Transcript Document

Object-Oriented Programming
Concepts
Good Questions
What are Objects?
 What are Classes?
 What are Messages?
 What is Inheritance?

What Are Objects?

Software objects model read-world objects or
abstract concepts
• dog, bicycle, queue

Real-world objects have states and behaviors
• Dogs' states: name, color, breed, hungry
• Dogs' behaviors: barking fetching

How do Software objects implement real-world
objects?
• Use variables to implement states
• Use methods to implement behaviors

An object is a software bundle of variables and
related methods
Visual Representation of A
Software Object
Variable
Method
Software Bicycle
 Instance Variables and Instance Method
Encapsulation
The objects' variables make up the center of the
object.
 Methods surround and hide the object's center
from other objects.
 Benefit of encapsulation

• Modularity
• Information hiding

For implementation or efficiency reasons, an object
may wish to expose some of its variables or hide
some of its methods.
What Are Classes?
A class is a blueprint or prototype defining the
variables and methods common to all objects of a
certain kind.
 An object is an instance of a certain class.
 After you have created a class, you must create an
instance of it before you can use.
 Class variables and class methods
 The benefit of Classes: Reusability

Visual Representation of A
Software Class
What Are Messages?
 Software objects interact and communicate with each other
by sending messages to each other.
More on Messages

Components of a Message
• The object to whom the message is addressed (Your
Bicycle)
• The name of the method to perform(changeGears)
• Any parameters needed by the method (low gear)

The Benefits of Messages
• Messages passing supports all possible interactions
between objects (aside from direct variable access)
• Objects don’t need to be in the same process or even on
the same machine to send and receive messages.
What is Inheritance?

Inheritance allows classes to be defined in terms of
other classes
• superclass and subclass
Each subclass inherits variables and methods from
its superclass.
 Subclasses can add variables and methods to the
ones they inherit from the superclass.
 Subclasses can also override inherited method and
provide specialized implementations for those
methods.
 Inheritance or class hierarchy

Benefits of Inheritance
Programmers can reuse the code in the superclass
many times.
 Programmers can implement superclasses called
abstract classes

• Abstract class defines “generic” behaviors
• Define and may partially implement the behavior but
much of the class is undefined and unimplemented.
Other programmers fill in the details with specialized
subclasses.
Message Example
Inheritance Example
Object-Oriented Development
in Java
Agenda
Java Class and Object Declaration
 The Life Cycle of an Object
 Controlling Access to Members of a Class

Java Classes and Objects
Class: templates for specifying the
state and behavior of an object at
runtime
Object: instances of a class
The concepts of class/object provide a
mechanism for encapsulation
Basic Structures of a Class
Class Declaration
 Variable

• Instance Variable
• Class Variable
Constructor
 Method

• Instance Method
• Class Method
Cleanup
 Rectangle2.java

Point and Rectangle
Rectangle2.java
The Class Declaration
Constructor

A method in a class that initialize an instance of an
object before it's used.
• The same name as the class and have no return type

Multiple Constructors: the name name but a
different number of arguments or different typed
arguments
• Method Overloading
Java Provides default constructors.
 The special variable, this, can be used inside a
method to refer to the object instance.
 Rectangle.java

Member Variables
Declaration
Instance and Local Variable


Local variable is defined
inside a block of code or a
method.
Example (FirstClass.java)
• Instance Variable:
firstVariable
• Local Variable: half

public int getHalf() {
int half; // local variable
half = firstVariable / 2;
return half;
}
Methods
Method Declaration
Return a Value from a
Method
Use return operator in the method to return the
value.
 Methods can return a primitive type or a reference
type.
 The class of the returned object must be either a
subclass of or the exact class of the return type.

Method Overload
Signature of a Method: return value, name,
parameter list
 Method Overloading: Use the same method name
with different arguments to group together related
methods.
 Constructors can also be overloaded.

• this(parameters) : Call another constructor within a
constructor

Example: FirstClass.java, SecondClass.java,
Rectangle.java
Passing Information into a
Method

Argument types
• primitive and reference data type: Yes
• method: No

Argument Names
• Can have the same name as one of the class's member
variable
 Use this to refer to the member variable
Primitive arguments are passed by value.
 Reference arguments are passed by reference.
 FirstClass.java

The Life Cycle of an Object
Creating Objects
Using Objects
Cleaning Up Unused Objects
Creating Objects

Rectangle r = new Rectangle(5,5,100,200);
• Declaration: Rectangle r
(Type name)
• Instantiation: new
 Allocate memory for the object
 Initialize instance variables
 Call a constructor
• Initialization by Calling a Constructor
 Rectangle(5,5,100,200)
Using Objects

Manipulate or inspect its variables
 objectReference.variable
 r.x = 50
 r.y = 80

Call its methods
 objectReference.methodName(argumentList)
 r.move(20,30)

Java provides an access control mechanism
whereby classes can restrict or allow access to its
variables and methods.
Clean Up

When all references to an object are dropped, the
object is no longer required, and become eligible
for garbage collection.
• Call finalize() to release system resources such as open
files or open sockets before the object is collected.
• Release references to other objects
• protected void finalize() throws Throwable
 Rectangle2.java
Controlling Access to
Members of a Class
Specifier
Class
Subclass
Package
private
O
protected
O
O
O
public
O
O
O
package
O
O
world
O
Private
class Alpha {
class Beta {
private int iamprivate;
void accessMethod() {
private void privateMethod()
Alpha a = new Alpha();
{
a.iamprivate = 10;
System.out.println
a.privateMethod();
("privateMethod");
}
}
}
}
How About one Alpha object access the private member of another
Alpha object?
Protected
package Greek;
class Alpha {
protected int iamprotected;
protected void
protectedMethod() {
System.out.println
("protectedMethod");
}
}
package Greek;
class Gamma {
void accessMethod() {
Alpha a = new Alpha();
a.iamprotected = 10;
a.protectedMethod();
}
}
Protected (II)
import Greek.*;
package Latin;
class Delta extends Alpha {
void accessMethod(Alpha a, Delta d) {
a.iamprotected = 10;
d.iamprotected = 10;
a.protectedMethod();
d.protectedMethod();
}
}
Public
package Greek;
public class Alpha {
public int iampublic;
public void publicMethod()
{
System.out.println("publicM
ethod");
}
}
import Greek.*;
package Roman;
class Beta {
void accessMethod() {
Alpha a = new Alpha();
a.iampublic = 10;
a.publicMethod();
}
}
Package
package Greek;
class Alpha {
int iampackage;
void packageMethod() {
System.out.println("packag
eMethod");
}
}
package Greek;
class Beta {
void accessMethod() {
Alpha a = new Alpha();
a.iampackage = 10;
a.packageMethod();
}
}
Inhereitance in Java
A mechanism you can use to create a
new class by extending the definition
of another class
Increase the reusability of codes
Single Inheritance
Simple Class Hierarchy
Object
Vehicle
Car
Van
Building
Bike
House
Office
Truck
Superclass and Subclass
More About Inheritance
Use the extends keyword to create a subclass.
 Method Override

• Use method overriding when you need a subclass to
replace a method of its superclass.
• Define a new method that replaces the superclass
method that has the same signature.

Calling Superclass Methods
• super.<method>(parameters)
• super(parameters) (Calling superclass constructors)

Example: GraphicsProgram.java