Download 4. Object-oriented programs

Chapter 4
How to write
object-oriented programs
CS170 ygao JAVA, C4
Slide 1
Objectives
 Describe the concept of encapsulation and explain its importance to
object-oriented programming.
 Describe a signature of a constructor or method, and explain what
overloading means.
 Code the instance variables, constructors, and methods of a class
that defines an object.
 Code a class that creates objects from a user-defined class and then
uses the methods of the objects to accomplish the required tasks.
 Code a class that contains static fields and methods, and call these
fields and methods from other classes.
 Add two or more classes to a package and make the classes in that
package available to other classes.
 Code javadoc comments for a class, and generate the documentation
for the class. Then, use your web browser to review that
documentation.
CS170 ygao JAVA, C4
Slide 2
Concept of Encapsulation
• What is encapsulation?
- data and functions/methods are packaged together in
the class normally with the same scope of the
accessibility in the memory
• Why encapsulation
- Objects no longer need to be dependent upon any
“outside” functions and data after the creation
- Provide ADT (Abstract Data Types)
- Easier in coding
- Promote standards in OOP
CS170 ygao JAVA, C4
Slide 3
Classes vs. Objects
• Class is a data structure that encapsulates data and
functions.
• Data in a class are called member data, class variables,
instance variables, or fields.
• Functions in a class are called member functions or
methods. Constructors are special methods.
• Objects are the instance of a class. Creation of the objects
and instantiation of the objects are all referring to memory
allocation to hold the data and methods of the objects. In
Java, all objects are created dynamically using new operator.
For example:
ClassName ObjName = new ClassName();
CS170 ygao JAVA, C4
Slide 4
A class diagram for the Book class
Book
-code: String
-title: String
-price : double
+setTitle(code: String)
+setPrice(code: String)
+getCode(): String
+getTitle(): String
+getPrice(): double
CS170 ygao JAVA, C4
class name
attributes /member
data
operations / methods
Slide 5
The relationship between a class and its objects
Book
-code: String
-title: String
-price: double
book1
-code = "WARP"
-title = "War and Peace"
-price = 14.95
book2
-code = "MBDK"
-title = "Moby Dick"
-price = 12.95
Note: Diagrams do not include methods, just for demo
CS170 ygao JAVA, C4
Slide 6
The Book class
public class Book{
private String code;
private String title;
private double price;
Instance variables/member
data
public Book(String bookCode){
code = bookCode;
setTitle(bookCode);
setPrice(bookCode);
}
Constructor
public void setTitle(String bookCode){
if (bookCode.equalsIgnoreCase("WARP"))
title = "War and Peace";
else if
(bookCode.equalsIgnoreCase("MBDK"))
title = "Moby Dick";
else
title = "Not Found";
}
CS170 ygao JAVA, C4
Method
Slide 7
The Book class (continued)
public void setPrice(String bookCode){
if (bookCode.equalsIgnoreCase("WARP"))
price = 14.95;
else if
(bookCode.equalsIgnoreCase("MBDK"))
price = 12.95;
else
price = 0.0;
}
public String getCode(){
return code;
}
More methods
public String getTitle(){
return title;
}
public double getPrice(){
return price;
}
}
CS170 ygao JAVA, C4
Slide 8
The syntax for declaring instance variables
public|private|protected primitiveType|ClassName variableName;
Examples
private double price;
private int quantity;
private String title;
private Book bookObject;
CS170 ygao JAVA, C4
Slide 9
Characteristics of Constructors
• A special method that has the same name of the
class without return type, not even void.
• Constructor will be automatically called whenever
an object is created.
• Like a method, constructor can be overloaded for the
flexibility of the object creation.
• A super class constructor can be called by a sub
class object as:
super();
CS170 ygao JAVA, C4
Slide 10
A constructor without any parameters
public Book(){
code = "";
title = "";
price = 0.0;
}
A constructor with one parameter
public Book(String bookCode){
code = bookCode;
setTitle(bookCode);
setPrice(bookCode);
}
A constructor with three parameters
public Book(String bookCode, String bookTitle,
double bookPrice){
code = bookCode;
title = bookTitle;
price = bookPrice;
}
CS170 ygao JAVA, C4
Slide 11
How to create an object in one statement
ClassName objectName = new ClassName(optionalArguments);
An example with no arguments
Book book = new Book();
An example with one literal argument
Book book = new Book("WARP");
An example with one variable argument
Book book = new Book(code);
An example with three arguments
Book book = new Book(code, title, price);
CS170 ygao JAVA, C4
Slide 12
set methods
• In OOP, particularly in Java, set() means to set object’s
member data
• set() usually has void return and takes argument(s) to set
the member data
• set() also checks the validity of the data before it sets
• Use meaningful name for a set() method
A set method with one parameter
public void setTitle(String bookCode){
if (bookCode.equalsIgnoreCase("WARP"))
title = "War and Peace";
else if (bookCode.equalsIgnoreCase("MBDK"))
title = "Moby Dick";
else
title = "Not Found";
}
CS170 ygao JAVA, C4
Slide 13
get methods
• In OOP, particularly in Java, get() means to return one of object’s
member data
• get() usually has a return type and takes no argument
• Use meaningful name for a get() method
A get method that returns a String object
public String getTitle(){
return title;
}
A get method that returns a double type
public double getPrice(){
return price;
}
CS170 ygao JAVA, C4
Slide 14
Complete Example Analysis
The BookOrder class
import java.text.*;
public class BookOrder{
private Book book; //Book class defined before in slide 7 & 8
private int quantity;
private double total;
public BookOrder(String bookCode, int orderQuantity){
book = new Book(bookCode);
quantity = orderQuantity;
setTotal();
}
public void setTotal(){
total = quantity * book.getPrice();
}
public Book getBook(){
return book;
}
CS170 ygao JAVA, C4
Slide 15
Complete Example Analysis (continue)
The BookOrder class (continued)
public int getQuantity(){
return quantity;
}
public double getTotal(){
return total;
}
public String toString(){
NumberFormat currency=NumberFormat.getCurrencyInstance();
String orderString = "Code: " + book.getCode() + "\n"
+ "Title: " + book.getTitle() + "\n"
+ "Price: "
+ currency.format(book.getPrice())
+ "\n" + "Quantity: " + quantity+ "\n"
+ "Total: " + currency.format(total)
+ "\n";
return orderString;
}
}
CS170 ygao JAVA, C4
Slide 16
Complete Example Analysis (continue)
The code for the driver program BookOrderApp class
import javax.swing.JOptionPane;
public class BookOrderApp{
public static void main(String[] args){
String choice = "";
try{
while (!(choice.equalsIgnoreCase("x"))){
String code = JOptionPane.showInputDialog(
"Enter a book code:");
String inputQuantity = JOptionPane.showInputDialog(
"Enter a quantity:");
int quantity = parseQuantity(inputQuantity);
BookOrder bookOrder = new BookOrder(code,quantity);
String message = bookOrder.toString() + "\n"
+ "Press Enter to continue or enter 'x' to exit:";
choice = JOptionPane.showInputDialog(null, message,
"Book Order", JOptionPane.PLAIN_MESSAGE);
}//end while
catch(NullPointerException e){
System.exit(0);
}
System.exit(0); }
CS170 ygao JAVA, C4
Slide 17
Complete Example Analysis (continue)
The code for the driver program (continued)
private static int parseQuantity(String quantityString){
int quantity = 0;
boolean tryAgain = true;
while(tryAgain){
try{
quantity = Integer.parseInt(quantityString);
while (quantity <= 0){
quantityString = JOptionPane.showInputDialog(
"Invalid order total. \n"
+ "Please enter a positive number: ");
quantity = Integer.parseInt(quantityString);
}
tryAgain = false;
}
catch(NumberFormatException e){
quantityString = JOptionPane.showInputDialog(
"Invalid quantity.\n"
+ "Please enter an integer.");
}
}
return quantity; }
}
CS170 ygao JAVA, C4
Slide 18
Static Fields
• What is a static field?
– a variable that is not associated with any instance of the
class, therefore it’s a class-wide field/variable/data
– a variable that can be used without creation of the object
– a private static field can be only use within the current
class
– can be constant (final), so it’s value cannot be modified;
must be initialized when declaring
• Why static fields?
– keep tracking class-wide information
– Convenient to use and save memory
– Be careful data encapsulation in using static fields
CS170 ygao JAVA, C4
Slide 19
How to declare static fields
private static int numberOfObjects = 0;
private static double majorityPercent = .51;
public static final int DAYS_IN_JANUARY = 31;
public static final float EARTH_MASS_IN_KG = 5.972e24F;
A class that contains static constants
public class DateConstants{
public static final int DAYS_IN_JANUARY = 31;
public static final int DAYS_IN_FEBRUARY = 27;
...
}
CS170 ygao JAVA, C4
Slide 20
A class that contains a static method that makes a
calculation
public class FinancialCalculations{
public static double calculateFutureValue(double monthlyPayment,
int months, double monthlyInterestRate){
int i = 1;
double futureValue = 0;
while (i <= months) {
futureValue = (futureValue + monthlyPayment) *
(1 + monthlyInterestRate);
i++;
}
return futureValue;
}
}
CS170 ygao JAVA, C4
Slide 21
A class that contains a static variable and a static
method
public class BookOrder{
private Book book;
private int quantity;
private double total;
private static int orderObjectCount = 0;
public BookOrder(String bookTitle, int bookQuantity){
book = new Book(bookTitle);
quantity = bookQuantity;
setTotal();
orderObjectCount++;
}
public static int getOrderObjectCount(){
return orderObjectCount;
}
...
CS170 ygao JAVA, C4
Slide 22
The syntax for calling a static field or method
className.fieldName
className.methodName(optionalArgumentList)
CS170 ygao JAVA, C4
Slide 23
How to call static fields
From the Java API
Math.PI
Math.E
JOptionPane.INFORMATION_MESSAGE
JOptionPane.PLAIN_MESSAGE
From a user-defined class
DateConstants.DAYS_IN_JANUARY
CS170 ygao JAVA, C4
Slide 24
How to call static methods
From Java classes
String inputQuantity = JOptionPane.showInputDialog(
"Enter a quantity:");
int quantity = Integer.parseInt(inputQuantity);
From user-defined classes
double futureValue = FinancialCalculations.calculateFutureValue(
monthlyPayment, months, monthlyInterestRate);
int orderCount = BookOrder.getOrderObjectCount();
CS170 ygao JAVA, C4
Slide 25
Static Initialization Block
• what is a static initialization block?
– a block of code in a class starting with keyword static
– the block will be automatically executed when any
method of the class is called first time
• why static initialization blocks?
– used when a static object/field cannot be initialized
by a single statement
– ensure the object/filed is ready/available to the rest of
the methods in the class starting at the point of any
method in the class that is called
CS170 ygao JAVA, C4
Slide 26
The syntax for coding a static initialization block
public class className{
any field declarations
static{
any initialization statements
}
the rest of the class
CS170 ygao JAVA, C4
Slide 27
Simple Example of Static Block
1.
public class TestStaticApp{
2.
private static int y;
3.
//private int x = 0; //illigal to delcare regular variable here in app
4.
5.
6.
7.
8.
static {
y = 99;
//declaring a static varaible
//static initialization block
//initializing the static variable
}
public static void main(String[] args){
System.out.println(“Output from main()");
9.
test(); // calling a method
10.
}
11.
12.
13.
public static void test() //must be static method in app
{
System.out.println("static field value in test():" + y);
14.
}
15. }
CS170 ygao JAVA, C4
Slide 28
A class that uses a static initialization block
public class BookDB{
private static Connection connection;
static{
try{
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String url = "jdbc:odbc:MurachBooks";
String user = "Admin";
String password = "";
connection = DriverManager.getConnection(url,
user, password);
}
catch (ClassNotFoundException e){
System.err.println("Driver not found.");
}
catch (SQLException e){
System.err.println("Error connecting to database.");
}
}
CS170 ygao JAVA, C4
Slide 29
A class that uses a static initialization block (continued)
// other static methods that use the Connection object
public static void close(){}
public static void addRecord(Book book){}
public static void updateRecord(Book book){}
public static void deleteRecord(String bookCode){}
public static Book findOnCode(String bookCode){}
}
CS170 ygao JAVA, C4
Slide 30
Packages
• what is a package?
– is used to organize the classes both API and
programmer defined classes
• why package?
– is able programs to import classes as API
– provides convention for unique class names to avoid
the name conflict
CS170 ygao JAVA, C4
Slide 31
Examples of package names
Internet domain
Package name
Subdirectory path
java.sun.com
com.sun.java.text
\com\sun\java\text
murach.com
com.murach.orders
\com\murach\orders
Other examples of package names
CS170.ygao
CS170 ygao JAVA, C4
ygao.CS170.classes
\ygao\CS170\classes
Slide 32
How to code a package statement
package com.murach.orders;
How to compile the package
CS170 ygao JAVA, C4
Slide 33
How to make a package available to any class in
your program
1. Start the command prompt and navigate to the root directory that
holds your classes.
2. Use the jar command to create a JAR file for the *.class file or files.
3. Move the JAR file to the c:\jdk1.3\jre\lib\ext directory.
CS170 ygao JAVA, C4
Slide 34
Syntax for creating a JAR file for a package
c:\anydirectory>jar cvf JARFilename.jar
classDirectory\*.class
Example
c:\java>jar cvf orders.jar com\murach\orders\*.class
Result
CS170 ygao JAVA, C4
Slide 35
How to make a package available to other classes
import packageName;
Example
import com.murach.orders.*;
CS170 ygao JAVA, C4
Slide 36
javadoc tool
• what is javadoc tool?
– allows you to create documentation for your classes
in the same way java API does
– is a command in JDK that does the documentation
• why javadoc?
– create professional-like documentation
– Other programmers can use to learn about the
fields, constructors, and methods in the classes
CS170 ygao JAVA, C4
Slide 37
Code from the Book class with javadoc comments
/*******************************************************
* The <code>Book</code> class represents a book and is
* used by the <code>BookOrder</code> class.
**********************************************************/
public class Book{
private String code;
private String title;
private double price;
/*******************************************************
* Constructs a Book object from a book code.
*******************************************************/
public Book(String bookCode){
code = bookCode;
setTitle(bookCode);
setPrice(bookCode);
}
CS170 ygao JAVA, C4
Slide 38
HTML tags for Java documentation
HTML tags
Description
<code></code>
Displays all text between these tags with a special font.
CS170 ygao JAVA, C4
Slide 39
How to generate the API documentation for a class
c:\programDirectory>
javadoc –d documentationDirectory ClassNameList
Examples
c:\java\com\murach\orders>
javadoc –d c:\java\docs Book.java
c:\java\com\murach\orders>
javadoc –d c:\java\docs Book.java BookOrder.java
c:\java\com\murach\orders>
javadoc –d c:\java\docs *.java
CS170 ygao JAVA, C4
Slide 40
The documentation that’s generated for the Book class
CS170 ygao JAVA, C4
Slide 41