OOP Inheritance

Inheritance

Anastasia Rashtchian

A Bit About Me….

Computer Consultant, Trainer and Educator

Master of Science – Computer Science

Illinois Institute of Technology

• Artificial Intelligence and Expert Systems

• Current Interest: Machine Learning

Master of Education – Education, Policy, Organization, Leadership

University of Illinois – Urbana-Champaign

• eLearning in Higher Education

• Current Interest: Automated Adaptive Learning Systems

Let’s Review

Let’s Review

• What are some recent topics you have covered?

Let’s Review

• What are some recent topics you have covered?

• What are some programs you created?

Learning Objectives

• To understand the Object Oriented programming concept - inheritance

Learning Objectives


• To identify class hierarchies, single and multiple inheritance

Learning Objectives



• To define the syntax of inheritance in Java

Learning Objectives




• To analyze super and sub classes

Learning Objectives




• To analyze super and sub classes

• To examine the effect of inheritance on encapsulation and constructors

What is Inheritance?


Inheritance is a relationship between two or more classes where derived classes inherit behaviours and attributes of pre-existing (base) classes.


Inheritance is a relationship between two or more classes where derived classes inherit behaviours and attributes of pre-existing (base) classes.

What are some Benefits of Inheritance?

• Intended to help reuse existing code with little or no modification.




• Easy to implement real world models



• Easy to implement real world models

• Is of a transitive nature


What is a Class Hierarchy?


• The class hierarchy is usually drawn as an inverted (upside-down) tree.





• The tree can have any number of levels.




• The class at the top (base) of the inverted tree is called the root class.




• The class at the top (base) of the inverted tree is called the root class.

Root Class

Java Class Hierarchy

• In Java, the root class is called Object.



• Every class is a descendant, direct or indirect, of the Object class.




• Every class you use or write inherits the instance methods of Object.









• The Object class is defined in the java.lang package

What Would You Do?

What Would You Do?

How would you know what attributes and methods to define for the various levels in the hierarchy?

Inheritance Concepts - Terms


• OOP languages provide specific mechanisms for defining inheritance relationships between classes.



• (Derived) (Child) (Sub) Class - a class that inherits characteristics of another class.




• (Base) (Parent) (Super) Class - a class from which characteristics are inherited by one or more other classes.









• What attributes are inherited?





• A derived class inherits data and function members from ALL of its base classes.

Examples of Single and Multiple Inheritances

Each Java class has one

(and only one) superclass.




C++ allows for multiple

inheritance





inheritance

Classes higher in the

hierarchy are more

general and more

abstract





inheritance

Classes higher in the

hierarchy are more

general and more

abstract

Classes lower in the

hierarchy are more

specific and concrete

Inheritance can be continuous…

42

Inheritance can be continuous…Derived class can inherit from a base class

43


The derived class can act as a base class so another class can inherit from it

44



All features of the base class are available in the derived class

45




46

If you change the base class, all derived classes also change




47

If you change the base class, all derived classes also changeAny changes in the derived class do not change the base class




Also, the additional features in the derived class are not available in the base class

48





Also, the additional features in the derived class are not available in the base class

49


What really happens?

The object is created using new



The system allocates enough

memory to hold all its instance

variables.



The system allocates enough

memory to hold all its instance

variables.This includes any inherited instance

variables

Inheritance and Encapsulation

• Three levels of access control


• Three levels of access control• Public: members (data and methods)

can be used by the class and everybody else (other classes, functions, etc.)

• Protected: members can be accessed by the class (and its friends) and its derived classes

• Private: members can be accessed only by the class (and its friends)


• Three levels of access control• Public: members (data and methods)

can be used by the class and everybody else (other classes, functions, etc.)

• Protected: members can be accessed by the class (and its friends) and its derived classes

• Private: members can be accessed only by the class (and its friends)

• Remark: without inheritance private and protected are the same


• private member– Is accessible only via the base class

• public member– Is accessible everywhere (base class, derived class,

other classes)

• protected member– Is accessible by the base class and derived classes

What Are Constructors?


• Classes use constructors to initialize instance variables



• When a subclass object is created, its constructor is called.



• When a subclass object is created, its constructor is called

• It is the responsibility of the subclass constructor to invoke the appropriate superclass constructors



• When a subclass object is created, its constructor is called

• It is the responsibility of the subclass constructor to invoke the appropriate superclass constructors

• This ensures instance variables defined in the superclass are properlyinitialized

Calling the Super Class Constructor

Superclass constructors can be called using the "super" keyword


Superclass constructors can be called using the "super" keywordIt must be the first line of code in the sub class constructor


Superclass constructors can be called using the "super" keywordIt must be the first line of code in the sub class constructorIf a call to super is not made, the system will automatically attempt to invoke the no-argument constructor of the superclass.

Constructors - Example

public class BankAccount

{

private String ownersName;

private int accountNumber;

private float balance;

public BankAccount(int anAccountNumber, String aName)

{

accountNumber = anAccountNumber;

ownersName = aName;

}

[...]

}

public class OverdraftAccount extends BankAccount

{

private float overdraftLimit;

public OverdraftAccount(int anAccountNumber, String aName, float aLimit)

{

super(anAccountNumber, aName);

overdraftLimit = aLimit;

}

}



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

What is this?



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

Constructor



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

What is this?



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

extends indicates inheritance



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

What is this?



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

What is this?



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

Subclass Constructor



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

Subclass Constructor

additional parameter



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

What is this?



{





{


ownersName = aName;

}

[...]

}


{



{



}

}

Call to super class constructor

A portion of a Shape class hierarchy.

Shape

TwoDimensionalShape ThreeDimensionalShape

Circle Square Triangle Sphere Cube Tetrahedron

The Shape and Circle ClassesSide by Side

The Shape and Circle

ClassesSide by

Side

Abstract Classes & Interfaces

Definitions

• abstract methods = Methods that are declared, with no implementation

• abstract class = A class with abstract methods, not meant to be instantiated

• interface = A named collection of method definitions (without implementations)

Examples

• Food is an abstract class. Can you make an instance of food? No, of course not. But you can make an instance of an apple or a steak or a peanut butter cup, which are types of food. Food is the abstract concept; it shouldn’t exist.

• Skills are interfaces. Can you make an instance of a student, an athlete or a chef? No, but you can make an instance of a person, and have that person take on all these skills. Deep down, it’s still a person, but this person can also do other things, like study, sprint and cook.


ClassesSide by

Side


ClassesSide by

Side


ClassesSide by

Side


ClassesSide by

Side

Abstract and Interface Classes Revisited

Abstract Classes and Interfaces

cannot be instantiated

may contain a mix of methods declared with or without an implementation.

With abstract classes,

you can declare fields that are not static and final

you can define public, protected, and private concrete methods.

With interfaces,

all fields are automatically public, static, and final

all methods that you declare or define (as default methods) are public.


ClassesSide by

Side


ClassesSide by

Side


ClassesSide by

Side


ClassesSide by

Side


ClassesSide by

Side

Method Overriding

• Method Overriding is when the subclass modifies the implementation of a method defined in the superclass.

• It can only be done if the superclass method is public.

• It cannot be done if the superclass method is static.

• Runtime polymorphism is essentially referred as method overriding and can only be implemented through inheritance

Overriding vs. Overloading

public class Test {

public static void main(String[] args) {

A a = new A();

a.p(10);

a.p(10.0);

}

}

class B {

public void p(double i) {

System.out.println(i * 2);

}

}

class A extends B {

// This method overrides the method in B


System.out.println(i);

}

}

public class Test {

public static void main(String[] args) {

A a = new A();

a.p(10);

a.p(10.0);

}

}

class B {


System.out.println(i * 2);

}

}

class A extends B {

// This method overloads the method in B

public void p(int i) {

System.out.println(i);

}

}


ClassesSide by

Side

The Shape and Rect

ClassSide by Side

The Shape and Rect

ClassSide by Side

The Shape and Rect

ClassSide by Side

The Shape and Rect

ClassSide by Side

The Shape and Rect

ClassSide by Side

The Shape and Rect

ClassSide by Side

The Shape and Rect

ClassSide by Side

TestShapes Class

TestShapes Class

Which is from Shape and which is from Circ or Rect?

Output

CIRCLE

(X,Y) Position: (4,4)

Radius: 5

Area: 78.53981633974483

RECTANGLE


Width & Height: 5 10

Area: 50.0

Output

CIRCLE


Radius: 5

Area: 78.53981633974483

RECTANGLE


Width & Height: 5 10

Area: 50.0

Which is from Shape and which is from Circ or Rect?

How to do Multiple Inheritance in Java?

• Java does not support multiple inheritance.


• Java does not support multiple inheritance.• You can achieve partial multiple inheritance with the help of interfaces.


• Java does not support multiple inheritance.• You can achieve partial multiple inheritance with the help of interfaces. • You can extend only one class (inheritance) but can implement any number

of interfaces.



of interfaces.

Example: under the assumption that Car and Automobile are interfaces.



of interfaces.

Example: under the assumption that Car and Automobile are interfaces.

public class FerrariF12011 extends Ferrari implements Car, Automobile

{

}

Summary

• We explored the Object Oriented programming concept - Inheritance

• We identified class hierarchies, single and multiple inheritance

• We defined the syntax of inheritance in Java

• We analyzed super and sub classes

• We examined the effect of inheritance on encapsulation and constructors

OOP Inheritance

Software