Chapter 03

Chapter 3 Manipulating Data Using Methods

Chapter 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

Chapter 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

Chapter 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

Chapter 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

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

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

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

Problem Analysis
Convert user input to metric measurements
Calculate the BMI
Display the result

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

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

Coding the Program

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

Declaring Variables

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

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

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

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
Example: height = dataIn.readLine();
inches = Integer.parseInt(height);

Assignment Statements
General syntax: location = value

Arithmetic Operators

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

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

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

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 (==, !=)

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

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

The Math Class

Using Variables in Output

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
Print the source code and screen images for documentation

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

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

Swing Dialog Boxes

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

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

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

Moving to the Web
Every event class has one or more associated listener interfaces

Moving to the Web

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

The init() Method
Initializes the window color and graphic
Adds components to the applet window
Registers the Button’s ActionListener

The actionPerformed() Method
When a click event occurs, the ActionListener’s actionPerformed() method is triggered
Input values are retrieved with getText()
Calculations are performed
Output is sent to a label with setText()

The paint() Method
Draws the initialized image in the applet window

Creating an HTML Host Document for an Interactive Applet
Compile the applet
Write an HTML Host Document to execute the applet
Use the <APPLET> 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

File Management
Coding and compiling an application creates several files on your storage device
File naming conventions and the operating system’s 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

Chapter Summary
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

Chapter Summary
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

Chapter Summary
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

Chapter Summary
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

Chapter 3 Complete

Chapter 03

by Chaffey College

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