question: private int readArgumentIndex ( final String pattern , final ParsePosition pos ) { final int start = pos . getIndex ( ) ; seekNonWs ( pattern , pos ) ; final StringBuilder result = new StringBuilder ( ) ; boolean error = false ; for ( ; ! error && pos . getIndex ( ) < pattern . length ( ) ; next ( pos ) ) { char c = pattern . charAt ( pos . getIndex ( ) ) ; if ( Character . isWhitespace ( c ) ) { seekNonWs ( pattern , pos ) ; c = pattern . charAt ( pos . getIndex ( ) ) ; if ( c != START_FMT && c != END_FE ) { error = true ; continue ; } } if ( ( c == START_FMT || c == END_FE ) && result . length ( ) > 0 ) { try { return Integer . parseInt ( result . toString ( ) ) ; } catch ( final NumberFormatException e ) { } } error = ! Character . isDigit ( c ) ; result . append ( c ) ; } if ( error ) { throw new IllegalArgumentException ( "Invalid format argument index at position " + start + ": " + pattern . substring ( start , pos . getIndex ( ) ) ) ; } throw new IllegalArgumentException ( "Unterminated format element at position " + start ) ; }
target:  Read the argument index from the current format element