Manipulating Data Using Methods

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Chapter 3

• Manipulating Data Using Methods
  

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Objectives

• Identify, declare, and use primitive data types
• Use the System class to create data streams
• Instantiate the BufferedReader class in code
• Use the readLine() method to handle user input

3
Objectives

• Convert strings to numbers using the parse()
  method
• Use assignment statements to store data with
  proper identifiers
• Use operators and parentheses correctly in
  numeric and conditional expressions
• Round an answer using the round() method of the
  Math class

4
Objectives

• Use Swing components to build the GUI for a Swing
  program
• Use the exit() method to close a Swing program
• Implement an ActionListener to handle events
• Add interface components to an applet

5
Objectives

• Use the init() and paint() methods to load the
  applet interface
• Use the actionPerformed() method
• Run and test an interactive applet
• Manage Java source code files and Java class files

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Introduction

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

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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
• Mobile devices use the command prompt
• Notebooks use 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
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Problem Analysis

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

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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

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Coding 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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Coding the Program

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Storing Data

• Java is strongly typed
• 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

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User Input - Streams

• 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 cont.

• 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

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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
• Example height dataIn.readLine()
• inches Integer.parseInt(height
  )

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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
• Addition and/or subtraction

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Conditional Expressions

• Conditional expression evaluate to either true or
  false
• 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
  (, ) 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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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
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Testing 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
• Print the source code and screen images for
  documentation

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Using Swing Components

• Save the previous version of the Body Mass Index
  Calculator with a new filename
• Import the javax.swing.JOptionPane class
• 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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Swing Dialog Boxes

• Dialog boxes are created with the JOptionPane
  show methods
• The showInputDialog() and showConfirmDialog
  return a String containing the user input

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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

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Testing the Swing Program

• 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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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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Adding Interface Components

• 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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The init() Method

• Initializes the window color and graphic
• Adds components to the applet window
• Registers the Buttons ActionListener

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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()

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The paint() Method

• Draws the initialized image in the applet window

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Creating an HTML Host Document for an Interactive
Applet

• Compile the applet
• Write an HTML Host Document to execute the applet
• Use the tag to specify the bytecode
  file, and width and height of the window
• Use the same sample data to test the applet
• Document the source code

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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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Summary

• Variable declarations
• Data types
• Assignment statements
• Input streams
• InputStreamReader
• BufferedReader
• readline()
• Arithmetic operators
• Operator precedence
• Math class methods

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Summary

• Output using System.out
• Concatenation of mixed data types
• Java Swing components
• JOptionPane
• AWT Components and method constructors
• Labels, TextFields, and Buttons
• Events
• ActionEvent, ActionListener, actionPerformed()

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Chapter 3 Complete