Transcript Day 4-Concept 2-General OO Concepts

General OO Concepts
Objectives For Today:
• Discuss the benefits of OO Programming
• Inheritance and Aggregation
• Abstract Classes
• Encapsulation
• Introduce Visual Age for Java
General OO Concepts
• Java is an Object Oriented Language.
• What makes OO languages so appealing to developers?
• Object Oriented technology improves software development in
the following areas:
1) Code Quality
2) Maintenance
3) Application Scalability
4) Time to Market
General OO Concepts
Code Quality and Maintenance
• Code quality and maintainability improves with OO
programming because code is written inside of Classes.
• Code inside of a Class is written to undertake the tasks
that the Class is responsible for.
• So, we know exactly what that code is suppose to be
doing and how to find it quickly.
• In Java, once code is written, it can be re-used
over and over again.
• This re-use reduces the amount of code you have to write
greatly improving quality and maintainability.
General OO Concepts
Scalability and Time to Market
• Classes make up our programs, but are
completely independent of one another.
• This means that if you want to add Classes to expand
your program, you don’t have to tear apart
and change what you already have.
• You can just add additional Classes to your program.
• All these characteristics make the application development
process much faster.
OO Concepts
Inheritance
• We have already seen and created our own Classes.
• A well designed class can be used over and over again.
• In fact, it becomes a re-usable piece of code.
• The Java platform comes with a set of classes that you can
use in your programs. You have already seen a few of these
classes: String, Integer etc.
• Not only can we re-use classes, we can also build our own
classes from existing classes.
OO Concepts
Inheritance
• Creating classes from existing Classes is called Inheritance.
• When you create a Class based on another Class, the new class
becomes the sub class and the original is referred to as the
super class.
• The sub class inherits all the fields and methods of its parent.
• This allows the developer to use the existing methods and fields
and also allows him/her to build more methods and fields to make
a class more specialized.
OO Concepts
Inheritance
• For example, if you create a motor vehicle class,
car and motorcycle would be appropriate subclasses.
Motor Vehicle
Motorcycle
Car
OO Concepts
• Classes can have two kinds of relationships between them.
• The super class sub class relationship is referred to as an
is-a relationship.
ie. A car is-a motor vehicle (Inheritance)
• Classes can have fields that are in themselves objects. This is
referred to as a has-a relationship.
ie. A car has-a motor.
(Also known as Aggregation or Containment)
• Demonstrate motor vehicle diagrams
OO Concepts
Inheritance
• Creating a new class based on another class is called
extending a class.
• We implement inheritance by extending a class.
• All the fields and methods are inherited from the super class
to the subclass.
• You often add additional fields and methods to specialize the
subclass so it has the implementation you are looking for.
• We extend a Class by using the keyword extends.
OO Concepts
The Cosmic super class:
• All Classes built in Java have one common ancestor.
• The Object class found in the java.lang package is directly or
indirectly related to all Classes in the Java language.
• In Java the Object class is at the top of the class hierarchy
• All classes descend from the Class called Object.
• The java.lang package is imported by default into
all classes that you create.
OO Concepts
The Cosmic super class:
• Lets do an exercise that demonstrates Inheritance.
OO Concepts
Abstract Classes
• An abstract class is a class that can never be used to
create multiple objects.
• Do this to design a class for use only as a super class.
• Abstract classes are designed to define the behavior and
fields common to all subclasses in an inheritance hierarchy.
• We work with abstract classes through extending the class
since it cannot be instantiated.
• The keyword abstract is used to define a Class
as being abstract.
OO Concepts
Abstract Classes
• Classes are declared as being abstract as follows:
public abstract class MotorVehicle {
}
• You can also declare methods to be abstract, but only within
the confines of an abstract Class.
• Abstract methods have no implementation (code) and are
forced to be implemented by sub classes of the parent. Why?
OO Concepts
Abstract Classes
• We may have a method that we want subclasses to
inherit and implement, but we are not sure how to implement
it in the super class.
• To solve this problem define an abstract method in an
abstract class that provides no implementation and make it
abstract.
• Methods can be declared in abstract classes as follows:
public abstract class MotorVehicle {
public abstract void steer();
}
• Let’s re-visit our Motor Vehicles and explore Abstract Classes.
OO Concepts
Encapsulation
Encapsulation is a process of hiding details of an object. A
kind of protective capsule that stops direct access from
other objects. Forces access to an objects state through its
Encaps ulation
public methods.
public get X();
public int set X();
privat e int x=0;
privat e int y=0;
public get Y();
public int set Y();
OO Concepts
Encapsulation
• It is common to represent an object as a donut.
• The center of the donut contains the instance variables (fields).
• The instance variables are protected and only accessible
through the methods that the object provides.
• The methods are represented on the outer rim of the
donut surrounding the variables on the inside
OO Concepts
Encapsulation
• For your objects to be truly encapsulated, you should declare
your fields as private when creating your classes.
• Private fields can only be accessed by other objects via
methods of the Class where the fields are declared.
• This ensures that other Objects cannot manipulate these fields
directly.
• Other Objects should only be able to access these fields through
public methods defined in the class.
OO Concepts
Encapsulation Example
public class Person {
private String name;
//Accessor method used to get the name of the person
public String getName() {
return name;
}
//Mutator method used to set the name of the person.
public void setName(String aName) {
name = aName;
}
}
• Lets do an example demonstrating encapsulation
OO Concepts
• Objects communicate to one another by calling each others
methods. In the example below, the Person object is sending
a message to the Car object by calling the start method
Person Object
Send Message
Car Object
name
carMake
age
horsePower