标志，System.out.format Java

Question

Talking about flags, it's described in the javadoc that the flag ' ' (empty space) will insert automatically an empty space instead of the sign "+" just at the beginning of the number. However when I try it it works only when the first optional argument of the format specifier is filled.

  System.out.printf("% d%n %1$ d%n",0x3);

Even though there is an empty space in both the formatters, it works only in the latter one %1$ d%n whereas in the former % d%n the leading empty space meaning the positive number is absent (even though the flag ' ' is on).

Is there any specific reason why it happens like that? Thanks in advance.

Answer 1

Here is the output of that line of code:

 3
  3

Both lines have a leading space, however, the second line has two spaces because you have a space between the second and third conversion, ie if I split the String to make it more readable:

"% d" + "%n" + " " + "%1$ d" + "%n"