Understanding the $ in Java's format strings

Question

 StringBuilder sb = new StringBuilder();
 // Send all output to the Appendable object sb
 Formatter formatter = new Formatter(sb, Locale.US);

 // Explicit argument indices may be used to re-order output.
 formatter.format("%4$2s %3$2s %2$2s %1$2s", "a", "b", "c", "d")
 // -> " d  c  b  a"

In this case, why is a 2 appended to $?

Answer 1

The 2$ means put the second argument from the list here. The $ follows a number not precedes it. Similarly, 4$ means put the forth argument here.

To clarify, we can break down the %2$2s format into its parts:

• % - indicates this is a format string

• 2$ - shows the second value argument should be put here

• 2 - the format is two characters long

• s - format the value as a String

You can find more information in the documentation.

Answer 2

The 2 has nothing to do with the $:

• % = Start of format string
• 4$ = Fourth argrument ('d')
• 2 = width of two
• s = String

Answer 3

Those are positional arguments where %4$2s signals to format the fourth argument as a string with width 2. This is especially helpful when providing strings for localization where arguments need to be reordered without touching the source code.

The format specifiers for types which are used to represents dates and times have the following syntax:

%[argument_index$][flags][width]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. —Formatter documentation