1. Object-Oriented Programming Concepts

2. Good Questions What are Objects? What are Classes? What are Messages?
What is Inheritance?

3. 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

4. Visual Representation of A Software Object
Variable
Method

5. Software Bicycle
Instance Variables and Instance Method

6. 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.

7. 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

8. Visual Representation of A Software Class

9. What Are Messages?
Software objects interact and communicate with each other by sending messages to each other.

10. More on Messages Components of a Message The Benefits of Messages
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.

11. 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

12. 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.

13. Message Example

14. Inheritance Example

15. Object-Oriented Development in Java

16. Agenda Java Class and Object Declaration The Life Cycle of an Object
Controlling Access to Members of a Class

17. 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

18. Basic Structures of a Class
Class Declaration
Variable
Instance Variable
Class Variable
Constructor
Method
Instance Method
Class Method
Cleanup
Rectangle2.java

19. Point and Rectangle
Rectangle2.java

21. The Class Declaration

22. 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

23. Member Variables Declaration

24. 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; }

25. Methods

26. Method Declaration

27. 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.

28. 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

29. 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

30. The Life Cycle of an Object
Creating Objects
Using Objects
Cleaning Up Unused Objects

31. 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)

32. Using Objects Manipulate or inspect its variables Call its methods
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.

33. 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

34. Controlling Access to Members of a Class

35. Private class Alpha { private int iamprivate;
private void privateMethod() {
System.out.println ("privateMethod"); }
class Beta {
void accessMethod() {
Alpha a = new Alpha();
a.iamprivate = 10;
a.privateMethod(); }
How About one Alpha object access the private member of another
Alpha object?

36. 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(); }

37. 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(); }

38. Public package Greek; public class Alpha { public int iampublic;
public void publicMethod() {
System.out.println("publicMethod"); }
import Greek.*;
package Roman;
class Beta {
void accessMethod() {
Alpha a = new Alpha();
a.iampublic = 10; a.publicMethod(); }

39. Package package Greek; class Alpha { int iampackage;
void packageMethod() {
System.out.println("packageMethod"); }
package Greek;
class Beta {
void accessMethod() {
Alpha a = new Alpha();
a.iampackage = 10;
a.packageMethod(); }

40. Increase the reusability of codes
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

41. Simple Class Hierarchy
Object
Vehicle
Building
Car
Bike
House
Office
Van
Truck
Superclass and Subclass

42. 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