More on Inheritance Chapter 11 Continued

Reminders Overloading – different signatures Overriding – same signatures Preventing overriding – use final –public final void message()

Access Specifications public - everyone private – – methods of class – methods of subclass protected –methods of class –methods of subclass –methods of any class in same packagepackage

Special Note IF you do not specify package visibility the default package visibility is directory.

Protected Protected is between public and private Next 2 slides show examples of use.

** This class determines the grade for a final exam. The numeric score is rounded up to the next whole number if its fractional part is.5 or greater. */ public class FinalExam2 extends GradedActivity2 { private int numQuestions; // Number of questions private double pointsEach; // Points for each question private int numMissed; // Number of questions missed public FinalExam2(int questions, int missed) { double numericScore; // To hold a numeric score numQuestions = questions; numMissed = missed; pointsEach = / questions; numericScore = (missed * pointsEach); setScore(numericScore); adjustScore(); // adjusts the score } public double getPointsEach() { return pointsEach; } public int getNumMissed() { return numMissed; } private void adjustScore() { double fraction; fraction = score - (int) score; score = score + (1.0 - fraction); } }

/** A class that holds a grade for a graded activity. */ public class GradedActivity2 { protected double score; // Numeric score /** The setScore method sets the score s The value to store in score. */ public void setScore(double s) { score = s; } /** The getScore method returns the The value stored in the score field. */ public double getScore() { return score; } /** The getGrade method returns a letter grade determined from the score The letter grade. */ public char getGrade() { char letterGrade; if (score >= 90) letterGrade = 'A'; else if (score >= 80) letterGrade = 'B'; else if (score >= 70) letterGrade = 'C'; else if (score >= 60) letterGrade = 'D'; else letterGrade = 'F'; return letterGrade; } }

Access Subtleties IF you do not provide an access specifier for a class member, the class member is given package access by default. Example public class Circle { double radius; int centerX, centerY; // methods omitted here } Any method in the same package as the Circle class may directly access radius, centerX and centerY. Protected members may be accessed by methods in a subclass even if the subclasses are in a different package.

Access Specifiers Summarized Access Modifier Accessible to a subclass inside the same package? Accessible to all other classes inside the same package? default (no modifier) Yes PublicYes ProtectedYes PrivateNo Access Modifier Accessible to a subclass outside the package? Accessible to all other classes outside the package? default (no modifier) No PublicYes ProtectedYesNo PrivateNo

General UML diagram layout Class name goes in this box Fields go in this box Methods are listed here

Access Specification in UML diagrams Class name goes in this box Fields go in this box + indicates public - indicates private # indicates protected Methods are listed here

Specific UML diagrams GradedActivity2 # score : double + setScore(s : double) : void + getScore() : double + getGrade() : char FinalExam2 - numQuestions : int - pointsEach : double - numMissed : int + FinalExam(questions : int, missed : int) : + getPointsEach() : double + getNumMissed() : int - adjustScore() : void

Chains of Inheritance Example See the code in your text and on your CD Code Listings: 11-1 GradedActivity 11-21PassFailActivity 11-22PassFailExam PassFailExamDemo Object PassFailActivity PassFailExam GradedActivity

Polymorphism Skipping this topic for now

Abstract Methods An abstract method appears in a superclass An abstract method has no body, only a header An abstract method must be overriden in a subclass Header example: public abstract void setValue(int value); –NOTE: semicolon is required

Abstract Classes Abstract Classes An abstract class can not be instantiated, i.e. You can NOT create an object of an abstract class. Any class that contains an abstract method is an abstract class. Abstract classes are extended by other classes (i.e. subclasses) Header Example: public abstract class Student

Code Examples from text abstract class Student.java (Code Listing 11-26) subclass CompSciStudent.java (Code Listing 11-27) demo program CompSciStudentDemo.java (Code Listing 11-28)

Interfaces An interface specifies behavior for a class An interface is a class that has all abstract methods. An interface can not be instantiated. All of the methods in an interface must be written elsewhere. Methods in the interface has no bodies only headers terminated by semicolons. Example interface header public interface InterfaceName

Interface example /** Relatable interface */ public interface Relatable { boolean equals(GradedActivity g); boolean isGreater(GradedActivity g); boolean isLess(GradedActivity g); } See Code Listings Relatable.java FinalExam3.java InterfaceDemo.java

Fields & Methods in an Interface An interface can contain field declarations. All fields in an interface are final and static. You must provide an initialization for each of them. By convention, their names should be all upper case. Example interface with fields: public interface Doable { int FIELD1 = 1; int FiELD2 = 2; // method headers go here } All methods in an interface are public by default

Implementing Multiple Interfaces Remember: –A class can extend only one superclass. –Java does not have multiple inheritance. However: –A class my implement multiple interfaces. –When it implements multiple interfaces, it must provide the methods specified by all of them. Example header for a class that implements multiple interfaces public class MyClass implements InterfaceA, InterfaceB, InterfaceC

Interfaces in UML GradedActivity RelatableFinalExam3 A dashed line with an arrow indicates implementation of an interface.