Parsing and Formatting

1
Parsing and Formatting

• Obvious fact the internal representation of an
  object (a Scanner, a double, a date) is not
  easily readable by a human
• generally called the "human-readable" or
  "printable" version of the object
• Another obvious fact there is not a unique
  relationship between the object and the printable
  version
• 2 versus 2 versus 002
• 3.02 versus 03.02222 versus 30.02E-01 versus
  3.02 versus 302
• July 4, 1776 versus 07/04/1776
• Parsing is the process of going from a String
  (the printable version) to the Object (an int or
  double or Date)
• Scanner already does parsing for various numeric
  types!
• Formatting is the process of going from an object
  (int or double or Date) to a String
  representation
• toString() and println already do formatting for
  various numeric types!

2
Parsing Numbers

• Most of this is already done / known, because
  the Scanner already does parsing for all the
  number types (along with reading and tokenizing)
• However, the "real Java" way to parse a number is
  ask the corresponding numeric class to do it for
  you
• int anInteger Integer.parseInt("-3.2")
• System.out.println(Double.parseDouble("-03.62000E0
  5"))
• Output is -362000.0

3
Formatting Numbers

• This is a little more complicated because
• every valid String representation of a number
  maps into a unique number (parsing is
  unambiguous)
• but a number does not have a unique String
  equivalent (formatting is ambiguous)
• so formatting has to be done under the control of
  a "template" that says
• leading plus sign?
• leading zeros?
• how many decimal places?
• scientific notation?
• also, there are some special cases of interest
  because they are convenient
• format this number like a dollar amount
  (currency)
• format this number like a percentage

4
Number Formatting Examples

• General formatting
• read an integer and print it with a leading sign
  and exactly 9 additional digits
• read a double and print it with at most 3 decimal
  places accuracy
• how about exactly 3 decimal places accuracy?
• Special cases currency and percentage
• read two wage values and print "Your new salary
  of x.xx is an increase of yyy."

5
Dates and Date Formatting

• The motivating problem
• Suppose an order is placed at some point in time.
• If the order time is in the future, that's an
  error!
• Otherwise, if the order time is before noon
  today, the order will be ready at 5PM today.
• Otherwise, the order will be ready at noon
  tomorrow.
• Print "Your order will be ready at Friday January
  21 at 1200 PM"
• The starting point
• dates represent an instant in time
• dates are a lot like numbers you can compare
  them (is one date after another
• arithmetic is more complicated too adding an
  hour, or a week, or subtracting a minute
• parsing and formatting dates is much more
  complicated
• Tuesday January 15 at 325AM PDT
• 01/05/2004 16.42
• the Scanner doesn't parse dates
• the history / technology behind Java dates,
  times, and calendars is really twisted