2. Goals To learn about inheritance
To implement subclasses that inherit and override superclass methods
To understand the concept of polymorphism
To be familiar with the common superclass Object and its methods
Fundamental Principles of OOP:
-     Encapsulation
Encapsulation is the packing of data and functions into a single component. The features of encapsulation are supported using classes.
This allow us to hide unnecessary details in our classes and provide a clear and simple interface for working with them.
-     Inheritance
Inheritance is the process by which one object acquires the properties of another object.
class hierarchies improve code readability and enable the reuse of functionality.
-     Polymorphism
how to work in the same manner with different objects, which define a specific implementation of some abstract behavior.
-     Abstraction
Any model that includes the most important, essential, or distinguishing aspects of something while suppressing or ignoring less important, immaterial, or diversionary detai
deal with objects considering their important characteristics and ignore all other details. Classification is considered to be the most important abstraction principle.

3. Inheritance Hierarchies
the relationship between a more general class (superclass) and a more specialized class (subclass).
IS-A relationship
Figure 1 An Inheritance Hierarchy of Vehicle Classes
Vehicle
Motorcycle
Car
Truck
Natural hierarchy: IS-A relation to Person class
STUDENT (attributes=name, address, major | methods= toString (polymorphic), registercourse)
PROFESSOR (attributes=name, address, ssn, department | methods=toSring, teachCourse)
UNDRGRAD()
GRADUATE(atributes= thesis topic)
Some cannot be in same hierarchy:
Truck
Employee
Sedan
SUV

4. QUESTION Inheritance = IS-A relation
Which ones have inheritance relation?
Which ones are the super class and subclass?
Person , Student
Car ,Person
Car ,Vehicle

5. ANSWER Inheritance = IS-A relation
Which ones have inheritance relation?
Person is a Student
Student is a Person
Student = Subclass
Person = super class
Car is a Person
Car is a Vehicle
Car= Subclass
Vehicle= super class
Superclass
Subclass

6. Inheritance Hierarchies
A subclass inherits:
Superclass Non-private data attributes
Superclass Non-private methods
Inherited variables and methods behave as though they were declared in the subclass.
A subclass does NOT inherit:
Superclass constructors
Superclass private variables
Superclass private methods.
The subclass may define additional variables and methods that were not present in the superclass.
Inheritance lets you can reuse code instead of duplicating it.
Employee
-name
-Id
+getId()
+getName()
+calculateSalary()
+setName(name)
+setId(id)
SalariedEmployee
-weeklySalary
+calculateBonus()
CommissionEmployee
-grossSales
-commissionRate
+getCommissionRate()

7. The subclass can: Add new functionality Use inherited functionality
Override inherited functionality (modify)
Employee
-name
-Id
+claculateSalary()
+getName()
+setID(id)
SalariedEmployee
-weeklySalary
+calculateBonus()
CommissionEmployee
-grossSales
-commissionRate
+getCommissionRate()

8. The subclass can: Add new functionality Use inherited functionality
Override inherited functionality (modify)
Employee
-name
-Id
+claculateSalary()
+getName()
+setID(id)
SalariedEmployee
-weeklySalary
+calculateBonus()
CommissionEmployee
-grossSales
-commissionRate
+getCommissionRate()
parent/supercalss of SalariedEmployee class and CommissionEmployee
Additional attributes
Additional methods
Inherited + overridden by subclasses
Inherited and used as it is by subclasses

9. Superclass: Employee.java public class Employee { private String name;
private int id;
public double calculateSalary(){return ;}
public int getId() { return id; }
public String getName(){ return name;}
public void setId(int id){ this.id = id;}
public void setName(String name) { this.name = name; } }
Employee.java

10. Inheritance in java
In Java, inheritance is accomplished by extending an existing class (using keyword extends).
A superclass can be any previously existing class, including a class in the Java API.
E.g.:
public class SalariedEmployee extends Employee { … }
Subclass of Employee
Superclass of
salariedEmployee

11. Writing a Subclass Example
Superclass:
public class Employee {
private String name;
private int id;
public double calculateSalary(){return ;}
public int getId() { return id; }
public String getName(){ return name;}
public void setId(int id){ this.id = id;}
public void setName(String name) { this.name = name; } }
Employee.java
Subclass:
Public class SalariedEmployee extends Employee {
private double weeklySalary;
public double calculateSalary(){return this.weeklySalary*4;}
public double calculateBonus(){return this.weeklySalary*0.5; } }
Public class SalariedEmployee extends Employee {
private double weeklySalary;
public double getSalary(){return this.weeklySalary*4;}
public double calculateBonus(){
System.out.println(id); //Error: id has private access in Employee
System.out.println(setId(10)); //Error: ONLY IF setId method has private access in Employee
returnthis.weeklySalary*0.5; }
SalariedEmployee.java

12. Question Which of the following code lines gives compilation errors?
Public class SalariedEmployee extends Employee {
private double weeklySalary;
public double calculateBonus(){
System.out.println(“calculationg bonus of employee with
name:”+getName()+” id:”+id);
return this.weeklySalary*0.5; }

13. Answer
Print statement will result in compiler error as it is private in Employee class.
Public class SalariedEmployee extends Employee {
private double weeklySalary;
public double calculateBonus(){
System.out.println(“calculationg bonus of employee with
name:”+getName()+” id:”+id);
return this.weeklySalary*0.5; }

15. Corrected version public class SalariedEmployee extends Employee {
private double weeklySalary;
public double calculateSalary()
return this.weeklySalary*4; }
public double calculateBonus()
System.out.println("calculationg bonus of employee with name:"+getName()+" id:"+getId());
return this.weeklySalary*0.5;

16. Question Which of these statements will result in compiler error?
public class EmployeeDemo {
public static void main(String args[])
SalariedEmployee salEmp = new SalariedEmployee();
salEmp.calculateBonus();
salEmp.setId(12345);
salEmp.getCommissionRate();
System.out.print(salEmp.id);
System.out.print(salEmp.weeklySalary); }
Employee
-name
-Id
+getSalary()
+getName()
+setID(id)
SalariedEmployee
-weeklySalary
+calculateBonus()
CommissionEmployee
-grossSales
-commissionRate
+getCommissionRate()

17. Answer
Which of these statements will result in compiler error? Correct them
public class EmployeeDemo {
public static void main(String args[])
SalariedEmployee salEmp = new SalariedEmployee();
salEmp.calculateBonus();
salEmp.setId(12345);
salEmp.getCommissionRate();
System.out.print(salEmp.Id);
System.out.print(salEmp.weeklySalary); }
Employee
-name
-Id
+getSalary()
+getName()
+setID(id)
SalariedEmployee
-weeklySalary
+calculateBonus()
CommissionEmployee
-grossSales
-commissionRate
+getCommissionRate()

18. Se The substitution principle:
You can always use a subclass object when a superclass object is expected.
Person
Try in interaction pane.
Try calling some methods
(e.g. setId, getId, calculateBonus)
Employee
SalariedEmployee
CommissionEmployee
Employee emp = new SalariedEmployee();
Person person = new SalariedEmployee();

19. You can only call methods in reference variable class(Employee) even if object is a SalariedEmployee object

20. Question
Consider classes Manager and Employee. Which should be the superclass and which should be the subclass?

21. Answer
Because every manager is an employee but not the other way around, the Manager class is more specialized. It is the subclass, and Employee is the superclass.
Manager is a Employee
(subclass) (superclass)
Employee is a Manager

22. Question
Consider the method doSomething(Car c). List all vehicle classes from Figure 1 whose objects cannot be passed to this method.
Vehicle
Motorcycle
Car
Truck
Sedan
SUV

23. Answer Answer: Vehicle, Truck, Motorcycle doSomething(Car c) Vehicle
Sedan
SUV
Answer: Vehicle, Truck, Motorcycle

24. Writing Subclass Constructors
A subclass (E.g. Salaried employee) DOES NOT inherit constructors from its superclass
So it will need its own constructors.
Two things are done in subclass (E.g SalariedEmployee) constructor:
Initialize variables of superclass (likely to be private)
E.g. name, id in Employee class
Initialize variables declared in subclass
E.g. weeklySalary in SalariedEmployee class

25. Writing Subclass Constructors
Say Employee class already has a 2-arg constructor:
A first attempt at writing the constructor:
Is above an OK way to write subclass constructor?
Public class Employee{
private String name;
private int id;
public Employee(String name, int id){this name = name; this.id=id; } … }
public class SalariedEmployee extends Employee{
double weeklySalary;
public SalariedEmployee(String empName, int empId, double empWeekSal) {
name = empName;
id = empId;
weeklySalary = empWeekSal; }
...

26. No. You will get compile time errors because name and id are declared private in employee class.
public class SalariedEmployee extends Employee {
double weeklySalary;
public SalariedEmployee(String empName, int empId, double empWeekSal )
name = empName;
id = empId;
weeklySalary = empWeekSal; }

27. Writing Subclass Constructors
Solution: super keyword
Call superclass constructor within subclass constructor
E.g. SalariedEmployee constructor:
public class SalariedEmployee extends Employee{
double weeklySalary;
public SalariedEmployee(String empName, int empId, double empWeekSal){
super(empName, empId);
weeklySalary = empWeekSal; }
Super Must be the first statement in the subclass constructor

28. Writing Subclass Constructors
What if subclass constructor fails to call super?
Then the compiler will automatically insert super() at the beginning of the constructor.
public SalariedEmployee(String empName, int empId, double empWeekSal) {
weeklySalary = empWeekSal; }

29. What if a subclass has no constructors at all?
Then the compiler will create a no argument constructor in subclass that contains super().
This will be the only statement in constructor.

30. Programming Question
Write a class SavingsAccount that extends the BankAccount class. In addition to the public methods and variables declared in BankAccount class, SavingsAccount class need to define following:
Instance variable: interestRate
Constructor that initializes interestRate and all variables in BankAccount class.
Instance method: setInterestRate that set interest rate to parameter value
Use BankAccount class code is in next slide 

31. BankAccount.java public class BankAccount { private double balance;
private int accountNumber;
private static int lastAssignedNumber = 0;
public BankAccount(double initialBalance) {
balance = initialBalance;
lastAssignedNumber++;
accountNumber = lastAssignedNumber; }
public void deposit(double amount)
balance += amount;
public void withdraw(double amount)
balance -= amount;
public double getBalance()
return balance;
public String toString()
return "Bank Account No:"+accountNumber+" balance:"+balance;

32. Answer SavingsAccount.java
public class SavingsAccount extends BankAccount {
private double interestRate;
public SavingsAccount(double balance, double interestRate)
super(balance);
this.interestRate = interestRate; }
public void setInterestRate(double rate)
interestRate = rate;

33. Programming Question
Write a tester class SavingsAccountDemo to test the functionality of SavingsAccount class.
In the main method do following:
Create a savings account object with balance =10000 and interest rate=2.5
Update the interest rate of savings account to 4.25
Deposit 500
Print the final balance
Sample run:

34. Answer SavingsAccountDemo.java Can we replace this statement by:
public class SavingsAccountDemo {
public static void main(String args[])
SavingsAccount savingsAcct = new SavingsAccount(10000,2.50);
savingsAcct.setInterestRate(4.25);
savingsAcct.deposit(500.00);
System.out.println("Balance: " +savingsAcct.getBalance()); }
Can we replace this statement by:
BankAccount savingsAcct = new SavingsAccount(10000,2.50);
Test it. What happens?

35. You cannot call methods of subclasses from a Reference of a superclass type
Comment line7

36. Polymorphism Polymorphism: Polymorphism in OOP is achieved in 2 ways:
Ability of an object to take on many forms.
Polymorphism in OOP is achieved in 2 ways:
Method overloading (static polymorphism)
Method overriding (dynamic polymorphism)
Most common form of polymorphism

37. Overloading methods
when two or more methods in the same class have the same name but different parameter types.
Called static polymorphism.
Can be determined at compile time

38. Example 3 overloaded and methods public class Calculator {
public int add(int a, int b)
return a+b; }
public double add(double a, double b)
public int add(int a, int b, int c)
return a+b+c;
3 overloaded and methods

39. Overriding Methods
An overriding method can extend or replace the functionality of the superclass method.
Replace:
provide completely different implementation
Extend:
extend the functionality provided by super class
BankAccount toString(){return “BankAccount balance=”+balance;}
SavingsAccount toString(){ return super.toString()+ “ InterestRate=”+interestRate;}

40. Example
calculateSalary method in Employee class overridden in both subclasses
Employee.java
public class Employee {
private String name;
private int id;
public Employee(String name, int id) {
this.name = name;
this.id = id; }
public double calculateSalary()
return ;
public int getId() { return id; }
public String getName() { return name; }
public void setId(int id) { this.id = id; }
public void setName(String name) { this.name = name; }

41. Overridden implementation in SalariedEmployee subclass
public class SalariedEmployee extends Employee {
private double weeklySalary;
public SalariedEmployee(String empName, int empId, double weeklySalary)
super(empName, empId);
this.weeklySalary = weeklySalary; }
public double calculateSalary()
return this.weeklySalary*4;
public double calculateBonus()
//System.out.println("calculationg bonus of employee with name:"+getName()+" id:"+id);
return this.weeklySalary*0.5;
SalariedEmployee.java
Overridden implementation in SalariedEmployee subclass

42. Overridden implementation in CommissionEmployee subclass
public class CommissionEmployee extends Employee {
private double grossSales;
private double commissionRate;
public CommissionEmployee(String empName, int empId, double grossSales, double commissionRate)
super(empName, empId);
this.grossSales = grossSales;
this.commissionRate = commissionRate; }
public double calculateSalary()
return this.grossSales*this.commissionRate;
public double getCommissionRate()
return this.commissionRate;
CommissionEmployee.java
Overridden implementation in CommissionEmployee subclass

43. Same Employee reference variable calls calculateSalary method 2 times.
Calls getSalary method in SalariedEmployee class
Calls getSalary method in CommissionEmployee class
Same Employee reference variable calls
calculateSalary method 2 times.
But execute 2 different behaviors depending
on actual object it points to

44. Syntax 9.2 Calling a Superclass Method
Example of extending functionality of deposit method in SavingsAccount class

45. Example Base implementation in superclass
Extended implementation in subclass

46. Question
Assuming SavingsAccount is a subclass of BankAccount, which of the following code fragments are valid in Java? a. BankAccount account = new SavingsAccount(); b. SavingsAccount account2 = new BankAccount(); c. BankAccount account = null; SavingsAccount account2 = account;

47. Answer
Assuming SavingsAccount is a subclass of BankAccount, which of the following code fragments are valid in Java? a. BankAccount account = new SavingsAccount(); b. SavingsAccount account2 = new BankAccount(); c. BankAccount account = null; SavingsAccount account2 = account;
Answer: a only.

48. Question
What is the super class of PrintStream class in Java class library?
What is the parent class of Object class?

49. Answer Class hierarchy Indirect Super classes of PrintStream class
Direct Super class of PrintStream class
Object=root
Check any other api page for java library classes to verify this
What is the parent class of Object class
Subclasses of PrintStream

51. ACM Java library –inheritance example

52. Object: The Cosmic Superclass
The Object Class Is the (direct or indirect) Superclass of Every Java Class
Every class defined without an explicit extends clause automatically extend Object
E.g. Your BankAccount class extends Object class
for any object x, the expression: x.clone() != x is true
We did overriding without knowing: Rectangle equals() method
hashCode():
All the classes inherit a basic hash scheme from the fundamental base class java.lang.Object, but instead many override this to provide a hash function that better handles their specific data. Classes which provide their own implementation must override the object method public int hashCode().
The general contract for overridden implementations of this method is that they behave in a way consistent with the same object's equals() method: that a given object must consistently report the same hash value (unless it is changed so that the new version is no longer considered "equal" to the old), and that two objects which equals() says are equal must report the same hash value. There's no requirement that hash values be consistent between different Java implementations, or even between different execution runs of the same program, and while two unequal objects having different hashes is very desirable, this is not mandatory (that is, the hash function implemented doesn't need to be a perfect hash).[1]
For example, the class Employee might implement its hash function by composing the hashes of its members:
public class Employee {
int employeeId; String name; Department dept;
// other methods would be in
public int hashCode() {
int hash = 1;
hash = hash * 17 + employeeId;
hash = hash * 31 + name.hashCode();
hash = hash * 13 + (dept == null ? 0 : dept.hashCode());
return hash; }

53. compares 2 objects with each other
yields a string
describing the object

54. Example

55. Overriding the toString Method
Override the toString method in your classes to yield a string that describes the object’s state.
You already did this!
E.g. BankAccount class toString method:
public String toString() {
return "BankAccount[balance=" + balance + "]”; }
This is overridden implementation of Object (superclass) class toString implementation
Object.toString prints class name and the hash code of the object:
BankAccount momsSavings = new BankAccount(5000);
String s = momsSavings.toString();
// Sets s to something like

56. Programming Question
Override the toString method in SavingsAccount class so that text returned include accountNumber, balance and interestRate.
Modify SavingsAccountDemo class to call toString method:
public class BankAccountDemo{
public static void main(String args[]){
SavingsAccount savingsAcct = new SavingsAccount(10000,2.50);
savingsAcct.setInterestRate(4.25);
savingsAcct.deposit(500.00);
System.out.println("Balance: " +savingsAcct); }
Sample output:

57. Answer SavingsAccount.java
public class SavingsAccount extends BankAccount {
private double interestRate;
public SavingsAccount(double balance, double interestRate){
super(balance);
this.interestRate = interestRate; }
public double getInterestRate() {
return interestRate;
public void setInterestRate(double rate) {
interestRate = rate;
public String toString(){
return super.toString() + " interestRate="+interestRate;

58. Overriding the equals Method
equals method checks whether two objects have the same content:
if (stamp1.equals(stamp2)) . . .
// Contents are the same
== operator tests whether two references are identical - referring to the same object:
if (stamp1 == stamp2) . . .
// Objects are the same

59. Overriding the equals Method
To implement the equals method for a Stamp class -
Override the equals method of the Object class:
public class Stamp{
private String color;
private int value;
. . .
public boolean equals(Object otherObject) {
//your code here }
Cannot change parameter type of the equals method - it must be Object

60. Overriding the equals Method
After casting, you can compare two Stamps
public boolean equals(Object otherObject) {
Stamp otherStamp = (Stamp) otherObject;
//now compare this stamp object with otherStamp object }
How to compare?
Check if each corresponding instance variable values are same
If so, return true, Otherwise return false
return this.color.equals(otherStamp.color) && this.value==otherStamp.value;

61. The equals method can access the instance variables of any Stamp object.
public boolean equals(Object otherObject) {
Stamp otherStamp = (Stamp) otherObject;
return color.equals(otherStamp.color) && value == otherStamp.value; }
The access otherStamp.color and otherStamp.value is legal here (ONLY in equals method)

62. Programming Question
Modify BankAccount class to override equals method.
Add a tester class to call equals method:
Sample output

63. Answer public class BankAccount { private double balance;
private int accountNumber;
private static int lastAssignedNumber = 0;
public BankAccount(double initialBalance) {
balance = initialBalance;
lastAssignedNumber++;
accountNumber = lastAssignedNumber; }
public void deposit(double amount) { balance += amount; }
public void withdraw(double amount) { balance -= amount; }
public double getBalance() { return balance; }
public String toString() {
return "Bank Account No:"+accountNumber+" balance:"+balance;
public boolean equals(Object anotherAccount) {
BankAccount other = (BankAccount) anotherAccount;
return (accountNumber == other.accountNumber) && (balance==other.balance);

64. Homework: Programming Question
Modify your Rectangle.java class so you override the equals method and toString method.
equals method should return true only if x,y,width and height matches to rectangle in parameter.
To string method should print the x,y,width and height parameters of object
Sample output:

65. Answer public class Rectangle { int x, y, width, height;
public Rectangle(int x, int y, int width, int height)
this.x = x;
this.y = y;
this.width = width;
this.height = height; }
/**
* overriden equals method * */
public boolean equals(Object object)
Rectangle r = (Rectangle)object;
if(x==r.x && y==r.y && width==r.width && height==r.height)
return true;
return false;
public String toString()
return "Rectangle [x:"+x+" y:"+y+" width:"+width+" height:"+height+"]";
public static void main(String args[])
Rectangle r1 = new Rectangle(10,20,50,50);
Rectangle r2 = new Rectangle(10,20,50,50);
Rectangle r3 = new Rectangle(10,50,50,50);
System.out.println("r1= "+r1);
System.out.println("r2= "+r2);
System.out.println("r3= "+r3);
System.out.println("r1.equals(r2)"+r1.equals(r2));
System.out.println("r1.equals(r3)"+r1.equals(r3));
Answer

66. The instanceof Operator
It is legal to store a subclass reference in a superclass variable:
BankAccount acct1 = new SavingsAccount(1000.0, 2.5);
Object obj = acct1;
Sometimes you need to convert from a superclass reference to a subclass reference.
If you know a variable of type Object actually holds a SavingsAcount reference, you can cast
SavingsAccount acct1 = (SavingsAccount) obj;
If obj refers to an object of an unrelated type, “class cast” exception is thrown.
public class BankAccountDemo{
public static void main(String args[]) {
BankAccount bankAccount = new SavingsAccount(10000,2.50);
Object object = new SavingsAccount(10000,2.50);
object = new String("Hello");
SavingsAccount savingsAccount = (SavingsAccount)object; }
*******************output**********************************************
* > run BankAccountDemo
java.lang.ClassCastException: java.lang.String cannot be cast to SavingsAccount
at BankAccountDemo.main(BankAccountDemo.java:9)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at edu.rice.cs.drjava.model.compiler.JavacCompiler.runCommand(JavacCompiler.java:272)
>
**********************************************************************

67. Example : legal cast Cast obj to subclass BankAccount
SavingsAccount
Since obj is pointing to a SavingsAccount object it can ONLY be safely casted to:
SavingsAccount
BankAccount OR
Object
Same class of object type (SavingAccount)
Direct/Indirect superclass of object type (SavingAccount)

68. Example: illegal cast

69. The instanceof Operator
The instanceof operator tests whether an object belongs to a particular type.
obj instanceof BankAccount
Using the instanceof operator, a safe cast can be programmed as follows:
if (obj instanceof BankAccount) {
BankAccount acct1 = (BankAccount) obj; }
public class BankAccountDemo{
public static void main(String args[]) {
Object object = new SavingsAccount(10000,2.50);
if(object instanceof SavingsAccount) {
SavingsAccount savingsAccount = (SavingsAccount)object;
System.out.println("object instanceof SavingsAccount"); }
if(object instanceof BankAccount) {
BankAccount bankAccount = (BankAccount)object;
System.out.println("object instanceof BankAccount");
if(object instanceof Object) {
System.out.println("object instanceof Object");
*********************output*******************
Welcome to DrJava. Working directory is /net/people/faculty/cs/rdissanayaka/CS160/s15/lec05
> run BankAccountDemo
object instanceof SavingsAccount
object instanceof BankAccount
object instanceof Object
*********************************************

70. Example Direct/indirect superclasses = TRUE Same class= TRUE
Direct/indirect subclasses = FALSE
Any other class = FALSE

71. Question
Assuming that x is an object reference, what is the value of x instanceof Object?

72. Answer Answer: The value is false if x is null and true otherwise.

73. When x is null
When x is not null