String.format("%1$s %1$s %1$s %1$s %1$s %1$s", hello);

Another option is to use relative indexing: The format specifier references the same argument as the last format specifier.

For example:

String.format("%s %<s %<s %<s", "hello")

results in hello hello hello hello.

You need to use the index argument %[argument_index$] as the following:

String hello = "Hello";
String.format("%1$s %1$s %1$s %1$s %1$s %1$s", hello);

Result: Hello Hello Hello Hello Hello Hello

One common case for reusing an argument in String.format is with a separator (e.g. ";" for CSV or tab for console).

System.out.println(String.format("%s %2$s %s %2$s %s %n", "a", ";", "b", "c"));
// "a ; ; ; b"

This isn't the desired output. "c" doesn't appear anywhere.

You need to use the separator first (with %s) and only use the argument index (%2$s) for the following occurences :

System.out.println(String.format("%s %s %s %2$s %s %n", "a", ";", "b", "c"));
//  "a ; b ; c"

Spaces are added for readability and debugging. Once the format appears to be correct, spaces can be removed in the text editor:

System.out.println(String.format("%s%s%s%2$s%s%n", "a", ";", "b", "c"));
// "a;b;c"