Manipulating Data Using Methods

1
Chapter 3

• Manipulating Data Using Methods
  

2
Introduction

• Data are collections of raw facts or figures
• A program performs operations oninput data to
  output information
• Input data can come from a variety of sources
• The program itself
• Users of the program
• External files

3
The Body Mass Index Calculator

• An interactive program
• Accepts the weight and height from the user
• Calculates the BMI to gauge total body fat
• Displays the result
• Three versions
• Input/Output using the command prompt
• Input/Output using dialog boxes
• Web environments use an applet interface

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(a) console application in a command prompt window
(b) console application using dialog boxes
(c) applet
5

6
Problem Analysis

• Convert user input to metric measurements
• Calculate the BMI
• Display the result

7
Design the Solution

• Design the three kinds of user interfaces with
  storyboards
• Design the logic of the program
• Use pseudocode for sequential flow for all
  programs
• Use an event diagram for the applet
• Validate the design
• Compare the program design with the original
  requirements

8

pseudocode
event diagram
9
Code the program

• Open TextPad and save as BodyMass.java
• Enter beginning comments (lines 1 9)/
  Chapter 3 Body Mass Index Calculator
  Programmer your name Date todays date
  Filename BodyMass.java Purpose This project
  calculates the body mass index based on a
  person's height and weight./
• Line 10import java.io.

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Code the program

• Add class header and main method header(lines 12
  15)public class BodyMass
• public static void main(String args) throws
  IOException

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Code the program

• Import the java.io package
• Provides classes to support system input and
  output
• Add a throws IOException clause to the method
  header
• Warns the compiler that the possibility of input
  or output errors exists
• Gives the program the opportunity to handle input
  or output errors during run-time without aborting

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Storing Data

• Java is a strongly typed language
• Variables must be declared with a data type
• Variable locations can hold only that data type
• Java has two categories of data types
• Primitive data types hold single data items
• Integers, characters, floating point, and
  booleans are primitive types
• Reference data types hold a value that refers to
  the location of the data
• All Objects and arrays are reference types

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Declaring Variables
15
Code the program

• Declare and construct variables (lines16
  19)// declare and construct variablesString
  strHeight, strWeightint intInches,
  intPoundsdouble dblKilograms, dblMeters,
  dblIndex

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User Input Streams and the System Class

• The act of data flowing in and out of a program
  is called a stream
• The System class creates three streams when a
  program executes

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User Input Streams and the System Class

• Data from input streams are first sent to a
  buffer
• The java.io package contains several stream
  classes
• InputStreamReader
• Decodes the bytes from the System.in buffer into
  characters
• BufferedReader
• Increases efficiency by temporarily storing the
  input received from another class, such as
  InputStreamReader
• Aids in platform independence by simplifying the
  process of reading text and numbers from various
  input sources

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Using the BufferedReader class

• Call the BufferedReader constructor to
  instantiate a BufferedReader object
• The argument of the BufferedReader( ) method
  instantiates an InputStreamReader
• BufferedReader( ) returns a reference to the
  input data from System.in
• Line 20 BufferedReader dataIn new
  BufferedReader (new InputStreamReader
  (System.in))

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User Prompts, Inputs, and Conversions

• The readLine() method reads a line of input text
  and returns a String containing the line
• The returned String must be explicitly converted
  if the data is to be used as another data type
• Each primitive data type has a wrapper class
  allowing the primitive to be treated as an object
• The wrapper classes provides a parse() method to
  convert Strings to primitives, and vice versa

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Code the program

• Lines 22 - 30// print prompts and get input
• System.out.println("\tTHE SUN FITNESS CENTER
  BODY MASS INDEX CALCULATOR")
• System.out.println()
• System.out.print("\t\tEnter your height to the
  nearest inch ")
• strHeight dataIn.readLine()
• intInches Integer.parseInt(strHeight)
• System.out.print("\t\tEnter your weight to the
  nearest pound ")
• strWeight dataIn.readLine()
• intPounds Integer.parseInt(strWeight)

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Assignment Statements

• General syntax location value

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Arithmetic Operators
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Arithmetic Operators

• The order of operator precedence is a
  predetermined order that defines the sequence in
  which operators are evaluated in an expression
• Addition, subtraction, multiplication, and
  division can manipulate any numeric data type
• When Java performs math on mixed data types, the
  result is always the larger data type
• Casts allow programmers to force a conversion
  from one primitive type to another

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Comparison Operators

• A comparison operation results in a true or false
  value that can be stored in a boolean variable

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Numeric Expressions

• Numeric expressions evaluate to a number
• Only numeric primitive data types may be used in
  a numeric expression
• A value and variable must be separated by an
  arithmetic operator
• Unless parentheses supercede, an expression is
  evaluated left to right with the following rules
  of precedence
• Multiplication and/or division
• Integer division
• Modular division ( modulus)
• Addition and/or subtraction

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Conditional Expressions

• Conditional expression evaluate to either true or
  false (10 lt 5 true)
• Comparison operators, values, variables, methods,
  and Strings may be used in a conditional
  expression
• Two operands must be separated by a comparison
  operator
• Unless parentheses supercede, an expression is
  evaluated left to right with relational operators
  (lt, lt, gt, gt) taking precedence over equality
  operators (, !)

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Parentheses in Expressions

• Parentheses may be used to change the order of
  operations
• The part of the expression within the parentheses
  is evaluated first
• Parentheses can provide clarity in complex
  expressions
• Numeric and conditional expressions should be
  grouped with parentheses
• Parentheses can be nested
• Java evaluates the innermost expression first and
  then moves on to the outermost expression

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Construction of Error-Free Expressions

• Java may not be able to evaluate a validly formed
  expression due to the following logic errors
• Dividing by zero
• Taking the square root of a negative value
• Raising a negative value to a non-integer value
• Using a value too great or too small for a given
  data type
• Comparing different data types in a conditional
  expression

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The Math Class

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Using Variables in Output
32
Code the program

• Lines 32 35
• // calculations
• dblMeters intInches / 39.36
• dblKilograms intPounds / 2.2
• dblIndex dblKilograms / Math.pow(dblMeters, 2)

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Code the program

• Lines 37 - 42
• // output
• System.out.println()
• System.out.println("\tYOUR BODY MASS INDEX
  IS " Math.round(dblIndex) ".")
• System.out.println()

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Compiling, Running, and Documenting the
Application

• Compile the Body Mass Index Calculator program
• Execute the program
• Test the program by entering the sample input
  data supplied in the requirements phase at the
  prompts
• Verify the results

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Sample Results
36
Using Swing Components

• Save the previous version of the Body Mass Index
  Calculator with a new filename
  BodyMassSwing.java
• Use Search/Replace to find BodyMass and change
  them all to BodyMassSwing in the program.

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Using Swing Components

• Remove the old import statement and replace with
  import javax.swing.JOptionPane
• Contains methods to create dialog boxes for
  input, confirmation, and messages
• Delete the IOException and BufferedReader code
• The swing dialog boxes buffer data from the user
  and handle IO errors

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Code the program

• Lines 10 22
• import javax.swing.JOptionPane
• public class BodyMassSwing
• public static void main(String args)
• // declare and construct variables
• String strHeight, strWeight
• int intInches, intPounds
• double dblKilograms, dblMeters, dblIndex
• // print prompts and get input
• System.out.println("\tTHE SUN FITNESS CENTER
  BODY MASS INDEX CALCULATOR")

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Swing Dialog Boxes

• Dialog boxes are created with the JOptionPane
  show methods
• The showInputDialog() and showConfirmDialog
  return a String containing the user input
• First argument null means to center in the
  active window
• Second argument is the display area inside the
  dialog box
• Third argument is the caption on the title bar
• Fourth argument is a constant refering to the
  type of icon

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OptionPane.showMessageDialog(null, "message",
"caption", one of the arguments below)
JOptionPane.ERROR_MESSAGE
JOptionPane. INFORMATION_ MESSAGE
JOptionPane. WARNING_ MESSAGE
JOptionPane. QUESTION_ MESSAGE
JOptionPane. PLAIN_ MESSAGE
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Code the program
42
Closing Programs that use Swing

• System.exit() terminates an application that
  displays a GUI
• The command prompt window closes when this method
  is called
• System.exit accepts an integer argument that
  serves as a status code
• 0 indicates successful termination
• 1 indicates abnormal termination
• Add Line 39 System.exit(0)

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Saving, Compiling, and Running the Swing Version

• Verify that the file name matches the class name
  at the beginning of the code
• Compile the source code
• Test with the same sample data for all versions
  to compare output results
• If incorrect or unrealistic data is entered by
  the user, errors will occur
• Errors and exception handling will be discussed
  in a later chapter

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Run the application
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Moving to the Web

• The applet version of the Body Mass Index
  Calculator has four kinds of objects
• Image, Labels, TextFields, and Buttons
• Import three packages
• Java.applet
• Java.awt
• Java.awt.event
• Implement an ActionListener interface in the
  class header
• Informs the program to respond to user-driven
  events

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Moving to the Web

• Every event class has one or more associated
  listener interfaces

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Code the program

• Using TextPad, start a new program
• Save the program as BodyMassApplet.java
• Enter the first 13 lines
• /
• Chapter 3 Body Mass Index Calculator
• Programmer your name here
• Date todays date
• Filename BodyMassApplet.java
• Purpose This project calculates the body mass
  index based
• on a person's height and weight.
• /
• import java.applet.
• import java.awt.
• import java.awt.event.

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Code the program

• Lines 14 - 19
• public class BodyMassApplet extends Applet
  implements ActionListener
• // declare variables
• Image logo // declare an image object
• int intInches, intPounds
• double dblKilograms, dblMeters, dblIndex

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Adding Interface Components to an Applet

• Label
• Displays text in the applet window
• TextField
• Displays a text box for users to enter text
• Buttons
• Displays a command button for users to click

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Code the program

• Lines 21 - 28
• //construct components
• Label companyLabel new Label("THE SUN FITNESS
  CENTER BODY MASS INDEX
  CALCULATOR")
• Label heightLabel new Label("Enter your height
  to the nearest inch")
• TextField heightField new TextField(10)
• Label weightField new Label("Enter you weight
  to the nearest pound")
• TextField weightField new TextField(10)
• Button calcButton new Button("Calculate")
• Label outputLabel new Label("Click the
  Calculate Button to see your body mass
  index.")

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The init() Method

• Initializes the window color and graphic
• Adds components to the applet window
• Registers the Buttons ActionListener
• Enter lines 30 42

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The actionPerformed() Method

• When a click event occurs, the ActionListeners
  actionPerformed() method is triggered
• Input values are retrieved with getText()
• Calculations are performed
• Output is sent to a label with setText()
• Enter lines 44 52
• public void actionPerformed(ActionEvent e)
• intInches Integer.parseInt(heightField.getText(
  ))
• intPounds Integer.parseInt(weightField.getText(
  ))
• dblMeters intInches / 39.36
• dblKilograms intPounds / 2.2
• dblIndex dblKilograms / Math.pow(dblMeters,
  2)
• outputLabel.setText("YOUR BODY MASS INDEX IS "
  Math.round(dblIndex)
  ".")

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The paint() Method

• Draws the initialized image in the applet window
• Lines 54 - 58

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Creating an HTML Host Document for an Interactive
Applet

• Save and Compile the applet
• Fix any errors and compile again
• In TextPad create a new document and save as
  BodyMassApplet.html
• ltHTMLgt
• ltAPPLET CODE "BodyMassApplet.class" WIDTH
  375 HEIGHT 300gt
• lt/APPLETgt
• lt/HTMLgt

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Run the java applet
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File Management

• Coding and compiling an application creates
  several files on your storage device
• File naming conventions and the operating
  systems capability of displaying icons can help
  the programmer maintain a logical order
• Three java files named after the program purpose
  and user interface type
• Three class files after compilation
• HTML host document
• Image file

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Chapter 3 Homework

• Short Answer, page 201
• 1 9, 11 - 15
• Only type the answers, not the questions
• All answers must be typed and printed out
• Debugging Assignment, page 205
• Programming Assignments
• Pages 208 - 215
• 3, 4, 6, 10