question: private String addSpaceToCardNumber ( ) { final int splitBy = 4 ; final int length = mCardNumber . length ( ) ; if ( length % splitBy != 0 || length <= splitBy ) { return mCardNumber ; } else { final StringBuilder result = new StringBuilder ( ) ; result . append ( mCardNumber . substring ( 0 , splitBy ) ) ; for ( int i = splitBy ; i < length ; i ++ ) { if ( i % splitBy == 0 ) { result . append ( " " ) ; } result . append ( mCardNumber . charAt ( i ) ) ; } return result . toString ( ) ; } }
target:  Adds space after every 4 characters to the card number if the card number is divisible by 4