Chapter 13. Inheritance and Polymorphism. CS 180 Sunil Prabhakar Department of Computer Science Purdue University

Transcription

1 Chapter 13 Inheritance and Polymorphism CS 180 Sunil Prabhakar Department of Computer Science Purdue University

2 Introduction Inheritance and polymorphism are key concepts of Object Oriented Programming. Inheritance facilitates the reuse of code. A subclass inherits members (data and methods) from all its ancestor classes. The subclass can add more functionality to the class or replace some functionality that it inherits. Polymorphism simplifies code by automatically using the appropriate method for a given object. Polymorphism also makes it easy to extend code. 2

3 Inheritance A superclass corresponds to a general class, and a subclass is a specialization of the superclass. E.g. Account, Checking, Savings. Behavior and data common to the subclasses is often available in the superclass. E.g. Account number, owner name, data opened. Each subclass provides behavior and data that is relevant only to the subclass. E.g. Minimum balance for checking a/c, interest rate and computation for savings account. The common behavior is implemented once in the superclass and automatically inherited by the subclasses. 3

4 Defining Classes with Inheritance Case Study: Suppose we want implement a class roster that contains both undergraduate and graduate students. Each student s record will contain his or her name, three test scores, and the final course grade. The formula for determining the course grade is different for graduate students than for undergraduate students. 4

5 Modeling Two Types of Students There are two ways to design the classes to model undergraduate and graduate students. We can define two unrelated classes, one for undergraduates and one for graduates. We can model the two kinds of students by using classes that are related in an inheritance hierarchy. Two classes are unrelated if they are not connected in an inheritance relationship. NOTE: all classes inherit from Object 5

6 Classes for the Class Roster For the Class Roster sample, we design three classes: Student UndergraduateStudent GraduateStudent The Student class will incorporate behavior and data common to both UndergraduateStudent and GraduateStudent objects. The UndergraduateStudent class and the GraduateStudent class will each contain behaviors and data specific to their respective objects. 6

7 Creating a subclass We use the keyword extends in the class header to declare that it is a subclass: public class GraduateStudent extends Student { Student is the superclass, parent, or base class. A parent (of a parent ) is an ancestor class. GraduateStudent is the subclass, child, or derived class. A child (of a child...) is a descendent class. If a class does have the extend keyword, then it derives from the Object class. 7

8 Example: Student class Student { protected final static int NUM_OF_TESTS = 3; protected String name; protected int[] test; protected String coursegrade; public Student( ) { this("no Name"); public Student(String studentname) { name = studentname; test = new int[num_of_tests]; coursegrade = "****"; public String getcoursegrade( ) { return coursegrade; public String getname( ) { return name; public int gettestscore(int testnumber) { return test[testnumber-1]; public void setname(string newname) { name = newname; public void settestscore(int testnumber, int testscore) { test[testnumber-1] = testscore; public void computecoursegrade(); 8

9 Example: GraduateStudent class GraduateStudent extends Student{ public void computecoursegrade() { int total = 0; for (int i = 0; i < NUM_OF_TESTS; i++) { total += test[i]; if (total / NUM_OF_TESTS >= 80) { coursegrade = "Pass"; else { coursegrade = "No Pass"; 9

10 Example: UndergraduateStudent class UndergraduateStudent extends Student{ public void computecoursegrade() { int total = 0; for (int i = 0; i < NUM_OF_TESTS; i++) { total += test[i]; if (total / NUM_OF_TESTS >= 70) { coursegrade = "Pass"; else { coursegrade = "No Pass"; 10

11 Inheritance Hierarchy Student New visibility modifier: # protected + NUM_OF_TESTS # name # test # coursegrade + Student(): void + Student(String): void + getcoursegrade(): String + getname(): String + gettestscore(int): int + setname(string): void + settestscore(int, int): void UndergraduateStudent GraduateStudent + computecoursegrade(): void + computecoursegrade(): void 11

12 Overriding All non-private members of a class are inherited by derived classes This includes instance and class members A derived class may however, override an inherited method Data members can also be overridden but should be avoided since it only creates confusion. To override a method, the derived class simply defines a method with the same name, number and types of parameters (same signature) An overridden method cannot change the return type! A subclass may overload any method (inherited or otherwise) by using the same name, but different signature. 12

13 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

14 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

15 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

16 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

17 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

18 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); sub.methoda(i); sub.methoda(c); sup.methoda(c); overloaded Sup methoda(int i){ Sub methoda(char c){ 13

19 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

20 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

21 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; Sup methoda(int i){ sup.methoda(); sup.methoda(i); sub.methoda(); sub.methoda(i); sub.methoda(c); sup.methoda(c); overridden Sub methoda(char c){ 13

22 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

23 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

24 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); sub.methoda(i); sub.methoda(c); sup.methoda(c); inherited Sup methoda(int i){ Sub methoda(char c){ 13

25 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

26 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

27 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; Sup methoda(int i){ sup.methoda(); sup.methoda(i); sub.methoda(); sub.methoda(i); sub.methoda(c); sup.methoda(c); overridden & overloaded Sub methoda(char c){ 13

28 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

29 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

30 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Overloaded & Auto cast Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

31 Overriding and overloading Sup sup = new Sup(); Sub sub = new Sub(); int i=9; char c = 'c'; sup.methoda(); sup.methoda(i); sub.methoda(); Sup methoda(int i){ Sub methoda(char c){ sub.methoda(i); sub.methoda(c); sup.methoda(c); 13

32 Limiting inheritance and overriding If a class is declared to be final, then no other classes can derive from it. public final class ClassA If a method is declared to be final, then no derived class can override this method. A final method can be overloaded in a derived class though. public final void methoda() 14

33 The protected Modifier The modifier protected makes a data member or method visible and accessible only to instances of the class and descendant classes. public data members and methods are accessible to everyone. private data members and methods are accessible only to instances of the class. protected is similar to: public for descendant classes private for any other class 15

34 Inheritance and Member Accessibility We use the following visual representation of inheritance to illustrate data member accessibility. public protected private Accessible Inaccessible 16

35 Inheritance and Member Accessibility We use the following visual representation of inheritance to illustrate data member accessibility. public protected private Super Accessible Inaccessible Sub Class Hierarchy 16

36 Inheritance and Member Accessibility We use the following visual representation of inheritance to illustrate data member accessibility. public protected private Super :Super Instances Accessible Inaccessible Sub :Sub :Super This shows the inherited components of the superclass are part of the subclass instance Class Hierarchy 16

37 The Effect of the Visibility Modifiers public protected private :Super Accessible Inaccessible :Client :Sub :Super Accessibility from a Client method 17

38 The Effect of the Visibility Modifiers public protected private :Super Accessible Inaccessible :Client :Sub :Super Accessibility from a Client method Only public members, those defined for the class and those inherited, are visible from outside. All else is hidden from the outside. 17

39 The Effect of the Visibility Modifiers public protected private :Super Accessible Inaccessible :Client :Sub :Super Accessibility from a Client method Only public members, those defined for the class and those inherited, are visible from outside. All else is hidden from the outside. 17

40 The Effect of the Visibility Modifiers public protected private :Super Accessible Inaccessible :Client Accessibility from a Client method :Sub :Super Only public members, those defined for the class and those inherited, are visible from outside. All else is hidden from the outside. 17

41 The Effect of the Visibility Modifiers public protected private :Super Accessible Inaccessible :Client Accessibility from a Client method Only public members, those defined for the class and those inherited, are visible from outside. All else is hidden from the outside. :Sub :Super 17

42 The Effect of the Visibility Modifiers public protected private :Client :Super Accessible Inaccessible Accessibility from a Client method Only public members, those defined for the class and those inherited, are visible from outside. All else is hidden from the outside. :Sub :Super 17

43 Accessibility of Super from Sub public protected private Accessibility from a method of the Sub class :Super :Sub 18

44 Accessibility of Super from Sub public protected private Accessibility from a method of the Sub class :Super :Sub From a method of the Sub, everything is visible except the private members of its super class. 18

45 Accessibility of Super from Sub public protected private Accessibility from a method of the Sub class :Super :Sub From a method of the Sub, everything is visible except the private members of its super class. 18

46 Accessibility of Super from Sub public protected private Accessibility from a method of the Sub class From a method of the Sub, everything is visible except the private members of its super class. :Super :Sub 18

47 Accessibility from Another Instance Data members accessible from an instance are also accessible from other instances of the same class. one:classa two:classa This could be private, Protected, or public. 19

48 Inheritance and Constructors Unlike members of a superclass, constructors of a superclass are not inherited by its subclasses. As always, each class that does not define a constructor is automatically given a default constructor. In addition, in each constructor of a derived class, we must make a call to the constructor of the base class by calling: super(); This must be the first statement in the constructor. If this statement is not present, the compiler automatically adds it as the first statement. You may optionally call some other constructor of the base class, e.g.: super( some string ); 20

49 Constructors and inheritance The constructor for each class can be used to initialize the variables defined in that class. For all classes, calls to the constructors are chained all the way back to the constructor for the Object class. Recall that it is also possible to call another constructor of the same class using the this keyword. However, this must also be the first statement of the constructor! A constructor cannot call another constructor of the same class and the base class. 21

50 Constructors Sup public Sup(){ public Sup(int i){ Sub public Sub(){ this( x ); public Sub(char c){ public Sub(int i){ super(i); 22

51 Constructors Sup public Sup(){ public Sup(int i){ Sup public Sup(){ super(); public Sup(int i){ super(); Sub public Sub(){ this( x ); public Sub(char c){ public Sub(int i){ super(i); Sub public Sub(){ this( x ); public Sub(char c){ super(); public Sub(int i){ super(i); Added by the compiler 22

52 Constructors Sup public Sup(){ public Sup(int i){ Sub public Sub(){ this( x ); public Sub(char c){ public Sub(int i){ super(i); Sup sup1, sup2; Sub sub1, sub2, sub3; sup1 = new Sup(); sup2 = new Sup(7); sub1 = new Sub(); sub2 = new Sub( y ); sub3 = new Sub(5); Sup public Sup(){ super(); public Sup(int i){ super(); Sub public Sub(){ this( x ); public Sub(char c){ super(); public Sub(int i){ super(i); Added by the compiler 22

53 Super keyword The super keyword is a call to the constructor of the parent class. It can also be used to call a method of the parent class: super.methoda(); This can be useful to call an overridden method. Similarly, it can be used to access data members of the parent. 23

54 super keyword example. Sup methoda(int i){ Sub methodb(){ methoda(); this.methoda(); super.methoda(); methoda(7); methoda( x ); 24

55 super keyword example. Sup methoda(int i){ Sub methodb(){ methoda(); this.methoda(); super.methoda(); methoda(7); methoda( x ); 24

56 super keyword example. Sup methoda(int i){ Sub methodb(){ methoda(); this.methoda(); super.methoda(); methoda(7); methoda( x ); 24

57 super keyword example. Sup methoda(int i){ Sub methodb(){ methoda(); this.methoda(); super.methoda(); methoda(7); methoda( x ); 24

58 super keyword example. Sup methoda(int i){ Sub methodb(){ methoda(); this.methoda(); super.methoda(); methoda(7); methoda( x ); 24

59 super keyword example. Sup methoda(int i){ Sub methodb(){ methoda(); this.methoda(); super.methoda(); methoda(7); methoda( x ); 24

60 Polymorphism Polymorphism allows a single variable to refer to objects from different subclasses in the same inheritance hierarchy For example, if Cat and Dog are subclasses of Pet, then the following statements are valid: 25

61 Polymorphism Polymorphism allows a single variable to refer to objects from different subclasses in the same inheritance hierarchy For example, if Cat and Dog are subclasses of Pet, then the following statements are valid: Pet mypet; mypet = new Dog();... mypet = new Cat(); 25

62 Creating the roster Array We can maintain our class roster using an array, combining objects from the Student, UndergraduateStudent, and GraduateStudent classes. Student roster = new Student[40];... roster[0] = new GraduateStudent(); roster[1] = new UndergraduateStudent(); roster[2] = new UndergraduateStudent();... 26

63 State of the roster Array The roster array with elements referring to instances of GraduateStudent or UndergraduateStudent classes. 27

64 State of the roster Array The roster array with elements referring to instances of GraduateStudent or UndergraduateStudent classes. 27

65 Sample Polymorphic Message To compute the course grade using the roster array, we execute 28

66 Sample Polymorphic Message To compute the course grade using the roster array, we execute for (int i = 0; i < numberofstudents; i++) { roster[i].computecoursegrade(); If roster[i] refers to a GraduateStudent, then the computecoursegrade method of the GraduateStudent class is executed. If roster[i] refers to an UndergraduateStudent, then the computecoursegrade method of the UndergraduateStudent class is executed. 28

67 Dynamic Binding At compile time, it is not known which version of a polymorphic method will get executed This is determined at run-time depending upon the class of the object This is called dynamic (late) binding Each object of a subclass is also an object of the superclass. But not vice versa! Do not confuse dynamic binding with overloaded methods. 29

68 Object Type Consider the inheritance hierarchy: Object A B An object of class B is also an object of classes A and Object. Thus we can use objects of class B wherever we can use objects of class A. The reverse is not true. A reference of type A can refer to an object of type B. However if we want to access the functionality of B on that object, we have to type cast to type B before doing that. 30

69 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); 31

70 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); 31

71 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub 31

72 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub :Sub 31

73 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub :Sub 31

74 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub :Sub 31

75 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub :Sub 31

76 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

77 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

78 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

79 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

80 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

81 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

82 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

83 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

84 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

85 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ sub1.methodb(); sub sup :Sub 31

86 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); Sub methodb(){ Note: sup.methodb( ) will not compile. sub sup :Sub 31

87 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); Sub methodb(){ Note: sup.methodb( ) will not compile. sub sup :Sub 31

88 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); Sub methodb(){ Note: sup.methodb( ) will not compile. sub sup :Sub 31

89 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ Note: sup.methodb( ) will not compile. sub1.methodb(); sub sup sub1 :Sub 31

90 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); Sub methodb(){ Note: sup.methodb( ) will not compile. sub1.methodb(); sub sup sub1 :Sub 31

91 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); sub sup sub1 Sub methodb(){ Note: sup.methodb( ) will not compile. Casting to Sub will work, but a runtime exception (ClassCastException) will be thrown if the object is not really a Sub object. :Sub 31

92 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); sub sup sub1 Sub methodb(){ Note: sup.methodb( ) will not compile. Casting to Sub will work, but a runtime exception (ClassCastException) will be thrown if the object is not really a Sub object. :Sub 31

93 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); sub sup sub1 Sub methodb(){ Note: sup.methodb( ) will not compile. Casting to Sub will work, but a runtime exception (ClassCastException) will be thrown if the object is not really a Sub object. :Sub 31

94 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); sub sup sub1 Sub methodb(){ Note: sup.methodb( ) will not compile. Casting to Sub will work, but a runtime exception (ClassCastException) will be thrown if the object is not really a Sub object. :Sub 31

95 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); sub sup sub1 Sub methodb(){ Note: sup.methodb( ) will not compile. Casting to Sub will work, but a runtime exception (ClassCastException) will be thrown if the object is not really a Sub object. :Sub 31

96 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); sub sup sub1 Sub methodb(){ Note: sup.methodb( ) will not compile. Casting to Sub will work, but a runtime exception (ClassCastException) will be thrown if the object is not really a Sub object. :Sub 31

97 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); sub sup sub1 Sub methodb(){ Note: sup.methodb( ) will not compile. Casting to Sub will work, but a runtime exception (ClassCastException) will be thrown if the object is not really a Sub object. :Sub 31

98 Polymorphism example Sup Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodB(); Sub sub1 = (Sub)sup; sub1.methoda(); sub1.methodb(); sub sup sub1 Sub methodb(){ Note: sup.methodb( ) will not compile. Casting to Sub will work, but a runtime exception (ClassCastException) will be thrown if the object is not really a Sub object. :Sub 31

99 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

100 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

101 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

102 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

103 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

104 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

105 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

106 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

107 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

108 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

109 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

110 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

111 Example Sub sub = new Sub(); Sup sup; sup = sub; sup.methoda(); ((Sub)sup).methodA(); sub = (Sub)sup; sub.methoda(); Sup methoda(string[] s){ Sub methoda(int i){ sub.methoda( test ); 32

112 The instanceof Operator The instanceof operator can help us discover the class of an object at runtime. The following code counts the number of undergraduate students. new undergradcount = 0; for (int i = 0; i < numberofstudents; i++) { if ( roster[i] instanceof UndergraduateStudent ) { undergradcount++; 33

113 Abstract Superclasses and Methods When we define a superclass, we often do not need to create any instances of the superclass. Depending on whether we need to create instances of the superclass, we must define the class differently. We will study examples based on the Student superclass defined earlier. 34

114 Definition: Abstract Class An abstract class is a class defined with the modifier abstract OR that contains an abstract method OR that does not provide an implementation of an inherited abstract method An abstract method is a method with the keyword abstract, and it ends with a semicolon instead of a method body. Private methods and static methods may not be declared abstract. No instances can be created from an abstract class. 35

115 Case 1 A Student must be Undergraduate or Graduate If a student must be either an undergraduate or a graduate student, we only need instances of UndergraduateStudent or GraduateStudent. Therefore, we should define the Student class so that no instances may be created of it. 36

116 Example: Abstract classes abstract class Student { protected final static int NUM_OF_TESTS = 3; protected String name; protected int[] test; protected String coursegrade; public Student( ) { this("no Name"); public Student(String studentname) { name = studentname; test = new int[num_of_tests]; coursegrade = "****"; public String getcoursegrade( ) { return coursegrade; public String getname( ) { return name; public int gettestscore(int testnumber) { return test[testnumber-1]; public void setname(string newname) { name = newname; public void settestscore(int testnumber, int testscore) { test[testnumber-1] = testscore; 37

117 Example: Abstract classes abstract class Student { protected final static int NUM_OF_TESTS = 3; protected String name; protected int[] test; protected String coursegrade; public Student( ) { this("no Name"); public Student(String studentname) { name = studentname; test = new int[num_of_tests]; coursegrade = "****"; public String getcoursegrade( ) { return coursegrade; public String getname( ) { return name; public int gettestscore(int testnumber) { return test[testnumber-1]; public void setname(string newname) { name = newname; public void settestscore(int testnumber, int testscore) { test[testnumber-1] = testscore; class UndergraduateStudent extends Student { public void computecoursegrade() { int total = 0; for (int i = 0; i < NUM_OF_TESTS; i++) { total += test[i]; if (total / NUM_OF_TESTS >= 70) { coursegrade = "Pass"; else { coursegrade = "No Pass"; 37

118 Example: Abstract classes abstract class Student { protected final static int NUM_OF_TESTS = 3; protected String name; protected int[] test; protected String coursegrade; public Student( ) { this("no Name"); public Student(String studentname) { name = studentname; test = new int[num_of_tests]; coursegrade = "****"; public String getcoursegrade( ) { return coursegrade; public String getname( ) { return name; public int gettestscore(int testnumber) { return test[testnumber-1]; public void setname(string newname) { name = newname; public void settestscore(int testnumber, int testscore) { test[testnumber-1] = testscore; class UndergraduateStudent extends Student { public void computecoursegrade() { int total = 0; for (int i = 0; i < NUM_OF_TESTS; i++) { total += test[i]; if (total / NUM_OF_TESTS >= 70) { coursegrade = "Pass"; else { coursegrade = "No Pass"; class GraduateStudent extends Student { public void computecoursegrade() { int total = 0; for (int i = 0; i < NUM_OF_TESTS; i++) { total += test[i]; if (total / NUM_OF_TESTS >= 80) { coursegrade = "Pass"; else { coursegrade = "No Pass"; 37

119 Abstract example (contd.) Non-private members of the abstract parent class are inherited. Note: constructors are not inherited! Default constructor calls super! public class Test { public static void main(string[ ] args){ Student s; GraduateStudent g; UndergraduateStudent u; s = new Student(); g = new GraduateStudent( John ); g = new GraduateStudent(); u = new UndergraduateStudent(); System.out.println(g.getName()); System.out.println(u.getName()); 38

120 Abstract example (contd.) Non-private members of the abstract parent class are inherited. Note: constructors are not inherited! Default constructor calls super! public class Test { public static void main(string[ ] args){ Student s; GraduateStudent g; UndergraduateStudent u; Cannot instantiate abstract class. s = new Student(); g = new GraduateStudent( John ); g = new GraduateStudent(); u = new UndergraduateStudent(); System.out.println(g.getName()); System.out.println(u.getName()); 38

121 Abstract example (contd.) Non-private members of the abstract parent class are inherited. Note: constructors are not inherited! Default constructor calls super! public class Test { public static void main(string[ ] args){ Student s; GraduateStudent g; UndergraduateStudent u; Cannot instantiate abstract class. s = new Student(); g = new GraduateStudent( John ); g = new GraduateStudent(); u = new UndergraduateStudent(); System.out.println(g.getName()); System.out.println(u.getName()); Error: constructor not inherited! 38

122 Abstract example (contd.) Non-private members of the abstract parent class are inherited. Note: constructors are not inherited! Default constructor calls super! public class Test { public static void main(string[ ] args){ Student s; GraduateStudent g; UndergraduateStudent u; Cannot instantiate abstract class. s = new Student(); g = new GraduateStudent( John ); g = new GraduateStudent(); u = new UndergraduateStudent(); System.out.println(g.getName()); System.out.println(u.getName()); Error: constructor not inherited! Inherited from abstract parent class. 38

123 Case 2 Student does not have to be Undergraduate or Graduate. In this case, we may design the Student class in one of two ways. We can make the Student class instantiable. We can leave the Student class abstract and add a third subclass, OtherStudent, to handle a student who does not fall into the UndergraduateStudent or GraduateStudent categories. 39

124 Which Approach to Use? The best approach depends on the particular situation. When considering design options, we can ask ourselves which approach allows easier modification and extension. 40

125 Inheritance versus Interface The Java interface is used to share common behavior (only method headers) among the instances of different classes. Inheritance is used to share common code (including both data members and methods) among the instances of related classes. In your program designs, remember to use the Java interface to share common behavior. Use inheritance to share common code. If an entity A is a specialized form of another entity B, then model them using inheritance. Declare A as a subclass of B. 41

126 Interface vs inheritance Interface Inheritance Data members? No Yes Methods? Only headers -- no body Yes Keyword implements extends Multiple? Yes No Instantiable? No Yes (if not abstract) An interface essentially provides compliance with some desired behavior. Inheritance allows sharing of code. 42

127 Interfaces A class does not extend an interface, instead it implements an interface. Implementing implies that the class must provide the bodies for all the methods specified in the interface i.e. with the same signatures & return types Remember: GUI interfaces ActionListener 43

128 Introduction to Data Structures A data structure is a specific organization of data for efficiency or ease of coding. E.g. an array allows us to manage a large number of similar items. Many different types of data structures are used in programming. CS251 deals only with this topic! We will look at two example data structures Linked list Queue 44

129 Linked List Recall that an array s capacity is fixed once it is created or initialized. The LinkedList class in java.util used a linked list to implement a variable size list of objects. How does this work? We will study it by creating our own version of the LinkedList class which can store a list of objects of any class, in order. 45

130 Objective The linked list will allow us to Create an empty list. Add items to the end of the list. Delete items from the end of the list. Iterate through the list from beginning to end. Note: no position indexing in this version. Our linked list will be implemented as a chain of Node objects. Each Node object will have An Object data member that is the value stored at that position. A reference to the next node in the list. 46

131 The Node Class class Node { private Node next; private Object content; public void Node() { next = null; content = null; public Object getcontent(){ return content; public void setcontent(object c){ content = c; public Node getnext(){ return next; public void setnext(node nextnode){ next = nextnode; :Node next content 47

132 The LinkedList class class LinkedList { private Node head; private Node iterator; public void LinkedList() { head = null; iterator = null; public void addtohead(object c){ Node n = new Node(); n.setcontent(c); n.setnext(head); head = n; public void deletefromhead throws Exception (){ if(head== null) throw new Exception( Empty List ); else head = head.getnext(); public void getfromhead throws Exception (){ if(head== null) throw new Exception( Empty List ); else return head.getcontent(); public void startscan throws Exception (){ if(head == null) throw new Exception( Empty List ); else iterator = head; public boolean hasmore(){ if(iterator.next == null) return false; else return true; public void moveahead(){ iterator = iterator.getnext(); public Object getcurrentitem throws Exception (){ if(iterator == null) throw new Exception( No Current Item ); return iterator.getcontent(); 48

133 Example LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); 49

134 Example list LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); 49

135 Example list :LinkedList head iterator LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); 49

136 Example list :LinkedList head iterator LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); s 49

137 Example list :LinkedList head iterator test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); s 49

138 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); s 49

139 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); s 49

140 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); s 50

141 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); test2 s 50

142 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; list.addtohead(s); s = test3 ; list.addtohead(s); test2 s 50

143 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; :Node content next test2 s list.addtohead(s); s = test3 ; list.addtohead(s); 50

144 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; :Node content next test2 s list.addtohead(s); s = test3 ; list.addtohead(s); 50

145 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; :Node content next test2 s list.addtohead(s); s = test3 ; list.addtohead(s); 51

146 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; :Node content next test2 s list.addtohead(s); s = test3 ; list.addtohead(s); test3 51

147 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; :Node content next test2 s list.addtohead(s); s = test3 ; list.addtohead(s); test3 51

148 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; :Node content next test2 s list.addtohead(s); s = test3 ; list.addtohead(s); :Node content test3 next 51

149 Example list :LinkedList head iterator :Node content next test1 LinkedList list; list = new LinkedList(); String s; s = test1 ; List.addToHead(s); s = test2 ; :Node content next test2 s list.addtohead(s); s = test3 ; list.addtohead(s); :Node content test3 next 51

150 Example list :LinkedList head iterator :Node content next test1 :Node list.startscan(); s = (String) list.getcurrentitem(); System.out.println(s); while(list.hasmore()){ list.moveahead(); s = (String) list.getcurrentitem(); System.out.println(s); content next :Node content next test2 test3 s 52

151 Example list :LinkedList head iterator :Node content next test1 :Node list.startscan(); s = (String) list.getcurrentitem(); System.out.println(s); while(list.hasmore()){ list.moveahead(); s = (String) list.getcurrentitem(); System.out.println(s); content next :Node content next test2 test3 s 52

152 Example list :LinkedList head iterator :Node content next test1 :Node list.startscan(); s = (String) list.getcurrentitem(); System.out.println(s); while(list.hasmore()){ list.moveahead(); s = (String) list.getcurrentitem(); System.out.println(s); content next :Node content next test2 test3 s 52

153 Example list :LinkedList head iterator :Node content next test1 :Node list.startscan(); s = (String) list.getcurrentitem(); System.out.println(s); while(list.hasmore()){ list.moveahead(); s = (String) list.getcurrentitem(); System.out.println(s); content next :Node content next test2 test3 s 52

154 Example list :LinkedList head iterator :Node content next test1 :Node list.startscan(); s = (String) list.getcurrentitem(); System.out.println(s); while(list.hasmore()){ list.moveahead(); s = (String) list.getcurrentitem(); System.out.println(s); content next :Node content next test2 test3 s 52

155 Example list :LinkedList head iterator :Node content next test1 :Node list.startscan(); s = (String) list.getcurrentitem(); System.out.println(s); while(list.hasmore()){ list.moveahead(); s = (String) list.getcurrentitem(); System.out.println(s); content next :Node content next test2 test3 s 52

156 Example list :LinkedList head iterator :Node content next test1 :Node list.startscan(); s = (String) list.getcurrentitem(); System.out.println(s); while(list.hasmore()){ list.moveahead(); s = (String) list.getcurrentitem(); System.out.println(s); content next :Node content next test2 test3 s 52

157 Example list :LinkedList head iterator :Node content next test1 :Node content next test2 list.deletefromhead(); :Node content next test3 53

158 Example list :LinkedList head iterator :Node content next test1 :Node content next test2 list.deletefromhead(); :Node content next test3 53

159 Example list :LinkedList head iterator :Node content next test1 :Node content next test2 list.deletefromhead(); test3 53

160 Example list :LinkedList head iterator :Node content next test1 :Node content next test2 list.deletefromhead(); 53

161 Queue A similar Node class can be used to implement another data structure called a First-In-First-out (FIFO) queue. A FIFO queue is often used by operating systems for processes, requests, etc. In a FIFO queue, items are added at one end and deleted from the other end. For this we need to have pointers going in both directions: Node2 54

162 The Node2 Class class Node2 { private Node2 next, prev; private Object content; public void Node2() { next = null; prev = null; content = null; public Object getcontent(){ return content; public void setcontent(object c){ content = c; public Node2 getnext(){ return next; public void setnext(node2 nextnode){ next = nextnode; public Node getprev(){ return prev; public void setprev(node2 prevnode){ prev = prevnode; :Node2 next content prev 55

163 The FifoQ class class FifoQ { private Node2 head, tail; public void FifoQ() { head = null; tail = null; public void addtoq(object c){ Node2 n = new Node2(); n.setcontent(c); n.setnext(head); head.setprev(n); head = n; public void deletefromq throws Exception (){ if(tail== null) throw new Exception( Empty Queue ); if(tail.getprev() == null){ tail = null; head = null; else { tail = tail.getprev(); tail.setnext(null); 56

164 Example FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); 57

165 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); 57

166 Example q :FifoQ FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); tail head 57

167 Example q :FifoQ FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); tail head test1 s 57

168 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev test1 s 57

169 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev test1 s 57

170 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev test1 s 57

171 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev test1 s 58

172 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev test1 test2 s 58

173 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev :Node2 next content prev test1 test2 s 58

174 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev :Node2 next content prev test1 test2 s 58

175 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev :Node2 next content prev test1 test2 s 58

176 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev :Node2 next content prev test1 test2 59

177 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev :Node2 next content prev test1 test2 59

178 Example q FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); :FifoQ tail head :Node2 next content prev :Node2 next content prev test1 test2 59

179 Example q :FifoQ FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); tail head :Node2 next content prev test2 59

180 Example q :FifoQ FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); tail head :Node2 next content prev test2 60

181 Example q :FifoQ FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); tail head :Node2 next content prev test2 60

182 Example q :FifoQ FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); tail head :Node2 next content prev test2 60

183 Example q :FifoQ FifoQ q; q = new FifoQ(); String s; s = test1 ; q.addtoq(s); s = test2 ; q.addtoq(s); while(!q.isempty()) q.deletefromq(); tail head 60

184 Problem Statement Write an application that reads in a text file organized in the manner shown below and displays the final course grades.the course grades are computed differently for the undergraduate and graduate students based on the formulas listed on page 710. The input text file format is as follows: A single line is used for information on one student. Each line uses the format <Type> <Name> <Test 1> <Test 2> <Test 3> where <Type> designates either a graduate or an undergraduate student,<name> designates the student s first and last name, and <Test i> designates the ith test score. of the End of input is designated by the word END. The case letters is insignificant. 61

185 Overall Plan Tasks Read an input file Compute the course grades Print out the result Input file format: <Type> <Name> <Test 1> <Test 2> <Test 3> U John Doe G Jill Jones G Jack Smith U Mary Hines U Mick Taylor END 62

186 Development Steps We will develop this program in five steps: 1. Start with the program skeleton.define the skeleton ComputeGrades classes. 2. Implement the printresult method.define any other methods necessary to implement printresult. 3. Implement the computegrade method.define any other methods necessary to implement computegrade. 4. Implement the readdata method.define any other methods necessary to implement readdata. 5. Finalize and look for improvements. 63

187 Step 1 Design We start with a program skeleton. We will define two constructors so the programmer can create a roster of default size or the size of her choice. 64

188 Step 1 Code Program source file is too big to list here. From now on, we ask you to view the source files using your Java IDE. Directory: Chapter13/Step1 Source Files: ComputeGrades.java 65

189 Step 1 Test We include a temporary output statement inside the (currently stub) method we define. We run the test main class and verify that the methods are called correctly. 66

190 Step 2 Design We design and implement the printresult method We use the helper class OutputBox for displaying the result. for each element i in the roster array { output the name of roster[i]; output the test scores of roster[i]; output the course grade of roster[i]; skip to the next line; 67

191 Step 2 Code Directory: Chapter13/Step2 Source Files: ComputeGrades.java 68

192 Step 2 Test We verify the temporary readdata method is working correctly. This confirms that we are using the correct student classes and using their methods correctly. We verify the printresult method does indeed display the data in our desired format. 69

193 Step 3 Design 70

194 Step 3 Design We design and implement the computegrade method. 70

195 Step 3 Design We design and implement the computegrade method. The code for actually determining the course grade is embedded in individual student classes 70

196 Step 3 Design We design and implement the computegrade method. The code for actually determining the course grade is embedded in individual student classes So the code to add to the ComputeGrades class is very simplistic. 70

197 Step 3 Design We design and implement the computegrade method. The code for actually determining the course grade is embedded in individual student classes So the code to add to the ComputeGrades class is very simplistic. This is a direct benefit of using polymorphism effectively. 70

198 Step 3 Code Directory: Chapter13/Step3 Source Files: ComputeGrades.java 71

199 Step 3 Test We will repeat the same test routines from Step 2. Instead of seeing four asterisks, we should be seeing the correct grades. We test both the passing and not passing test scores. 72

200 Step 4 Design We design and implement the core functionality of the program the readdata method We can express its logic as get the filename from the user; if (the filename is provided) read in data and build the roster array; else output an error message; 73

201 The buildroster Method set bufreader for input; while (!done ) { line = get next line; if (line is END) { done = true; else { student = createstudent( line ); if (student!= null) { roster[studentcount] = student; //add to roster studentcount++; 74

202 The buildroster Method The logic of the workhorse private method buildroster is as follows: set bufreader for input; while (!done ) { line = get next line; if (line is END) { done = true; else { student = createstudent( line ); if (student!= null) { roster[studentcount] = student; //add to roster studentcount++; 74

203 The createstudent Method StringTokenizer parser = new StringTokenizer( line ); String type; try { type = parser.nexttoken(); if (type.equals(under_grad) type.equals(grad)) { student = newstudentwithdata(type, parser); else { //invalid type is encountered student = null; catch (NoSuchElementException e) { //no token student = null; return student; 75

204 The createstudent Method We use the StringTokenizer class to break down items in a single line of input StringTokenizer parser = new StringTokenizer( line ); String type; try { type = parser.nexttoken(); if (type.equals(under_grad) type.equals(grad)) { student = newstudentwithdata(type, parser); else { //invalid type is encountered student = null; catch (NoSuchElementException e) { //no token student = null; return student; 75

205 Step 4 Code Directory: Chapter13/Step4 Source Files: ComputeGrades.java 76

206 Step 4 Test 77

207 Step 4 Test We run through a more complete testing routine in this step.we need to run the program for various types of input files. Some of the possible file contents are as follows: 77