it203 oop introduction to oop sjp art - …...2015/03/03 · java programming: oop 62 inheritance...
TRANSCRIPT
Inheritance
61
Java Programming: OOP 62
Inheritance
• On the surface, inheritance is a code re-use issue.
– we can extend code that is already written in a
manageable manner.
• Inheritance is more, it supports polymorphism at the
language level!
• Information is made manageable in a hierarchical
order
Inheritance cont.
• With Inheritance, new class can be derived from
existing classes as a building block
• New class Inherit properties and methods from the
existing class
• New class can also added Its own properties and
methods
• Keyword
– Extends
– Implements
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Java Programming: OOP 64
Inheritance cont.
• Take an existing object type (collection of fields and methods) and extend it.
– create a special version of the code without re-writing any of the existing code (or even explicitly calling it!).
– End result is a more specific object type, called the sub-class / derived class / child class.
– The original code is called the super class / parent class / base class.
Example
7/11/2015 Budditha Hettige ([email protected]) 65
Student
University
Student
Undergraduate Post graduate
Employee
Phone
Employee
-----------------
-
Name
Phone
FulltimeEmployee
------------------------
Salary
Office
Manager
Manager
------------------
CompanyCar
Phone
Contractor
------------------
HourlyRate,
ContractDuration
66
Introduction (Cont.)
• Class hierarchy
– Direct superclass
• Inherited explicitly (one level up hierarchy)
– Indirect superclass
• Inherited two or more levels up hierarchy
– Single inheritance
• Inherits from one superclass
– Multiple inheritance
• Inherits from multiple superclasses
Inheritance
67
Animal
Dog
public class Animal
{
}
public class Dog extends Animal
{ }
Introduction (Cont.)
68
Animal
Dog
Animal
Dog
Private
Public
Public
69
protectedprotectedprotectedprotected Members
• protected access
– Intermediate level of protection between public and private
– protected members accessible by
• superclass members
• subclass members
• Class members in the same package
– Subclass access to superclass member
• Keyword super and a dot (.)
Introduction (Cont.)
70
Animal
Dog Animal
Dog
Private
Public
Public
Protected
Protected
Example
71
IS-A Relationship (Example)
public class Animal
{
}
public class Mammal extends Animal
{ }
public class Reptile extends Animal
{ }
public class Dog extends Animal
{ }
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Animal
Mamal
extends
IS-A Relationship (Example)
public class Dog extends Mammal
{
public static void main(String args[])
{
Animal a = new Animal();
Reptile r = new Reptile();
Mammal m = new Mammal();
Dog d = new Dog();
System.out.println(m instanceof Animal);
System.out.println(d instanceof Mammal);
System.out.println(d instanceof Animal);
}
}
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Animal
Mamal Dog Reptile
IS-A Relationship
• In Object Oriented terms following are true:
– Animal is the superclass of Mammal class.
– Animal is the superclass of Reptile class.
– Mammal and Reptile are sub classes of Animal
class.
– Dog is the subclass of both Mammal and Animal
classes.
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IS-A Relationship
• IS-A relationship we can say:
– Mammal IS-A Animal
– Reptile IS-A Animal
– Dog IS-A Mammal
– Hence : Dog IS-A Animal as well
• Use of the extends keyword the subclasses will be
able to inherit all the properties of the superclass
except for the private properties of the superclass
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Java Programming: OOP 76
Accessing super class methods
• Can use super() to access all (non-private) super
class methods.
– even those replaced with new versions in the
derived class.
• Can use super() to call base class constructor.
Java Programming: OOP 77
Single Inheritance
• You can't extend more than one class!
– the derived class can't have more than one base
class.
• You can do multiple inheritance with interface
inheritance.
Java Programming: OOP 78
Inheritance Example cont.
• Employee: name, email, phone
– FulltimeEmployee: also has salary, office, benefits, …
• Manager: CompanyCar, can change salaries, rates contracts, offices, etc.
– Contractor: HourlyRate, ContractDuration, …
• A manager a special kind of FullTimeEmployee,
which is a special kind of Employee.
• The relationship modeled by inheritance is often
referred to as a “is a” relationship
Inheritance Example
79
Employee
------------------
Name
Phone
FulltimeEmployee
------------------------
Salary
Office
Manager
Manager
------------------
CompanyCar
Phone
Contractor
------------------
HourlyRate,
ContractDuration
Inheritance Example
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Employee
------------------
Name
Phone
FulltimeEmployee
------------------------
Salary
Office
Manager
Manager
------------------
CompanyCar
Phone
Contractor
------------------
HourlyRate,
ContractDuration
Is a
Is a Is a
