1. Inheritance In object-oriented programming, a mechanism called inheritance is used to design two or more entities that are different but share many common features. First we define a class that contains the common features of the entities. Then we define classes as an extension of the common class The common class is called the superclass and all classes that inherit from it subclasses. The superclass is an ancestor of the descendant subclass.

2. Sample Inheritance Hierarchy Account Savings Checking SuperSaver RegularStudent Interest Bearing ATM Checking

3. Defining Classes with Inheritance Suppose we want to model graduate and undergraduate students in maintaining a class roster. For both types of students we keep their name, three test scores, and the final course grade. The formula for deriving the final course grades are different for undergraduate and graduate students. How shall we implement the two types of students?

4. Student with Two Subclasses GraduateStudent computeGrade UndergraduateStudent computeGrade Student setTestScore getTestScore NUM_OF_TESTS 3 courseGrade name test[ ] … We will define three classes: Student GraduateStudent UndergraduateStudent We will define three classes: Student GraduateStudent UndergraduateStudent Implementation of computeGrade is unique to each subclass.

5. Inheritance Subclasses inherit from their superclass Everything that's in the superclass, and not in the subclass, is inherited by the subclass If a subclass defined something that is in the superclass, the subclass elements override the superclass elements Objects created from the subclass consist of the inherited components and any new components defined in the subclass

6. Accessng Superclass Data and Methods Constructors of a subclass can call constructors of their superclass using super() If a subclass constructor does not call the superclass constructor, then the compiler adds a call. Components of a superclass that have been overridden in a subclass can be accessed using super.

7. Example Program Ding … out to reality … Student.java, GraduateStudent.java, UndergraduateStudent.java, UseStudent1.java

8. Inheritance and Member Accessibility Which members of a superclass are accessible from its subclasses? There is a third visibility modifier called protected. public private protected Superclass Subclass

9. Access from the Subclass Methods Public and protected elements of the superclass (objects) are visible to its subclasses (objects). mySub Subclass Superclass inaccessible accessible

10. Access from the Outside Only the public elements are accessible from the outside objects. mySub Subclass Superclass mySuper Superclass Client test

11. Access from Another Instance All elements of an object are accessible from other instances of the same class. anInstance AClass anotherInstance AClass

12. Example Program Dong … out to reality … HouseOccupant.java, Human.java, Pet.java, UseHouseOccupant.java

13. Polymorphism Polymorphism allows a variable to refer to objects from different (but related by inheritance) classes. References to data or methods are correctly resolved according to the object’s class. Requires that the superclass have the inherited data or method Student student; student = new GraduateStudent(); student.computeGrade( ); Student student; student = new UndergraduateStudent(); student.computeGrade( ); This will call the method of GraduateStudent This will call the method of UndergraduateStudent

14. Creating the roster Array Student roster[ ] = new Student[40]; roster is an array of Student objects. Create instances of the subclasses of Student. roster[0] = new GraduateStudent( ); roster[1] = new UndergraduateStudent( ); roster[2] = new UndergraduateStudent( ); roster[3] = new GraduateStudent( ); 0123436373839 roster Graduate- Student Under- graduate- Student Graduate- Student

15. Processing the roster Array for (int i = 0; I < numberOfStudents; i++ ) { roster[i].computeGrade( ); } Use instanceOf to determine the class the object belongs to. int undergradCount = 0; for (int i = 0; i < numberOfStudents; i++ ) { if ( roster[i] instanceOf UndergraduateStudent ) { undergradCount++; } Notice how the polymorphism is used here.

16. Example Programs Dung … out to reality … Student.java, GraduateStudent.java, UndergraduateStudent.java, UseStudent2.java

17. Abstract Classes Often we want to place elements common to all subclasses in their superclass. But we do not want any instances to be created from the superclass. In such case, we designate the superclass as an abstract class. This is also a way to enforce existence of methods in subclasses.

18. Abstract Methods It is common for an abstract class to include an abstract method that must be implemented by the descendant classes. If a class includes an abstract method, then it is considered as an abstract class and must have the abstract modifier in the class declaration.

19. Sample Abstract Class Abstract method has no method body. abstract class Student {... abstract public void computeGrade( );... } Reserved word abstract in the class declaration. class GraduateStudent extends Student {... public void computeGrade( ) { //method body comes here }... } Abstract method is must be fully implemented in the descendant class.

20. Example Programs Deng … out to reality … H2O.java, Ice.java, Water.java, Steam.java, UseH2O.java