Getting Numbers From Input Streams

1
Getting Numbers From Input Streams

• So far we know how to hook a Scanner up to an
  input stream, and how to get an entire line of
  input into a String
• If the user types in "3.14159", what is the data
  type of the resulting object, and what operations
  can we perform on it?
• The important lesson there is no necessary
  relationship between
• the int 123 and the string "123"
• the double 3.14159 and the string "3.14159"
• the char '1' and the int 1
• the char '1' and the double 1.0
• the char '1' and the String "1"
• We need a general method for converting from
  Strings that "look like" numbers, and the
  equivalent numbers

Scanner aScanner new Scanner(System.in) String
wholeLine aScanner.nextLine()
2
Two Scanner Classes That Do Numeric Conversion

• First, suppose the user types in "12345". Will
  either of these work? Why or why not?
• The (correct) alternative asks the Scanner both
  to read a line of text from its input stream, and
  then to convert it to an int
• And the obvious analogue for floating-point
  numbers is

Scanner aScanner new Scanner(System.in) String
wholeLine aScanner.nextLine() int i
(int)wholeLine
Scanner aScanner new Scanner(System.in) String
wholeLine aScanner.nextLine() int i
wholeLine
Scanner aScanner new Scanner(System.in) int i
aScanner.nextInt()
Scanner aScanner new Scanner(System.in) double
d aScanner.nextDouble()
3
Scanner Errors

• When you scan a String, nothing can go wrong
• because anything the user can possibly type in is
  a sequence of valid string characters
• But that's not the case when you are scanning
  numbers
• an integer is an optional sign followed by 1 or
  more digits
• a double is an optional sign followed by a
  mantissa part followed by an optional decimal
  part
• What happens when your program is expecting an
  int, but gets something else
• a string with non-numeric characters
• a negative sign by itself
• a floating-point number

4
Multiple Input Tokens On A Line

• Suppose the user types in the following line in
  response to some Scanner input request
• " Mary Smith 20 20.25 "
• This line has four "tokens" on it, where a token
  is defined to be a sequence of non-whitespace
  characters separated by one or more whitespace
  characters
• There are two different sorts of Scanner methods
  that do two different things
• aScanner.nextLine() returns the whole line,
  regardless of how many tokens
• aScanner.next() returns the next token,
  regardless of what else is on the line
• aScanner.nextInt() extracts the next token
  then converts it to an int
• aScanner.nextDouble() extracts the next token
  then converts it to a double
• If a line has just one token on it, the
  difference is not important, but if a line might
  contain multiple tokens (and you care), you have
  to be aware of the distinction between
• one line, multiple tokens
• multiple lines, one token per line
• For example, what if the input specification says
  that an input line must contain first name, last
  name, hours worked, and wage rate (blank
  separated)?
• Instead, what if the input specification says
  that the name is on the first line, then the
  hours on the second line, then the wage rate on
  the third line?

5
Formatting Numbers

• No big issues in displaying strings on an output
  stream just print them out
• Formatting numbers is a bigger deal, because how
  they are displayed is significant (apart from the
  numeric value). For example, consider the double
  value 1.235
• 1.235 just itself to full precision
• 1.2350 padded to four decimal places
• 1.24 rounded to two decimal places
• 1.2 rounded to one decmial place
• 1.23 as currency
• 124 as a percentage
• Same value, different meanings
• There are some nice utilities that format things
  in all those ways, but the Java syntax is a
  little strange!

6
Using the NumberFormat class

• The NumberFormat class gives you two "formatter"
  objects that are specialized for formatting
  numbers as currency and percentages
• NumberFormat pct NumberFormat.getPercentInstance
  ()
• NumberFormat cur NumberFormat.getCurrencyInstanc
  e()
• And the DecimalFormat class gives you a
  "formatter" object that can be customized to
  format a decimal number
• DecimalFormat dec new DecimalFormat("0.00")
• a zero means a digit must be printed there pad
  with zeros if necessary
• a means a digit can be printed there if the
  number accuracy demands it
• so this example will always print at least one
  number before the decimal place and at least two
  numbers after the decimal place, and maybe a
  third decimal place
• So far, you know how to create instances of these
  objects, but what do they do?
• In particular, how do I tell them to format the
  double number 1.235 for me?
• The answer is simple all of them respond to a
  method format, which takes a number (double or
  int) as an argument, and produce a string
• pct.format(1.235)
• cur.format(1.235)
• dec.format(1.235)
• (But even that doesn't actually display the
  output we might still want to print the
  result.)
• Exercise take an employee name and old salary
  from the console. Then take a new salary from
  the console. Print the employee's name, old, and
  new salary, and how much of a raise that salary
  is.