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Chapter 3

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.

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Chapter 3

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  1. Chapter 3 Manipulating Data Using Methods

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. (a) console application in a command prompt window (b) console application using dialogboxes (c) applet

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

  10. 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

  11. 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

  12. Coding the Program

  13. 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

  14. Declaring Variables

  15. 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

  16. 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

  17. 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

  18. 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);

  19. Assignment Statements • General syntax: location = value

  20. Arithmetic Operators

  21. 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

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

  23. 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

  24. 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 (==, !=)

  25. 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

  26. 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

  27. The Math Class

  28. Using Variables in Output

  29. 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

  30. 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

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

  32. Swing Dialog Boxes

  33. 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

  34. 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

  35. 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

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

  37. Moving to the Web

  38. 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

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

  40. 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()

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

  42. 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

  43. 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

  44. 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

  45. 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

  46. 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

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