1. Intro to OOP with Java, C. Thomas Wu Inheritance and Polymorphism
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2. Intro to OOP with Java, C. Thomas Wu
Objectives
Define inheritance, overriding methods and polymorphism.
Define reusable classes based on inheritance and abstract classes and abstract methods.
Define methods, using the protected modifier.
Describe the concepts of constructor in inheritance.
Write programs that are easily extensible and modifiable by applying polymorphism in program design.
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3. Intro to OOP with Java, C. Thomas Wu
Inheritance
Inheritance
Software reusability
Create new class from existing class
Absorb existing class’s data and behaviors
Enhance with new capabilities
Subclass extends superclass
Subclass
More specialized group of objects
Behaviors inherited from superclass
Can customize
Additional behaviors
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4. Intro to OOP with Java, C. Thomas Wu
Inheritance
To design two or more entities that are different but share many common features.
Steps :
Define a class that contains the common feature of the entities
Define classes as an extension of common class inheriting everything form the common class
Notes :
Common class = superclass, ancestor, base class
Class inherit = subclasses ; descendant; derived class
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5. Intro to OOP with Java, C. Thomas Wu
Examples
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6. Intro to OOP with Java, C. Thomas Wu
Example
public class Employee{
public String name;
public double salary;
public Date birthDate;
public void getDetail(){
//... }
Employee
+name : String
+salary : double
+birthDate : Date
+getDetails () : String
public class Manager{
public String name;
public double salary;
public Date birthDate;
public String department;
public void getDetail(){
//... }
Manager
+name : String
+salary : double
+birthDate : Date
+department : String
+getDetails () : String
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7. Intro to OOP with Java, C. Thomas Wu
Inheritance Concept
Java programming language allows a class to extend only one other class
When a class inherits from only one class, it is called single inheritance
For multiple inheritance (inherits more than one classes) must used interfaces
Employee
+name : String
+salary : double
+birthDate : Date
+getDetails () : String
Manager
+department : String
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8. Intro to OOP with Java, C. Thomas Wu
Inheritance Syntax
public class Employee{
public String name;
public double salary;
public Date birthDate;
public void getDetail(){
//... }
public class Manager extends Employee{
public String department;
Employee
+name : String
+salary : double
+birthDate : Date
+getDetail () : String
Manager
+department : String
<modifier> class <className> extends <superclass> {
// <declaration> }
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9. Intro to OOP with Java, C. Thomas Wu
Inheritance
Tree
Employee
name : String
salary : double
birthDate : Date
getDetail () : String
Manager
Engineer
+department : String
Director
+ carAllowance : double
+ inceaseAllowance()
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10. Intro to OOP with Java, C. Thomas Wu
Another example
Case Study:
Suppose we want implement a class roster that contains both undergraduate and graduate students.
Each student’s record will contain his or her name, three test scores, and the final course grade.
The formula for determining the course grade is different for graduate students than for undergraduate students.
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11. Defining Classes with Inheritance
Intro to OOP with Java, C. Thomas Wu
Defining Classes with Inheritance
Type of Student Grading System
Undergraduate pass if (test1+test2+test3/3) >=70
Graduate pass if (test1+test2+test3/3) >=80
Steps:
Define two unrelated classes for undergraduate and graduate students
Define entities that share common data or behavior (avoid duplication)
Design these two classes using inheritance
Define 3 classes
Student (class to incorporate behavior and data common to both graduate and undergraduate
GraduateStudents (class to incorporate behavior specific to graduate students)
UndergraduateStudents (class to incorporate behavior specific to undergraduate students)
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12. Modeling Two Types of Students
Intro to OOP with Java, C. Thomas Wu
Modeling Two Types of Students
There are two ways to design the classes to model undergraduate and graduate students.
We can define two unrelated classes, one for undergraduates and one for graduates.
We can model the two kinds of students by using classes that are related in an inheritance hierarchy.
Two classes are unrelated if they are not connected in an inheritance relationship.
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13. Classes for the Class Roster
Intro to OOP with Java, C. Thomas Wu
Classes for the Class Roster
For the Class Roster sample, we design three classes:
Student
UndergraduateStudent
GraduateStudent
The Student class will incorporate behavior and data common to both UndergraduateStudent and GraduateStudent objects.
The UndergraduateStudent class and the GraduateStudent class will each contain behaviors and data specific to their respective objects.
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14. Inheritance Hierarchy
Intro to OOP with Java, C. Thomas Wu
Inheritance Hierarchy
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15. The Protected Modifier
Intro to OOP with Java, C. Thomas Wu
The Protected Modifier
The modifier protected makes a data member or method visible and accessible to the instances of the class and the descendant classes.
Intermediate level of protection between public and private.
Public data members and methods are accessible to everyone.
Private data members and methods are accessible only to instances of the class.
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16. Inheritance and Member Accessibility
Intro to OOP with Java, C. Thomas Wu
Inheritance and Member Accessibility
We use the following visual representation of inheritance to illustrate data member accessibility.
This shows the inherited
components of the
superclass are part of
the subclass instance
Instances
Class Hierarchy
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17. The Effect of Three Visibility Modifiers
Intro to OOP with Java, C. Thomas Wu
The Effect of Three Visibility Modifiers
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18. Accessibility of Super from Sub
Intro to OOP with Java, C. Thomas Wu
Accessibility of Super from Sub
Everything except the private members of the Super class is visible from a method of the Sub class.
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19. Accessibility from Another Instance
Intro to OOP with Java, C. Thomas Wu
Accessibility from Another Instance
Data members accessible from an instance are also accessible from other instances of the same class.
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20. Intro to OOP with Java, C. Thomas Wu
Accessibility
Modifier
Same class
Same Package
Subclass
Universe
private
Yes
default
protected
public
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21. Intro to OOP with Java, C. Thomas Wu
Overriding Method
A subclass can modify behavior inherited form parent class
A subclass can create a method with different functionality than the parent’s method but with the same:
> name
> return type
> argument list
Overriding is to adding additional features; modify existing behavior.
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22. Rules About Overriding Method
Intro to OOP with Java, C. Thomas Wu
Rules About Overriding Method
Must have a return types that is identical to the method it overrides
Cannot be less accessible that the method it overrides.
When override a method, real goal is not to replace the existing behavior but to extend that behavior in some way.
Can used super keyword – refers to the superclass of the class in which the keyword is used. It is used to refer to the member variables or methods of the superclass.
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23. Intro to OOP with Java, C. Thomas Wu
Overriding Method
public class Employee{ // class name
private String name;
private double salary;
public String getDetail(){ // method declaration
return “ Name : “+ name “\n” +
“ Salary : “ +salary; }
public class Manager extends Employee{ // class name
private String department;
public String getDetail(){
return super.getDetails() + //invoked the entire behavior “ Manager of : “ + department;
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24. Inheritance and Constructors
Intro to OOP with Java, C. Thomas Wu
Inheritance and Constructors
Unlike members of a superclass, constructors of a superclass are not inherited by its subclasses.
You must define a constructor for a class or use the default constructor added by the compiler.
The statement
super();
calls the superclass’s constructor.
If the class declaration does not explicitly designate the superclass with the extends clause, then the class’s superclass is the Object class.
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25. Intro to OOP with Java, C. Thomas Wu
Inheritance & Constructor
Example class definition:
class Person { // class name
public void sayHello(){ // method declaration
System.out.println (“ hello guys !”); }
Is equivalent to
public Person (){
super ();
public void sayHello(){
This statement calls the superclass’s constructor
Automatically added to the classes by compiler
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26. Intro to OOP with Java, C. Thomas Wu
Inheritance & Constructor
Example 2 : constructor definition:
class MyClass { // class name
private int myInt;
public myClass(){ // method declaration
myInt = 10; }
Compiler will rewrite the constructor to
public MyClass (){
Super ();
Automatically added to the classes by compiler
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27. Intro to OOP with Java, C. Thomas Wu
Inheritance & Constructor
Example 3 :
class Vehicle{ // class name
private String vin;
public Vehicle (String vehicleID){ // constructor declaration
vin = vehicleID; }
class Truck extends Vehicle{ // class name
private int cargoWeightLimit;
public Truck (int weightLimit, String vin){
super (vin);
cargoWeightLimit = weightLimit;
Notes : If a class has a superclass that is not the Object class, then a constructor of the class should make an explicit call to a constructor of the superclass.
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28. Case study – Company Payroll Application
Intro to OOP with Java, C. Thomas Wu
Case study – Company Payroll Application
Commission employee are paid a percentage of their sales
Base-salaried commission employee receives a base salary plus percentage of their sales.
Commission employee has first name, last name, social security number, commission rate and gross (total) sales amount.
Base-salaried commission employee has first name, last name, social security number, commission rate, gross (total) sales amount and base salary
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29. Intro to OOP with Java, C. Thomas Wu
Polymorphism
Polymorphism comes from Greek meaning “many forms.”
Polymorphism allows a single variable to refer to objects from different subclasses in the same inheritance hierarchy
For example, if Cat and Dog are subclasses of Pet, then the following statements are valid:
Pet myPet;
myPet = new Dog();
. . .
myPet = new Cat();
A variable of class X may not refer to an object from the superclass or sibling classes of X.
Sibling classes are those that share the common ancestor class.
For example, the following statements are invalid:
Dog myDog = new Cat();
Cat myCat = new Pet();
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30. Intro to OOP with Java, C. Thomas Wu
This is another example.
If Graduate Student and Undergraduate Student are subclasses of Student, then the following statements are valid:
Student student;
student = new Student();
student = new GraduateStudent();
. . .
student = new UndergraduateStudent();
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31. Creating the roster Array
Intro to OOP with Java, C. Thomas Wu
Creating the roster Array
We can also declare using array. Instead of using two separate array to represent both Graduate and Undergraduate Student, we can declare single array for them.
It is because the array elements are objects.
We can maintain our class roster using an array, combining objects from the Student, UndergraduateStudent, and GraduateStudent classes.
Student roster = new Student[40];
. . .
roster[0] = new GraduateStudent();
roster[1] = new UndergraduateStudent();
roster[2] = new UndergraduateStudent();
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32. State of the roster Array
Intro to OOP with Java, C. Thomas Wu
State of the roster Array
The roster array with elements referring to instances of GraduateStudent or UndergraduateStudent classes.
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33. Sample Polymorphic Message
Intro to OOP with Java, C. Thomas Wu
Sample Polymorphic Message
To compute the course grade using the roster array, we execute
for (int i = 0; i < numberOfStudents; i++) {
roster[i].computeCourseGrade(); }
If roster[i] refers to a GraduateStudent, then the computeCourseGrade method of the GraduateStudent class is executed.
If roster[i] refers to an UndergraduateStudent, then the computeCourseGrade method of the UndergraduateStudent class is executed.
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34. The instanceof Operator
Intro to OOP with Java, C. Thomas Wu
The instanceof Operator
The instanceof operator can help us learn the class of an object.
The following code counts the number of undergraduate students.
int undergradCount = 0;
for (int i = 0; i < numberOfStudents; i++) {
if ( roster[i] instanceof UndergraduateStudent ) {
undergradCount++; }
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35. Abstract Superclasses and Abstract Methods
Intro to OOP with Java, C. Thomas Wu
Abstract Superclasses and Abstract Methods
When we define a superclass, we often do not need to create any instances of the superclass.
Depending on whether we need to create instances of the superclass, we must define the class differently.
We will study examples based on the Student superclass defined earlier.
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36. Definition: Abstract Class
Intro to OOP with Java, C. Thomas Wu
Definition: Abstract Class
An abstract class is a class
defined with the modifier abstract OR
that contains an abstract method OR
that does not provide an implementation of an inherited abstract method
that is not intended to be used to create objects.
An abstract method is a method with the keyword abstract, and it ends with a semicolon instead of a method body.
Private methods and static methods may not be declared abstract.
abstract class Student{ ……
abstract public void computeCourseGrades() ; }
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37. Intro to OOP with Java, C. Thomas Wu
Case 1
Student Must Be Undergraduate or Graduate
If a student must be either an undergraduate or a graduate student, we only need instances of UndergraduateStudent or GraduateStudent.
Therefore, we must define the Student class so that no instances may be created of it.
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38. Intro to OOP with Java, C. Thomas Wu
Case 2
Student Does Not Have to Be Undergraduate or Graduate.
In this case, we may design the Student class in one of two ways.
We can make the Student class instantiable.
We can leave the Student class abstract and add a third subclass, OtherStudent, to handle a student who does not fall into the UndergraduateStudent or GraduateStudent categories.
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39. Intro to OOP with Java, C. Thomas Wu
Which Approach to Use
The best approach depends on the particular situation.
When considering design options, we can ask ourselves which approach allows easier modification and extension.
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40. Inheritance versus Interface
Intro to OOP with Java, C. Thomas Wu
Inheritance versus Interface
The Java interface is used to share common behavior (only method headers) among the instances of different classes.Example : class Person that implements multiple interfaces such as Driver, Commuter and Biker.
Inheritance is used to share common code (including both data members and methods) among the instances of related classes.
In your program designs, remember to use the Java interface to share common behavior. Use inheritance to share common code.
If an entity A is a specialized form of another entity B, then model them by using inheritance. Declare A as a subclass of B.
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