cis3023: programming fundamentals for cis majors ii summer 2010 viswanathan inheritance and...

CIS3023: Programming Fundamentals for CIS Majors IISummer 2010

Viswanathan

Inheritance and Polymorphism

Course Lecture Slides2nd June 2010

“ We are all gifted. That is our inheritance. ” --Ethel Waters

Ganesh

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 class or a parent class).

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

}

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;}

Inheritance and Methods

public class Main { public static void main(String[] args){ GraduateStudent s1 = new GraduateStudent(); s1.print(); }}

GraduateStudent class inherits all the attributes and the print( ) method

> java MainN/A N/A 0.0

Output:

public class Student { public String name = “”; public String major = “”; public 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() {

System.out.println(name + “ ” + major + “ ” + gpa);System.out.println(ugDegree + “ ” + ugInst);

}}

Method Overriding

public class GraduateStudent extends Student{ private String ugDegree; private String ugInstitution; public void print() {

super.print();System.out.println(ugDegree + “ ” + ugInst);

}}

public class Student { public String name = “”; public String major = “”; public float gpa = 0.0f; public void print() { System.out.println( name + “ ” + major + “ ” + gpa); }}

Method Overriding

Method Overriding

• Can change the implementation of the method but cannot change the method signature.

• If you change the signature of a method -- this results in method overloading.

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) {

// code to print the graduate student information // to the file called ‘filename’

}}

Method Overloading

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’

}}

public class Student { private String name = “”; private String major = “”; private float gpa = 0.0f; public void print() { System.out.println( name + “ ” + major + “ ” + gpa); }}

Inheritance and Methods

public class Main { public static void main(String[] args){ GraduateStudent s1 = new GraduateStudent(); s1.print(); // legal? }}

GraduateStudent class inherits all the attributes and the print( ) method

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!

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

Accessibility Summary

Modifier on members in a class

Accessed from the same class

Accessed from the same package

Accessed from a subclass

Accessed from a different package

public

protected -

default - -

private - - -

Inheritance and Constructors

Constructors are not inherited

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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!}

Inheritance and ConstructorsWe have to explicitly create constructors in the subclass

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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”); }

Inheritance and ConstructorsWe have to explicitly create constructors in the subclass

17


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(“?”);}}



• 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!


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setName(n); setSsn(s); } // no other constructor is defined // other details not shown here


setName(n); setSsn(s); setUgDegree(“?”); setUgInst(“?”);}

}


Error!

Student.java:19: cannot find symbolsymbol : constructor Std()location: class Std public GradStudent(String n, String s) { ^1 error

// line #19


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setName(n); setSsn(s); } // no other constructor is defined // other details not shown here


super(n,s); setUgDegree(“?”); setUgInst(“?”);}

}


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}

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 StudentJack is a Graduate Student

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}

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 symbolsymbol : method print()location: class Student studentBody[i].print(); // line #15. will it work? ^1 error

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

Casting between Superclass and Subclass

Implicit casting

Student z = new GraduateStudent();

Explicit casting

GraduateStudent gz = (GraduateStudent)z;

Class HierarchyOver time, we build up an inverted ‘tree’ of

classes that are interrelated through inheritance

parent class, superclass, ancestor;

child class, subclass, descendant

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

public class Circle { ... }

Equivalent public class Circle extends Object { ... }

Some Methods from Object class

• String toString()• boolean equals(Object object)• Class getClass()• Object clone()

The toString() method

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); }}

• Intended to return a string representation of

the object.

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( ));

Student@126b249Output:

Example

public class Main {

public static void main(String[] args) {

Course c1 = new Course(“Java Prog. II”, “CIS 3023”, 1);


// 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…}

public class Main {




// 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…}

Example

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 {





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; }}

}

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public class Main {





boolean b = (c1.equals(c2));

}

}



public boolean equals(Object o) { if(o instanceof Course) { if(courseNo == o.getCourseNo()) // is it ok?

return true;else

return false; } else {

return false; }}

}

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public boolean equals(Object o) { if(o instanceof Course) { Course c = (Course)o;

if(courseNo == c.getCourseNo()) // now ok!return true;

elsereturn false;

} else {return false; }

}}

public class Main {






}

}

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public class Main {


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


}

}



public boolean equals(Object o) { if(o instanceof Course) { Course c = (Course)o;

if(courseNo == c.getCourseNo()&& courseName.equals(c.getCourseName()))return true;

elsereturn false;

} else {return false;

}}

}

Comparing Strings• You have used the the equals() method of the String class to compare strings.

String s1 = new String("Hello World!");String s2 = new String("Hello World!");

if(s1 == s2) System.out.println("true");else System.out.println("false"); // will print false

if (s1.equals(s2)) System.out.println("true");else System.out.println("false"); // will print true

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

The clone() Method

• takes no arguments and returns a copy of the calling object.

Example:

int[] targetArray = (int[]) sourceArray.clone();

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.

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 { ...}

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

http://home.cogeco.ca/~ve3ll/jatutor5.htm



http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Object.html

http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Object.html

http://www.horstmann.com/corejava.html

cis3023: programming fundamentals for cis majors ii summer 2010 viswanathan inheritance and...

Documents

public class student

public string major

graduatestudent class

private string major

derived class

public float gpa

f public void print

extended class