Extending classes with inheritance
Learning objectives
By the end of this lecture you should be able to:
• explain the term inheritance;
• design inheritance structures using UML notation;
• implement inheritance relationships in Java;
• distinguish between method overriding and method overloading;
• explain the term type cast and implement this in Java;
• explain the use of the abstract modifier and the final modifier, when applied to both classes and methods;
• describe the way in which all Java classes are derived from the Object class.
Software Reuse
Employee
Full-time Employee
Part-time Employee
BankAccount
Interest Account
Defining inheritance
Inheritance is the sharing of attributes and methods among classes;
The inheritance relationship is also often referred to as an is-a-kind-of relationship;
PartTimeEmployee is a kind of Employee
InterestAccount is a kind of BankAccount
Car is a kind of Vehicle
Monitor is not a kind of Computer
UML notation for inheritance
Employee
number : String name : String
Employee(String, String) setName(String) getNumber() : String getName() : String
PartTimeEmployee
hourlyPay : double
PartTimeEmployee(String, String, double) setHourlyPay(double) getHourlyPay() :double calculateWeeklyPay(int) :double
superclass
subclass
base class
derived class
public class Employee{ private String number; private String name;
public Employee(String numberIn, String nameIn) { number = numberIn; name = nameIn; } public void setName(String nameIn) { name = nameIn; } public String getNumber() { return number; } public String getName() { return name; }}
public class PartTimeEmployee extends Employee { private double hourlyPay; public PartTimeEmployee(String numberIn, String nameIn, double hourlyPayIn) { super(numberIn, nameIn); hourlyPay = hourlyPayIn; } public double getHourlyPay() { return hourlyPay; } public void setHourlyPay(double hourlyPayIn) { hourlyPay = hourlyPayIn; } public double calculateWeeklyPay(int noOfHoursIn) { return noOfHoursIn * hourlyPay; }}
public class PartTimeEmployeeTester{
public static void main(String[] args) {
String number, name; double pay; int hours; PartTimeEmployee emp; System.out.print("Employee Number? "); number = EasyScanner.nextString(); System.out.print("Employee's Name? "); name = EasyScanner.nextString(); System.out.print("Hourly Pay? "); pay = EasyScanner.nextDouble(); System.out.print("Hours worked this week? "); hours = EasyScanner.nextInt();
emp = new PartTimeEmployee(number, name, pay); System.out.println();
System.out.println(emp.getName()); System.out.println(emp.getNumber()); System.out.println(emp.calculateWeeklyPay(hours));
}}
public class PartTimeEmployeeTester{
public static void main(String[] args) {
String number, name; double pay; int hours; PartTimeEmployee emp; System.out.print("Employee Number? "); number = EasyScanner.nextString(); System.out.print("Employee's Name? "); name = EasyScanner.nextString(); System.out.print("Hourly Pay? "); pay = EasyScanner.nextDouble(); System.out.print("Hours worked this week? "); hours = EasyScanner.nextInt();
emp = new PartTimeEmployee(number, name, pay); System.out.println();
System.out.println(emp.getName()); System.out.println(emp.getNumber()); System.out.println(emp.calculateWeeklyPay(hours));
}}
RUN
Employee Number?
Walter WallcarpetingA103456310.0
Employee's Name?
Hourly Pay?
Hours worked this week?
A103456
Walter Wallcarpeting
15.50
20
Extending the Oblong class
6.5
9.2
Oblong myOblong = new Oblong(6.5, 9.2);
******************************************************
ExtendedOblong
symbol : char
ExtendedOblong(double, double, char) setSymbol(char) draw() : String
Oblong
length : doubleheight : double
Oblong(double, double)setLength(double)setHeight(double)getLength() : doublegetHeight() : doublecalculateArea() : doublecalculatePerimeter() : double
public class ExtendedOblong extends Oblong{ private char symbol;
public ExtendedOblong (double lengthIn, double heightIn, char symbolIn) { super(lengthIn, heightIn); symbol = symbolIn; }
public void setSymbol(char symbolIn) { symbol = symbolIn; }
public String draw() {
// code to produce String goes here }}
The new-line character
************
**** **** ****<NEW LINE> <NEW LINE>‘\n’ ‘\n’
for (int j = 1; j <= length; j++){ s = s + symbol; }s = s + '\n';
for (int i = 1; i <= height; i++){ }
<NEW LINE>‘\n’
public String draw(){ String s ; int length, height; length = getLength(); height = getHeight(); for (int i = 1; i <= height; i++) { for (int j = 1; j <= length; j++) { s = s + symbol; } s = s + '\n'; } return s; }}
= “ “;= new String();
(int)(int)
public class ExtendedOblongTester
{
public static void main(String[] args)
{
ExtendedOblong extOblong = new ExtendedOblong(10,5,'*');
System.out.println(extOblong.draw());
extOblong.setSymbol('+');
System.out.println(extOblong.draw());
}
}
RUN
**************************************************
++++++++++++++++++++++++++++++++++++++++++++++++++
Method overriding
Customer
name : StringtotalMoneyPaid : doubletotalGoodsReceived : double
Customer(String)getName() : StringgetTotalMoneyPaid() : doublegetTotalGoodsReceived() : doublecalculateBalance() : doublerecordPayment(double)dispatchGoods(double) : boolean
GoldCustomer
creditLimit : double
GoldCustomer(String, double)getCreditLimit() : doublesetCreditLimit(double)dispatchGoods(double) : boolean
public class Customer
{
protected String name;
protected double totalMoneyPaid;
protected double totalGoodsReceived;
public Customer(String nameIn)
{
name = nameIn;
totalMoneyPaid = 0;
totalGoodsReceived = 0;
}
// more methods go here
}
public String getName(){ return name;} public double getTotalMoneyPaid(){ return totalMoneyPaid;} public double getTotalGoodsReceived(){ return totalGoodsReceived;} public double calculateBalance(){ return totalMoneyPaid - totalGoodsReceived;}
public void recordPayment(double paymentIn)
{
totalMoneyPaid = totalMoneyPaid + paymentIn;
}
public boolean dispatchGoods(double goodsIn)
{
if(calculateBalance() >= goodsIn)
{
totalGoodsReceived = totalGoodsReceived + goodsIn;
return true;
}
else
{
return false;
}
}
public class GoldCustomer extends Customer{ private double creditLimit; public GoldCustomer(String nameIn, double limitIn) { super(nameIn); creditLimit = limitIn; } public void setCreditLimit(double limitIn) { creditLimit = limitIn; } public double getCreditLimit() { return creditLimit; } // code for dispatchGoods}
public boolean dispatchGoods(double goodsIn)
{
if((calculateBalance() + creditLimit) >= goodsIn)
{
totalGoodsReceived = totalGoodsReceived + goodsIn;
return true;
}
else
{
return false;
}
}
Customer firstCustomer = new Customer("Jones");
GoldCustomer secondCustomer = new GoldCustomer("Cohen", 500);
// more code here
firstCustomer.dispatchGoods(98.76);
secondCustomer.dispatchGoods(32.44);
Effect of overriding methods
Abstract classes and methods
draw()
abstract draw()
draw()
Shape
Circle Triangle
abstract Shape
Employee
number : String name : String
Employee(String, String) setName(String) getNumber() : String getName() : String getStatus() : String
FullTimeEmployee
annualSalary : double
FullTimeEmployee(String, String, double) setAnnualSalary(double) getAnnualSalary() : double calculateMonthlyPay () : double getStatus() : String
PartTimeEmployee
hourlyPay : double
PartTimeEmployee(String,String, double) setHourlyPay(double) getHourlyPay() : double calculateWeeklyPay(int) : double getStatus() : String
An Example
public abstract class Employee{ // attributes as before // methods as before abstract public String getStatus(); }
public class PartTimeEmployee extends Employee{ // code as before public String getStatus() { return "Part-Time"; }}
public class FullTimeEmployee extends Employee
{
private double annualSalary;
public FullTimeEmployee(String numberIn, String nameIn, double salaryIn)
{
super(numberIn,nameIn);
annualSalary = salaryIn;
}
// other methods here
}
public void setAnnualSalary(double salaryIn){ annualSalary = salaryIn;}
public double getAnnualSalary(){ return annualSalary;}public double calculateMonthlyPay(){ return annualSalary/12;}
public String getStatus(){ return "Full-Time";}
Inheritance and Types Employee e ;
PartTimeEmployee p ; FullTimeEmployee f ;
getStatus()
abstract getStatus()
getStatus()
Employee
PartTimeEmployee FullTimeEmployee
public class StatusTester{ public static void tester(Employee employeeIn) { System.out.println(employeeIn.getStatus()); }}
public class RunStatusTester
{
public static void main(String[] args)
{
FullTimeEmployee fte = new FullTimeEmployee("100", "Patel", 30000);
PartTimeEmployee pte = new PartTimeEmployee("101", "Jones", 12);
StatusTester.tester(fte);
StatusTester.tester(pte);
}
}
RUN
Full-TimePart-Time
The final modifier
final double PI = 3.1417; public final class SomeClass{
// code goes here}
public final void someMethod(){ // code goes here}
SomeClass
someMethod()
AnotherClass
someMethod()
final SomeClass
final someMethod()
The Object class
Employee
PartTimeEmployee FullTimeEmployee
String BankAccount
Object
Generic Arrays
private BankAccount[] list; This array can only hold BankAccount objects
private Employee[] list; This array can only hold Employee objects
private Object[] list; This array can only hold any type of objects
Bank
EmployeeList
ObjectList
Generic Methods - add
public boolean add( BankAccount itemIn){ if (!isFull()) { list[total] = itemIn; total++; return true; } else { return false; }}
Object
objectList.add( new BankAccount(“007”, “James Bond”) ) ;
Generic Methods - getItem
public BankAccount getItem(int positionIn){ if(positionIn < 1 || positionIn > total) { return null; } else { return list[positionIn - 1]; }}
Object
BankAccount myAccount = objectList.getItem(3);
System.out.println(myAccount.getBalance());
(BankAccount) objectList.getItem(3);
Wrapper classes
Q How could you use an array of Objects to store a simple type such as an int or a char - or to pass such a type to a method that expects an Object?
A Use a wrapper class.
Integer
hidden int
Integer(int)
getValue():int
Character
hidden char
Character(char)
getValue():char
Double
hidden double
Double(double)
getValue():double
objectList.add( new Integer( 37)) ;
Autoboxing
Object[] anArray = new Object[20];
anArray[0] = new Integer(37);
Java 5.0 allows us to make use of a technique known as autoboxing:
anArray[0] = 37;
Unboxing
Integer intObject = (Integer) anArray[0];int x = intObject.getValue();
Java 5.0 allows us to make use of a technique known as unboxing:
int x = (Integer) anArray[0];
A mixed list
Third item
Second item
First item
Part-timeemployee
Full-timeemployee
Full-timeemployee
Employee[] employeeList = new Employee[3];
public class MixedListTester{ public static void main(String[] args) { Employee[] employeeList = new Employee[3]; String num, name; double pay; char status; for(int i = 0; i < employeeList.length; i++) { System.out.print("Enter the employee number: "); num = EasyScanner.nextString(); System.out.print("Enter the employee's name: "); name = EasyScanner.nextString(); System.out.print("<F>ull-time or <P>art-time? "); status = EasyScanner.nextChar(); // more code here}
if(status == 'f' || status == 'F') { System.out.print("Enter the annual salary: "); } else { System.out.print("Enter the hourly pay: "); } pay = EasyScanner.nextDouble(); if(status == 'f' || status == 'F') { employeeList[i] = new FullTimeEmployee(num, name, pay); } else { employeeList[i] = new PartTimeEmployee(num, name, pay); } System.out.println();} // end of loop
for(Employee item : employeeList)
{
System.out.println("Employee number: " + item.getNumber());
System.out.println("Employee name: " + item.getName());
System.out.println("Status: " + item.getStatus());
System.out.println();
}
public class MixedListTester{ public static void main(String[] args) { Employee[] employeeList = new Employee[3]; // declare local variables to hold values entered by user String num, name; double pay; char status; for(int i = 0; i < employeeList.length; i++) { System.out.print("Enter the employee number: "); num = EasyScanner.nextString(); System.out.print("Enter the employee's name: "); name = EasyScanner.nextString(); System.out.print("<F>ull-time or <P>art-time? "); status = EasyScanner.nextChar(); if(status == 'f' || status == 'F') { System.out.print("Enter the annual salary: "); } else { System.out.print("Enter the hourly pay: "); } pay = EasyScanner.nextDouble(); if(status == 'f' || status == 'F') { employeeList[i] = new FullTimeEmployee(num, name, pay); } else { employeeList[i] = new PartTimeEmployee(num, name, pay); } System.out.println(); } for(Employee item : employeeList) { System.out.println("Employee number: " + item.getNumber()); System.out.println("Employee name: " + item.getName()); System.out.println("Status: " + item.getStatus()); System.out.println(); } }}
RUN
Enter the employee number: 1
Enter the employee's name: Jones
<F>ull-time or <P>art-time? f
Enter the annual salary: 30000
Enter the employee number: 2
Enter the employee's name: Agdeboye
<F>ull-time or <P>art-time? f
Enter the annual salary: 35000
Enter the employee number: 3
Enter the employee's name: Sharma
<F>ull-time or <P>art-time? p
Enter the hourly pay: 15
RUN
Employee number: 1
Employee name: Jones
Status: Full-Time
Employee number: 2
Employee name: Agdeboye
Status: Full-Time
Employee number: 3
Employee name: Sharma
Status: Part-Time
