1 / 33

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.

bly
Download Presentation

Chapter 3

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related