Web Programming with Java Java - Object-Oriented Programming doc

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Web Programming with Java
Java Object-Oriented Programming
Huynh Huu Viet
Email:
Department of Information Systems
2008 © Department of Information Systems - University of Information Technology
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Outline
Objects and Classes
OOP Concepts
 Encapsulation
 Aggregation/Composition
 Inheritance & Polymorphism
Some useful Java classes
Exception Handling, I/O
Readings & Exercise
Discussion
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Objects and Classes
Basic OO concepts
 Object, class, state, behavior, message
Creating Java classes, objects,
properties and methods
Constructors
Package
Reference data type
Variable scope
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Basic OO concepts : Object
 Object: any thing, which can be described by
 State: property, or attribute
 Behavior: function/action, or method: ask the object to
perform a task
 Example
 A student, a desk, a circle, a button, and even a loan
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Basic OO concepts : Class
 Class: a set of objects with common attributes and behaviors
 An object is an instance of a class
 Class is a template or blueprint that defines what an object's data
and methods will be
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Basic OO concepts : Message
Objects interact (communicate) by
passing messages
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Object-Oriented Programming
 Class/object is the focus of OOP
 At design time
 Class is the basic programming unit; a program
consists of one or more classes
 Programming focuses on
• Defining classes and their properties
• Manipulating classes/objects properties and behaviors
• Handling objects interactions

 At execution time
 Programs are executed through predefined
manipulations and interactions
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Constructor
 Constructor is a special method used for
object creation
 Circle myCircle = new Circle();
 Default constructor
 A constructor without any parameter
 If a programmer doesn’t define any constructor
for a class, JRE will implicitly create a default
constructor
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Using Constructor
 Defining constructors
 Using constructor for default states and
behaviors of an object when it is initially
created
Three differences with methods:
 Must have the same name as the class itself.
 Do not have a return type - not even void.
 Invoked using the new operator when an
object is created.
ClassName() //no “void” or any other data
type
{
…

}
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Constructor Overloading
Like methods, constructors can be
overloaded
This offers greater flexibility and
convenience of creating objects
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Using Objects (1)
 Object initialization
 Using Member Variables
 Member variable declaration
• Any where in a class, outside all methods
 Used within the class
• Can be used/referenced anywhere directly
 Used with objects
• Using the "." operator and preceded with object name
• Ex: myCircle.radius
ClassName objectName;
objectName = new ClassName();
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Using Objects: using methods
Defining methods
 With a return value (type)
 Without a return value (type) – void
Calling Methods
 Within the class: Called by method name

directly
 Used with objects or outside the class
• Using the "." operator and preceded with
object name
• Examples:
–myCircle.getArea()
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The “main” Method
 Following OOP guidelines, the use of the
“main” method should be deemphasized
 The “main” method is merely a starting point of the
application
 There should not be too many statements in the main
method
 Using objects and methods effectively
 Typically (and ideally), you only do these
things in the main method
 Creating objects
 Calling class/object methods
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Package
Packages are used to group classes
Four reasons for using packages
 To locate classes
 To avoid naming conflicts
 To distribute software conveniently
 To protect classes
Packages may be within packages

(hierarchical )
One-to-one mapping of the package
name and the file system directory
structure
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Java Packages
 Java hierarchically organizes classes into
packages
 java.lang
 java.text
 java.util
 …
 Classes need to be referred using its complete
name (package + class name): for example,
java.util.Calendar
 Packages can be “imported” to avoid writing package
names every time you use a class (except java.lang)
import java.util.*;
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Using Package
Organizing your classes into packages
 A class can only be in one package
 No duplicate class definition in the same
package
 Put the package statement at the beginning
Examples:
package vn.edu.uit.java4web;
package vn.edu.uit.assignment;

package vn.edu.uit.lecture.web;
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Default Package
A class without any package defined
is in a “default package”
The default package is NOT the root
package!
 Classes in the default package cannot be
referenced outside the default package
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Reference Data Type
Reference data type stores memory
address (reference) as its value
Objects are reference data type
Variable index
object1: 0xf1
object2: 0xf2
object3 …
Data values
0xf1: object1’s content
0xf2: object2’s content
…
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Object Assignment
Objects are assigned by
reference
Object object1=new Object();

object2=object1;
Variable index
Object object1;
Object object2;
Object object3;
Object content
0xf1: new object1()
0xf2: new object2()
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Variable Scope
 Member variable
 Something like a global
variable within the class
 Local variable
 Method parameter
 Method level variable
 Block level variable
 A variable is effective at
its declaration level and
all sub-levels
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Variable Overriding
 Variables cannot be declared twice at the same
level and its sub-levels
 except class member variables (overridden by local
variables)
 Use "this" keyword to refer to the class/object
itself to access (the overridden) member variables

this.title
this.couseId
this.getCourseInfo() //”this” can be used with method
 It is good to use “this” explicitly for member
variables
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Outline
Objects and Classes
OOP Concepts
 Encapsulation
 Aggregation/Composition
 Inheritance & Polymorphism
Some useful Java classes
Exception Handling, I/O
Readings & Exercise
Discussion
2008 © Department of Information Systems - University of Information Technology
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Encapsulation
A concept to hide information or other
details from the outside programs
Why? Should classes and methods
properties be available to other
programs (classes)?
 Privacy …
 Cohesion, modularity, integrity
 Hides complexity, consistent and clear
interaction
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Access Modifiers
 Encapsulation is implemented through the use of access
modifiers/specifiers
 private - completely encapsulated within the class
private int length;
 default (no modifier) - within the same package
int length;
 public - completely open
public int length;
 (protected)
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Access Modifiers for Classes
If a class is defined public, then it can
be referenced anywhere (world
scope)
public class Book { … }
Otherwise, if it is defined “default”,
then it can be referenced only by
those in the same package
class Book { … }
Note: any .java source file can have
only one public class