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Chapter 6
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
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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
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Object and Reference Variables
• Primitive variables: directly store data into their
memory space
• Reference variables: store the address of the object
containing the data
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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
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The Operator new
• Statement:
Integer num;
num = new
Integer(78);
• Result:
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Garbage Collection
• Change value of num:
num = new Integer(50);
• Old memory space reclaimed
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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
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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)
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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
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The class String
• String variables are reference variables
• Given String name;
– Equivalent Statements:
name = new String("Lisa Johnson");
name = “Lisa Johnson”;
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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
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Commonly Used String Methods
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String(String str)
char charAt(int index)
int indexOf(char ch)
int indexOf(String str, int pos)
int compareTo(String str)
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Commonly Used String Methods
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String concat(String str)
boolean equals(String str)
int length()
String replace(char ToBeReplaced, char
ReplacedWith)
• String toLowerCase()
• String toUpperCase()
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Input/Output
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Input Data
Format Input
Output Results
String Tokenization
Format Output
Read From and Write to Files
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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);
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Parameters for the Method showMessageDialog
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JOptionPane Options for the Parameter messageType
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JOptionPane Example
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Tokenizing a String
• class StringTokenizer
– Contained in package java.util
– Tokens usually delimited by whitespace
characters (space, tab, newline, etc)
– Contains methods:
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public StringTokenizer(String str, String delimits)
public int countTokens()
public boolean hasMoreTokens()
public String nextToken(String delimits)
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Formatting the Output of Decimal Numbers
• Type float: defaults to
6 decimal places
• Type double: defaults
to 15 decimal places
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class Decimal Format
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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
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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
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File Input/Output
• Java file I/O process:
1.Import classes from package java.io
2.Declare and associate appropriate variables
with I/O sources
3.Use appropriate methods with declared
variables
4.Close output file
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Inputting (Reading) Data from a File
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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"));
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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"));
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Skeleton of I/O Program
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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:
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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
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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
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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
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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
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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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