question: private String inputAccruedNationalNumber ( ) { int lengthOfNationalNumber = nationalNumber . length ( ) ; if ( lengthOfNationalNumber > 0 ) { String tempNationalNumber = "" ; for ( int i = 0 ; i < lengthOfNationalNumber ; i ++ ) { tempNationalNumber = inputDigitHelper ( nationalNumber . charAt ( i ) ) ; } return ableToFormat ? appendNationalNumber ( tempNationalNumber ) : accruedInput . toString ( ) ; } else { return prefixBeforeNationalNumber . toString ( ) ; } }
target:  Invokes inputDigitHelper on each digit of the national number accrued, and returns a formatted string in the end.