1. University Of Ha’il Chapter 8 Abstract Class

2. Abstract Class • Sometimes, it does NOT make sense to create objects from specific classes • In such case, these classes should be created as abstract • An abstract class can only be used to derive other classes; you cannot create objects from an abstract class • An abstract class must have at least one abstract method

3. Abstract Method • An abstract method has a complete method heading, to which has been added the modifier abstract • It has no method body, and ends with a semicolon in place of its body public abstractlong getSerNumber(); • An abstract method cannot be private • An abstract class can have any number of abstract and/or fully defined methods

4. Abstract Classes (Cont’d) • Attempting to instantiate an object of an abstract class is a compilation error. • Example: abstract class Shape { … } Shape sh = new Shape();

5. Abstract Classes (Cont’d) • Failure to implement a superclass’s abstract methods in a subclass is a compilation error unless the subclass is also declared abstract. • Example: abstract class Shape { public abstract void drawHere(); } class Rectangle extends Shape {// The method drawHere() is NOT implemented! }

6. UML Inheritance Diagrams for Abstract Classes

7. Creating Abstract Superclass: Employee class • abstract superclass Employee • earnings is declared abstract • No implementation can be given for earnings in the Employeeabstract class • An array of Employee variables will store references to subclass objects • earnings method calls from these variables will call the appropriate version of the earnings method

8. The Employee Class Hierarchy

9. Abstract Class Employee: Outline // Employee abstract superclass. public abstract class Employee { private String firstName; private String lastName; private String socialSecurityNumber; // three-argument constructor public Employee(String first, String last, String ssn ) { firstName = first; lastName = last; socialSecurityNumber = ssn; } // end three-argument Employee constructor

10. Abstract Class Employee: Outline (Cont’d) public void setFirstName(String first) { firstName = first; } public String getFirstName() { return firstName; } public void setLastName( String last ) { lastName = last; } public String getLastName() { return lastName; } public void setSocialSecurityNumber( String ssn ) { socialSecurityNumber = ssn; } public String getSocialSecurityNumber() { return socialSecurityNumber; } public abstract double earnings(); // no implementation here } // end abstract class Employee

11. Extending Concrete Classes public class SalariedEmployee extends Employee { private double weeklySalary; … // calculate earnings; override abstract method earnings in Employee public double earnings() { return getWeeklySalary(); } // end method earnings … }

12. Extending Concrete Classes (Cont’d) public class CommissionEmployee extends Employee { private double grossSales; // gross weekly sales private double commissionRate; // commission percentage… // calculate earnings; override abstract method earnings in Employee public double earnings() { return getCommissionRate() * getGrossSales(); } // end method earnings … }

13. Extending Concrete Classes (Cont’d) public class HourlyEmployee extends Employee { private double wage; // wage per hour private double hours; // hours worked for week // calculate earnings; override abstract method earnings in Employee public double earnings() { if ( getHours() <= 40 ) // no overtime return getWage() * getHours(); else return 40 * getWage() + ( getHours() - 40 ) * getWage() * 1.5; } // end method earnings

14. Implementing an Abstract Method public abstract class Animal { protected String kind; // Cow, pig, cat, etc. public Animal() { } public String toString() { return "I am a " + kind + " and I go " + speak(); } public abstract String speak(); // Abstract method } • Define the speak() method as abstract in the superclass Animal. • Implement speak() differently in each subclass. public class Cat extends Animal { public Cat() { kind = "cat"; } public String speak() { return "meow"; } } public class Cow extends Animal { public Cow() { kind = "cow"; } public String speak() { return "moo"; } }