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Chapter 3

Chapter 3. Introduction to Objects and Input/Output. Chapter Objectives. Learn about objects and reference variables Explore how to use predefined methods in a program Become familiar with the class String Learn how to use input and output dialog boxes in a program. Chapter Objectives.

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Chapter 3

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  1. Chapter 3 Introduction to Objects and Input/Output

  2. Chapter Objectives • Learn about objects and reference variables • Explore how to use predefined methods in a program • Become familiar with the class String • Learn how to use input and output dialog boxes in a program

  3. Chapter Objectives • Learn how to tokenize the input stream • Explore how to format the output of decimal numbers with the class DecimalFormat • Become familiar with file input and output

  4. Object and Reference Variables • Primitive variables: directly store data into their memory space • Reference variables: store the address of the object containing the data

  5. Object and Reference Variables • Declare a reference variable of a class type • Use the operator new to: • Allocate memory space for data • Instantiate an object of that class type • Store the address of the object in a reference variable

  6. The Operator new • Statement: Integer num; num = new Integer(78); • Result:

  7. Garbage Collection • Change value of num: num = new Integer(50); • Old memory space reclaimed

  8. Using Predefined Classes and Methods in a Program • Many predefined packages, classes, and methods in Java • Library: Collection of packages • Package: Contains several classes • Class: Contains several methods • Method: Set of instructions

  9. Using Predefined Classes and Methods in a Program • To use a method you must know: • Name of class containing method (Math) • Name of package containing class (java.lang) • Name of method (pow), its parameters (int a, int b), and function (a^b)

  10. Using Predefined Classes and Methods in a Program • Example method call: import java.lang; //imports package Math.pow(2,3); //calls power method in //class Math • (Dot) . Operator: used to access the method in the class

  11. The class String • String variables are reference variables • Given String name; • Equivalent Statements: name = new String("Lisa Johnson"); name = “Lisa Johnson”;

  12. The class String • The String object is an instance of class string • The value “Lisa Johnson” is instantiated • The address of the value is stored in name • The new operator is unnecessary when instantiating Java strings • String methods are called using the dot operator

  13. Commonly Used String Methods • String(String str) • char charAt(int index) • int indexOf(char ch) • int indexOf(String str, int pos) • int compareTo(String str)

  14. Commonly Used String Methods • String concat(String str) • boolean equals(String str) • int length() • String replace(char ToBeReplaced, char ReplacedWith) • String toLowerCase() • String toUpperCase()

  15. Input/Output • Input Data • Format Input • Output Results • String Tokenization • Format Output • Read From and Write to Files

  16. Using Dialog Boxes for Input/Output • Use a graphical user interface (GUI) • class JOptionPane • Contained in package javax.swing • Contains methods: showInputDialog and showMessageDialog • Syntax: str = JOptionPane.showInputDialog(strExpression) • Program must end with System.exit(0);

  17. Parameters for the Method showMessageDialog

  18. JOptionPane Options for the Parameter messageType

  19. JOptionPane Example

  20. Tokenizing a String • class StringTokenizer • Contained in package java.util • Tokens usually delimited by whitespace characters (space, tab, newline, etc) • Contains methods: • public StringTokenizer(String str, String delimits) • public int countTokens() • public boolean hasMoreTokens() • public String nextToken(String delimits)

  21. Formatting the Output of Decimal Numbers • Type float: defaults to 6 decimal places • Type double: defaults to 15 decimal places

  22. class Decimal Format • Import package java.text • Create DecimalFormat object and initialize • Use method format • Example: DecimalFormat twoDecimal = new DecimalFormat("0.00"); twoDecimal.format(56.379); • Result: 56.38

  23. File Input/Output • File: area in secondary storage used to hold information • class FileReader is used to input data from a file • class FileWriter and class PrintWritersend output to files

  24. File Input/Output • Java file I/O process: • Import classes from package java.io • Declare and associate appropriate variables with I/O sources • Use appropriate methods with declared variables • Close output file

  25. Inputting (Reading) Data from a File • Use class FileReader • Specify file name and location • Create BufferedReader Object to read entire line • Example: BufferedReader inFile = new BufferedReader(new FileReader("a:\\prog.dat"));

  26. Storing (Writing) Output to a File • Use class FileWriter • Specify file name and location • Utilize methods print, println, and flush to output data • Example: PrintWriter outFile = newPrintWriter(new FileWriter("a:\\prog.out"));

  27. Skeleton of I/O Program

  28. Programming Example: Movie Ticket Sale and Donation to Charity • Input: movie name, adult ticket price, child ticket price, number of adult tickets sold, number of child tickets sold, percentage of gross amount to be donated to charity • Output:

  29. Programming Example: Movie Ticket Sale and Donation to Charity • Import appropriate packages • Get inputs from user using JOptionPane.showInputDialog • Parse and format inputs using DecimalFormat • Make appropriate calculations • Display Output using JOptionPane.showMessageDialog

  30. Programming Example: Student Grade • Input: file containing student’s first name, last name, five test scores • Output: file containing student’s first name, last name, five test scores, average of five test scores

  31. Programming Example:Student Grade • Import appropriate packages • Get input from file using BufferedReader and FileReader • Tokenize input from file using StringTokenizer • Format input and take average of test scores • Open and write to output file using PrintWriter and FileWriter • Close files

  32. Chapter Summary • Primitive type variables store data into their memory space • Reference variables store the address of the object containing the data • An object is an instance of a class • Operator new is used to instantiate an object • Garbage collection is reclaiming memory not being used

  33. Chapter Summary • To use a predefined method you must know its name and the class and package it belongs to • The . (dot) operator is used to access a certain method in a class • Methods of the class string are used to manipulate input and output data • Dialog boxes can be used to input data and output results • String tokenization can be used to format input strings from files into data • Data can be read from and written to files • Data can be formatted using class DecimalFormat

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