chapter 3 using classes and objects. © 2004 pearson addison-wesley. all rights reserved3-2 using...

Chapter 3

Using Classes and Objects

© 2004 Pearson Addison-Wesley. All rights reserved 3-2

Using Classes and Objects

• We can create more interesting programs using predefined classes and related objects

• Chapter 3 focuses on:

object creation and object references the String class and its methods the Java standard class library the Math classes formatting output enumerated types wrapper classes


Outline

Creating Objects

The GregorianCalendar Class

The String Class

Wrapper Classes

Next Class:

Packages

Formatting Output

Enumerated Types


Creating Objects

• A variable holds either a primitive type or a reference to an object

• A class name can be used as a type to declare an object reference variable

String title;

• No object is created with this declaration

• An object reference variable holds the address of an object

• The object itself must be created separately


Creating Objects

• Generally, we use the new operator to create an object

title = new String ("Java Software Solutions");

This calls the String constructor, which isa special method that sets up the object

• Creating an object is called instantiation

• An object is an instance of a particular class


Invoking Methods

• We've seen that once an object has been instantiated, we can use the dot operator to invoke its methods

count = title.length()

• A method may return a value, which can be used in an assignment or expression

• A method invocation can be thought of as asking an object to perform a service


References

• Note that a primitive variable contains the value itself, but an object variable contains the address of the object

• An object reference can be thought of as a pointer to the location of the object

• Rather than dealing with arbitrary addresses, we often depict a reference graphically

"Steve Jobs"name1

num1 38


Assignment Revisited

• The act of assignment takes a copy of a value and stores it in a variable

• For primitive types:

num1 38

num2 96Before:

num2 = num1;

num1 38

num2 38After:


Reference Assignment

• For object references, assignment copies the address:

name2 = name1;

name1

name2Before:

"Steve Jobs"

"Steve Wozniak"

name1

name2After:

"Steve Jobs"


Aliases

• Two or more references that refer to the same object are called aliases of each other

• That creates an interesting situation: one object can be accessed using multiple reference variables

• Aliases can be useful, but should be managed carefully

• Changing an object through one reference changes it for all of its aliases, because there is really only one object


Garbage Collection

• When an object no longer has any valid references to it, it can no longer be accessed by the program

• The object is useless, and therefore is called garbage

• Java performs automatic garbage collection periodically, returning an object's memory to the system for future use

• In other languages, the programmer is responsible for performing garbage collection


Outline

Creating Objects


The String Class

Wrapper Classes


Example of a Mutable Class

• GregorianCalendar Defined in java.util Contains several constructors

• GregorianCalendar() – Constructs a default GregorianCalendar using the

current time in the default time zone with the default locale.

• GregorianCalendar(int year, int month, int date)

– Constructs a GregorianCalendar with the given date set in the default time zone with the default locale.

• GregorianCalendar(int year, int month, int date, int hour, int minute)

– Constructs a GregorianCalendar with the given date and time set for the default time zone with the default locale.

http://java.sun.com/j2se/1.3/docs/api/java/util/GregorianCalendar.html#GregorianCalendar%28%29

http://java.sun.com/j2se/1.3/docs/api/java/util/GregorianCalendar.html#GregorianCalendar%28int,%20int,%20int%29

http://java.sun.com/j2se/1.3/docs/api/java/util/GregorianCalendar.html#GregorianCalendar%28int,%20int,%20int,%20int,%20int%29


More on the GregorianCalendar

• Contains 16 methods boolean isLeapYear(int year)

• Determines if the given year is a leap year. void add(int field, int amount).

• Inherits 31 method from the Calendar class int get(int field)

• What is a valid field? Constants defined in Calendar class Examples: YEAR, MONTH, DATE, DAY_OF_MONTH,

DAY_OF_YEAR, DAY OF WEEK, WEEK_OF_YEAR, WEEK_OF_MONTH

• Using the get method int year = today.get(Calendar.YEAR);

http://java.sun.com/j2se/1.3/docs/api/java/util/GregorianCalendar.html#isLeapYear%28int%29

http://java.sun.com/j2se/1.3/docs/api/java/util/GregorianCalendar.html#add%28int,%20int%29

http://java.sun.com/j2se/1.3/docs/api/java/util/GregorianCalendar.html#add%28int,%20int%29

http://java.sun.com/j2se/1.3/docs/api/java/util/Calendar.html#DATE


Date Example// Demonstrates the GregorianCalendar, a mutable class

import java.util.*; // for GregorianCalendarimport java.text.*; // for DateFormat Class

public class DateExample{

// Creates a GregorianCalendar object for the quizDay, outputs // information about the day, and modifies the day using class // methods

public static void main(String [] args) {

// Creates the quizday object to store the day of the quiz

int month, day; // store date attributes

GregorianCalendar quizDay = new GregorianCalendar(2005, Calendar.OCTOBER, 1);


Date Example // Get integer value for the characteristics day = System.out.println("Day: " + day); month = System.out.println("Month: " + month);

// Make the quiz 4 days earlier

// Get the new date day = System.out.println("Day 50 days after the quiz: " + day); month = System.out.println("Month 50 days after the quiz: " + month ); // Format the output of the date using DateFormat DateFormat fmt = DateFormat.getInstance(); System.out.println("Formatted Date: " + fmt.format(quizDay.getTime())); }}

quizDay.get(Calendar.DAY_OF_MONTH);

quizDay.get(Calendar.MONTH);

quizDay.add(Calendar.DAY_OF_MONTH, -4);

quizDay.get(Calendar.DAY_OF_MONTH);

quizDay.get(Calendar.MONTH);


Primitive Types vs. Objects with Assignment Statementsint x = 5;

int y = x;

x++;

GregorianCalendar d1 = new GregorianCalendar(2005, Caldendar.October, 1);

GregorianCalendar d2 = d1;

d1.add(Calendar.YEAR, 5);


Fix using clone method

GregorianCalendar d1 = new GregorianCalendar(2005, Caldendar.October, 1);

GregorianCalendar d2 = (GregorianCalendar) d1.clone();

d1.add(Calendar.YEAR, 5);


Outline

Creating Objects


The String Class

Wrapper Classes


The String Class

• Because strings are so common, we don't have to use the new operator to create a String object

title = "Java Software Solutions";

• This is special syntax that works only for strings

• Each string literal (enclosed in double quotes) represents a String object


String Methods

• Once a String object has been created, neither its value nor its length can be changed

• Thus we say that an object of the String class is immutable

• However, several methods of the String class return new String objects that are modified versions of the original

• See the list of String methods on page 119 and in Appendix M or the Java online documentation: http://java.sun.com/j2se/1.5.0/docs/api/index.html

http://java.sun.com/j2se/1.5.0/docs/api/index.html


String Indexes

• It is occasionally helpful to refer to a particular character within a string

• This can be done by specifying the character's numeric index

• The indexes begin at zero in each string

• In the string "Hello", the character 'H' is at index 0 and the 'o' is at index 4


//*************************************************************// StringMutation.java Author: Lewis/Loftus//// Demonstrates the use of the String class and its methods.//*************************************************************

public class StringMutation{ //---------------------------------------------------------- // Prints a string and various mutations of it. //---------------------------------------------------------- public static void main (String[] args) { String phrase = "Change is inevitable"; String mutation1, mutation2, mutation3, mutation4;

System.out.println ("Original string: \"" + phrase + "\""); System.out.println ("Length of string: " + phrase.length());

mutation1 = phrase.concat(", except from vending machines."); mutation2 = mutation1.toUpperCase(); mutation3 = mutation2.replace ('E', 'X'); mutation4 = mutation3.substring (3, 30);


// Print each mutated string System.out.println ("Mutation #1: " + mutation1); System.out.println ("Mutation #2: " + mutation2); System.out.println ("Mutation #3: " + mutation3); System.out.println ("Mutation #4: " + mutation4);

System.out.println ("Mutated length: " + mutation4.length()); }}

Original string: "Change is inevitable"Length of string: 20Mutation #1: Change is inevitable, except from vending machines.Mutation #2: CHANGE IS INEVITABLE, EXCEPT FROM VENDING MACHINES.Mutation #3: CHANGX IS INXVITABLX, XXCXPT FROM VXNDING MACHINXS.Mutation #4: NGX IS INXVITABLX, XXCXPT FMutated length: 27

Output:


String Methods (We are interested in) • str.length() – returns int • str.charAt(int) – returns char• str.substring(int) or str.substring(int, int) – returns

String • str.indexOf(String) or str.indexOf(String, int) – returns

int

• str.lastIndexOf(String)or str.lastIndexOf(String, int) – returns int

• str.toUpperCase() – returns String• str.toLowerCase() – returns String • str.trim() – returns String


Storing and Displaying Method Results

• Use assignment statements to store the result of a method call int posBlank = saying.indexOf(“ “);

•

String firstWord = saying.indexOf(0, posBlank);

•

• Including the method call in the output statement System.out.println (“The second blank is at position “ + saying.indexOf(“ “, 4));

String saying = “All roads lead to Rome”;


Tricks of the trade

• Use the results of one method as the input to another

char last = str.charAt(str.length()-1);

• Concatenating methods together int endFirstWord = str.trim().indexOf(“ “);


Outline

Creating Objects


The String Class

Wrapper Classes


Wrapper Classes

• The java.lang package contains wrapper classes that correspond to each primitive type:

Primitive Type Wrapper Class

byte Byte

short Short

int Integer

long Long

float Float

double Double

char Character

boolean Boolean

void Void


Wrapper Classes

• The following declaration creates an Integer object which represents the integer 40 as an object

Integer age = new Integer(40);

• An object of a wrapper class can be used in any situation where a primitive value will not suffice

• For example, some objects serve as containers of other objects

• Primitive values could not be stored in such containers, but wrapper objects could be


Wrapper Classes

• Wrapper classes also contain static methods that help manage the associated type

• For example, the Integer class contains a method to convert an integer stored in a String to an int value:

num = Integer.parseInt(str);

• The wrapper classes often contain useful constants as well

• For example, the Integer class contains MIN_VALUE and MAX_VALUE which hold the smallest and largest int values


Autoboxing

• Autoboxing is the automatic conversion of a primitive value to a corresponding wrapper object:

Integer obj;

int num = 42;

obj = int;

• The assignment creates the appropriate Integer object

• The reverse conversion (called unboxing) also occurs automatically as needed


Real Estate Commission Calculator

• Write a program that calculates the dollar amount of the commission based on user input Input: address, price, and commission percentage

• price: leading $ and no commas to group digits

• commission: percentage with a trailing %

Example: “5423 Seabrook Ave. $250000 5.5%” Display the result in a popup window

chapter 3 using classes and objects. © 2004 pearson addison-wesley. all rights reserved3-2 using...

Documents