1. Introduction to Objects and Input/Output
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 PrintWriter send 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 = new PrintWriter(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