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Introduction to Objects and Input/Output

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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);

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 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

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