Chapter 3b

1
Chapter 3b

• Standard Input and Output
• Sample Development

2
Topics

• Standard output
• Standard input
• GregorianCalendar class
• LoanCalculator development

3
Standard Output

• The showMessageDialog method is intended for
  displaying short one-line messages, not for a
  general-purpose output mechanism.
• Using System.out, we can output multiple lines of
  text to the standard output window - the console.

4
System.out

• System.out is a PrintStream object.
• Two useful methods
• the print method
• takes a String as its argument and prints it to
  the console
• the println method
• takes a String as its argument and prints it to
  the console with a newline character appended to
  the end
• Both methods will continue printing from the end
  of the currently displayed output.
• Both methods will do the necessary type
  conversion if we pass numerical data.

5
Overloaded Operator

• The operator can be used in 2 ways
• adding numbers
• concatenating Strings
• What happens if we mix its arguments

int x 1 int y 2 String output test x
y String output x y test
6
Standard Input

• The technique of using System.in to input data is
  called standard input.
• Associating a BufferedReader object to the
  System.in object allows us to read a single line
  of text.
• Then we can use that String
• as it was input if what we need is a String
• convert it to a value of a primitive data type
• Using the intermediate InputStreamReader object
  allows us to read a single character at a time.

7
Input classes

• How the sequence of I/O objects adds greater
  capabilities.
• InputStream has the capability to read bytes
• InputStreamReader has the capablity to read
  characters (2 bytes)
• BufferedReader can read an entire line

8
Using BufferedReader

• Declaration
• BufferedReader kbd
• Creation
• kbd new BufferedReader(
• new InputStreamReader(
• System.in))
• Use
• String input kbd.readLine()

9
Handling Input Errors

• Calling the readLine method of a BufferedReader
  can result in an error condition called an
  exception.
• The programmer is required to handle this
  potential error condition.
• For now we choose to ignore these problems by
  adding the clause throws IOException to the
  method declaration whenever a method includes a
  call to the readLine method.
• public static void main(String args) throws
  IOException
• ...
• String input bufReader.readLine()
• ...

10
Getting Numerical Input Values

• Java provides primitive data types for working
  with numbers.
• Wrapper classes exist that allow you to make an
  object out of a number.
• There is a wrapper class for each primitive type
  Integer, Double,
• These classes provide methods we can use to
  convert an appropriate String object to a
  numerical value.
• Integer.parseInt converts a String like "23" to
  an int
• Double.parseDouble converts a String like "2.3"
  to a double

11
Example Getting Numerical Input

• BufferedReader in
• new BufferedReader(
• new InputStreamReader( System.in))
• System.out.print("Enter radius")
• radiusStr in.readLine()
• radius Double.parseDouble(radiusStr)
• Similarly, you can use JOptionPane.showInputDialog
  to get a String to convert.

12
DecimalFormat

• Since floating point numbers can't be represented
  exactly in computer memory, they often print out
  with lost of digits after the decimal point.
• The DecimalFormat class lets you print numbers
  with a fixed number of digits after the decimal
  place.
• Use a "picture" string to show what you want the
  number to look like
• "0.000" means 3 dgiits after the decimal point
• Use the format method with the number you want
  formatted as an argument

13
DecimalFormat Example

• Create the DecimalFormat with a String
• DecimalFormat df
• new DecimalFormat( "0.000")
• Call the format method with the number as an
  argument
• double fp
• / assign a value to fp /
• System.out.print( df.format(fp))

14
The GregorianCalendar Class

• The GregorianCalendar class is useful in
  manipulating calendar information such as year,
  month, and day.
• You can create a GregorianCalendar for any date
  by giving the year, month and day
• cal
• new GregorianCalendar(2001, 8, 11)
• Caution the first month of the year, January, is
  represented by 0.

15
Sample Development Loan Calculator

• Problem Statement
• Write a loan calculator that computes monthly
  payments and total payments for a loan based on
  loan amount, annual interest rate and loan period
• Program flow
• Get three input values loanAmount, interestRate,
  and loanPeriod.
• Compute the monthly and total payments.
• Output the results.
• What classes do we need?

16
UML Diagram for Loan Calculator
17
Loan Calculator

• Steps in implementation
• Start with code to accept three input values.
• Add code to output the results.
• Add code to compute the monthly and total
  payments.
• Update or modify code and tie up any loose ends.

18
Loan Calculator Step 1

• Input Three Data Values
• Choices
• Call showInputDialog method for each of three
  input values
• Use a BufferedReader
• Question What format should the data be in?
• loan amount - use double for dollars and cents
• annual interest rate - percentage needs to be a
  double
• loan period - usually an even number of years so
  use an int

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Step 2 Output Values

• We must determine an appropriate format to
  display the computed results, so that the results
  will be comprehensible to the user.
• How much information do we need to show?
• For now, put placeholders in the position the
  actual results will go

20
Possible Display Formats

• Only the computed values displayed
• Monthly payment 143.47
• Total payment 17216.50
• Input values displayed.
• For
• Loan Amount 10000.00
• Annual Interest Rate 12
• Loan Period(years) 10
• Monthly payment is 143.47
• Total payment is 17216.50

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Step 3 Compute Loan Amount

• Complete the program by implementing the formula
  derived in the design phase.
• We must convert
• the annual interest rate (input value) to a
  monthly interest rate (per the formula)
• the loan period to the number of monthly
  payments.

22
Step 4 Finishing Up

• Finalize the program by making necessary
  modifications or additions.
• We will add a program description and format the
  monthly and total payments to two decimal places.