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CIS3023: Programming Fundamentals for CIS Majors II Summer 2010

CIS3023: Programming Fundamentals for CIS Majors II Summer 2010. Inheritance and Polymorphism. “ We are all gifted. That is our inheritance. ” --Ethel Waters . Course Lecture Slides 2 nd June 2010. Ganesh. Viswanathan. Inheritance.

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CIS3023: Programming Fundamentals for CIS Majors II Summer 2010

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  1. CIS3023: Programming Fundamentals for CIS Majors II Summer 2010 Inheritance and Polymorphism “ We are all gifted. That is our inheritance. ” --Ethel Waters Course Lecture Slides2nd June 2010 Ganesh Viswanathan

  2. Inheritance • Inheritance is the capability of a class to use the properties and methods of another class while adding its own functionality. • A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base classor a parent class).

  3. Inheritance • It allows us to define a general class and then specialize it simply by adding more attributes and adding/changing methods • Syntax: class <SpecialClass> extends<GeneralClass> { // new attributes // new or changed methods }

  4. public class Student { private String name = “N/A”; private String ssn = “N/A”; private float gpa = 0.0f; // constructors, get/set methods not shown here public void print() { System.out.println( name + “ " + ssn + " " + gpa); } } public class GraduateStudent extends Student{ private String ugDegree; private String ugInst; }

  5. Inheritance and Methods GraduateStudent class inherits all the attributes and the print() method public class Main { public static void main(String[] args){ GraduateStudent s1 = new GraduateStudent(); s1.print(); } } Output: > java Main N/A N/A 0.0

  6. public class Student { public String name = “”; public String major = “”; public float gpa = 0.0f; public void print() { System.out.println( name + “ ” + major + “ ” + gpa); } } MethodOverriding public classGraduateStudent extends Student{ private String ugDegree; private String ugInstitution; public void print() { System.out.println(name + “ ” + major + “ ” + gpa); System.out.println(ugDegree + “ ” + ugInst); } }

  7. public class Student { public String name = “”; public String major = “”; public float gpa = 0.0f; public void print() { System.out.println( name + “ ” + major + “ ” + gpa); } } MethodOverriding public classGraduateStudent extends Student{ private String ugDegree; private String ugInstitution; public void print() { super.print(); System.out.println(ugDegree + “ ” + ugInst); } }

  8. Method Overriding • Can change the implementation of the methodbut cannot change the method signature. • If you change the signature of a method -- this results in method overloading.

  9. public class Student { private String name = “”; private String major = “”; private float gpa = 0.0f; public void print() { System.out.println( name + “ ” + major + “ ” + gpa); } } Method Overloading public class GraduateStudent extends Student{ private String ugDegree; private String ugInstitution; public void print(String filename) { // code to print the graduate student information // to the file called ‘filename’ } }

  10. public class Student { private String name = “”; private String major = “”; private float gpa = 0.0f; public void print() { System.out.println( name + “ ” + major + “ ” + gpa); } } public class GraduateStudent extends Student{ private String ugDegree; private String ugInstitution; public void print(String filename) { int sName = name; // code to print the graduate student information // to the file called ‘filename’ } }

  11. Inheritance and Methods GraduateStudent class inherits all the attributes and the print() method public class Main { public static void main(String[] args){ GraduateStudent s1 = new GraduateStudent(); s1.print(); // legal? } }

  12. public class Student { private String name; private String studentId; private String majorField; private double gpa; private int credits; // Other details omitted. } public class GraduateStudent extends Student { String ugDegree; String ugInstitution; public void UpdateGpa(int c, int gr) { gpa += (gpa*credits + c*gr)/(credits + c); credits += c; } } Inheritance and Visibility Compiler Error because gpa and credits are private and hence can not be accessed directly!

  13. public class Student { private String name; private String studentId; private String majorField; protected double gpa; protected int credits; // Other details omitted. } public class GraduateStudent extends Student { String ugDegree; String ugInstitution; public void UpdateGpa(int c, int gr) { gpa += (gpa*credits + c*gr)/(credits + c); credits += c; } } Inheritance and Visibility Now, there is no Compiler Error here

  14. Accessibility Summary

  15. Inheritance and Constructors • Constructors are not inherited public class Student { protected String name; protected String ssn; public Student(String n, String s) { setName(n); setSsn(s); } public Student(){ this(“?”, “?”); } // other details not shown here } class GradStudent extends Student{ String ugDegree; String ugInst; // no constructor defined } static void Main(string[] args){ Student s = new Student("Joe“, “111-11-1111”); //ok! Student g = new GradStudent("Fred“, “111-11-1111”); // NOT OK! }

  16. Inheritance and Constructors • We have to explicitly create constructors in the subclass public class Student { protected String name; protected String ssn; public Student(String n, String s) { setName(n); setSsn(s); } public Student(){ this(“?”, “?”); } // other details not shown here } class GradStudent extends Student{ String ugDegree; String ugInst; public GradStudent(String n, String s){ setName(n); setSsn(s); setUgDegree(“?”); setUgInst(“?”);} } static void main(string[] args){ Student s = new Student("Joe“, “111-11-1111”); GradStudent s = new GradStudent("Fred“, “111-11-1111”); }

  17. Inheritance and Constructors • We have to explicitly create constructors in the subclass public class Student { protected String name; protected String ssn; public Student(String n, String s) { setName(n); setSsn(s); } public Student(){ this(“?”, “?”); } // other details not shown here } class GradStudent extends Student{ String ugDegree; String ugInst; public GradStudent(String n, String s){ super(n, s); setUgDegree(“?”); setUgInst(“?”);} } static void main(string[] args){ Student s = new Student("Joe“, “111-11-1111”); GradStudent s = new GradStudent("Fred“, “111-11-1111”); }

  18. Inheritance and Constructors • whenever a constructor for a subclass is called, constructor(s) for all of its ancestor class(es) are also called, either explicitly or by default – • in top-to-bottom order. • This is called Constructor Chaining!

  19. Inheritance and Constructors Student.java:19: cannot find symbol symbol : constructor Std() location: class Std public GradStudent(String n, String s) { ^ 1 error public class Student { protected String name; protected String ssn; public Student(String n, String s) { setName(n); setSsn(s); } // no other constructor is defined // other details not shown here } class GradStudent extends Student{ String ugDegree; String ugInst; public GradStudent(String n, String s){ setName(n);setSsn(s); setUgDegree(“?”); setUgInst(“?”);} } static void main(string[] args){ Student s = new Student("Joe“, “111-11-1111”); GradStudent s = new GradStudent("Fred“, “111-11-1111”); } // line #19 Error!

  20. Inheritance and Constructors public class Student { protected String name; protected String ssn; public Student(String n, String s) { setName(n); setSsn(s); } // no other constructor is defined // other details not shown here } class GradStudent extends Student{ String ugDegree; String ugInst; public GradStudent(String n, String s){ super(n,s); setUgDegree(“?”); setUgInst(“?”);} } static void main(string[] args){ Student s = new Student("Joe“, “111-11-1111”); GradStudent s = new GradStudent("Fred“, “111-11-1111”); } Woohoo! Fixed!

  21. public class Student { private String name = “”; private String ssn = “”; private String major = “”; private float gpa = 0.0f; public Student(String n) { setName(n);} public void print() { System.out.print(name); } // other details not shown here } public class GraduateStudent extends Student{ private String ugDegree; private String ugInst; public GraduateStudent(String n, String ug, String uI) { super(n); setUgDegree(ug); setUgInst(uI); } public void print() { super.print(); System.out.println(“ is a Graduate Student”); } // other details not shown here } public class UnderGraduateStudent extends Student{ private String highSchool; public UnderGraduateStudent(String n, String h) { super(n); highSchool = h; } public void print() { super.print(); System.out.println((“ is a UndergraduateStudent”); } // other details not shown here }

  22. static void main(string[] args) { Student[] studentBody = new Student[20]; UnderGraduateStudent u1 = new UnderGraduateStudent(“John”,“GNV High”); GraduateStudent g1 = new GraduateStudent(“Jack”, “CIS”, “UF”); studentBody[0] = u1; studentBody[1] = g1; for (int i = 0; i < 20; i++) if(studentBody[i]!= null) studentBody[i].print(); } Output: John is a Undergraduate Student Jack is a Graduate Student

  23. Polymorphism • The ability of two or more objects belonging to different classes to respond to exactly the same message in different class-specific ways. • It illustrates ‘late binding’.

  24. public class Student { protected String name = “”; private String ssn = “”; private String major = “”; private float gpa = 0.0f; public Student(String n) { setName(n);} // no print() method // other details not shown here } public class GraduateStudent extends Student{ private String ugDegree; private String ugInst; public GraduateStudent(String n, String ug, String uI) { super(n); setUgDegree(ug); setUgInst(uI); } public void print() { System.out.println(name + “ is a Graduate Student”); } // other details not shown here } public class UnderGraduateStudent extends Student{ private String highSchool; public UnderGraduateStudent(String n, String h) { super(n); highSchool = h; } public void print() { System.out.println(name + “ is a UndergraduateStudent”); } // other details not shown here }

  25. static void main(string[] args) { Student[] studentBody = new Student[20]; UnderGraduateStudent u1 = new UnderGraduateStudent(“John”,“GNV High”); GraduateStudent g1 = new GraduateStudent(“Jack”, “CIS”, “UF”); studentBody[0] = u1; studentBody[1] = g1; for (int i = 0; i < 20; i++) if(studentBody[i]!= null) studentBody[i].print(); // line #15. will it work? } Driver.java:15: cannot find symbol symbol : method print() location: class Student studentBody[i].print(); // line #15. will it work? ^ 1 error

  26. Inheritance: "Is A" Relationship • If GraduateStudent is a subclass of Student, then a GraduateStudent IS A Student • Anything that we say about a Student's behavior or data structure MUST also be true of a GraduateStudent

  27. Casting between Superclass and Subclass • Implicit casting Student z = new GraduateStudent(); Explicit casting GraduateStudent gz = (GraduateStudent)z;

  28. Class Hierarchy • Over time, we build up an inverted ‘tree’ of classes that are interrelated through inheritance parent class, superclass, ancestor; child class, subclass, descendant

  29. The Root of all Classes: Object Every class in Java has descended from the java.lang.Object class. If no inheritance is specified when a class is defined, the superclass of the class is java.lang.Object class

  30. Some Methods from Object class StringtoString() booleanequals(Object object) ClassgetClass() Objectclone()

  31. The toString() method • Intended to return a string representation of the object. public class Student { private String name = “”; private String ssn = “”; private String major = “”; private float gpa = 0.0f; public Student(String n) { name = n;} public String toString() { return(name + “ “ + ssn + “ “ + major + “ “ + gpa); } }

  32. The toString() method The default implementation returns a string consisting of a class name of which the object is an instance, the at sign (@), and a number representing this object. Student s = new Student(); System.out.println(s.toString()); Output: Student@126b249

  33. Example public class Main { public static void main(String[] args) { Course c1 = new Course(“Java Prog. II”, “CIS 3023”, 1); Course c2 = new Course(“Java Prog. II”, “CIS 3023”, 2); // Find out if both c1 and c2 are same course? // How can I do it? } } class Course { private String courseName; private int courseNo; private String offering; private int sectionNo; // other details not shown here… }

  34. Example public class Main { public static void main(String[] args) { Course c1 = new Course(“Java Prog. II”, “CIS 3023”, 1); Course c2 = new Course(“Java Prog. II”, “CIS 3023”, 1); // Find out if both c1 and c2 are same? boolean b = (c1 == c2); // will this work? System.out.println(“Is c1==c2? ” + b); } } class Course { private String courseName; private int courseNo; private String offering; private int sectionNo; // other details not shown here… }

  35. The equals() method intends to compare the contents (“values”) of two objects default implementation compares object references override equals() in a class to define when two objects of the that class will be considered as “equal”

  36. public class Main { public static void main(String[] args) { Course c1 = new Course(“Java Prog. II”, “CIS 3023”, 1); Course c2 = new Course(“Java Prog. II”, “CIS 3023”, 2); // Find out if both c1 and c2 are same? boolean b = (c1.equals(c2)); } } class Course { private String courseName; private int courseNo; private String offering; private int sectionNo; // other details not shown here… public boolean equals(Object o) { if(o instanceof Course) { if(courseNo == o.getCourseNo()) return true; else return false; } else { return false; } } } 36

  37. public class Main { public static void main(String[] args) { Course c1 = new Course(“Java Prog. II”, “CIS 3023”, 1); Course c2 = new Course(“Java Prog. II”, “CIS 3023”, 2); // Find out if both c1 and c2 are same? boolean b = (c1.equals(c2)); } } class Course { private String courseName; private int courseNo; private String offering; private int sectionNo; // other details not shown here… public boolean equals(Object o) { if(o instanceof Course) { if(courseNo == o.getCourseNo()) // is it ok? return true; else return false; } else { return false; } } } 37

  38. class Course { private String courseName; private int courseNo; private String offering; private int sectionNo; // other details not shown here… public boolean equals(Object o) { if(o instanceof Course) { Course c = (Course)o; if(courseNo == c.getCourseNo()) // now ok! return true; else return false; } else { return false; } } } public class Main { public static void main(String[] args) { Course c1 = new Course(“Java Prog. II”, “CIS 3023”, 1); Course c2 = new Course(“Java Prog. II”, “CIS 3023”, 2); // Find out if both c1 and c2 are same? boolean b = (c1.equals(c2)); } } 38

  39. public class Main { public static void main(String[] args) { Course c1 = new Course(“Java Prog. II”, 3023, 1); Course c2 = new Course(“Java Prog. II”, 3023, 2); // Find out if both c1 and c2 are same boolean b = (c1.equals(c2)); } } class Course { private String courseName; private int courseNo; private String offering; private int sectionNo; // other details not shown here… public boolean equals(Object o) { if(o instanceof Course) { Course c = (Course)o; if(courseNo == c.getCourseNo() && courseName.equals(c.getCourseName())) return true; else return false; } else { return false; } } } 39

  40. Comparing Strings You have used the the equals() method of the String class to compare strings. String s1 = newString("Hello World!"); String s2 = newString("Hello World!"); if(s1 == s2) System.out.println("true"); elseSystem.out.println("false");// will print false if (s1.equals(s2)) System.out.println("true"); elseSystem.out.println("false");// will print true

  41. The getClass() Method public class Main { public static void main(String[] args) { Course c = new Course(“Java Prog. II”, “CIS 3023”, 1); System.out.println( c.getClass() ); } } Output: class Course

  42. The clone() Method takes no arguments and returns a copy of the calling object. Example: int[] targetArray = (int[]) sourceArray.clone();

  43. Can we call the clone method on any object? The clone() Method No! Only objects of classes that implement the Cloneable interface can be cloned • Further material on interfaces coming up… a little later.

  44. The final Modifier can be used to prevent classes from being extended or a method from being overridden. Declaring a final class: The final method cannot be overridden by its subclasses. final class Math { ... }

  45. Get more info! Inheritance and Polymorphism: http://home.cogeco.ca/~ve3ll/jatutor5.htm Java Class hierarchy: http://java.sun.com/docs/books/tutorial/java/IandI/subclasses.html The Object Class in Java: http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Object.html A Good book! http://www.horstmann.com/corejava.html

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