1. Announcements Lab 8 Was Due Today Lab 7 Regrade Due Thursday
(no regrades)
Lab 7 Regrade Due Thursday
No class Friday

2. Today in COMP 110 Possible solution for Program 4
Inheritance recap from last time
More Inheritance

3. Program 4

4. Inheritance Define a general class
Later, define specialized classes based on the general class
These specialized classes inherit properties from the general class

5. Inheritance Hierarchies
Put these classes into an inheritance hierarchy
Crocodile
Human
Reptile
Animal
Mammal
Whale
Animal
Reptile
Mammal
Crocodile
Human
Whale

6. The is-a relationship
This inheritance relationship is known as an is-a relationship
A Doctoral student is a Grad student
A Grad student is a Student
A Student is a Person
Is a Person a Student?
Not necessarily!

7. The Keyword extends
public class Derived_Class_Name extends Base_Class_Name {
Declaration_of_Added_Instance_Variables
Definitions_of_Added_And_Overridden_Methods }
public class Student extends Person {
// stuff goes here
A derived (child) class inherits the public instance variables and public methods of its base (parent) class

8. Overriding Methods Java handles this situation as follows:
If a derived class defines a method with the same name, number and types of parameters, and return type as a method in the base class, the derived class’ method overrides the base class’ method
The method definition in the derived class is the one that is used for objects of the derived class

9. Overriding methods Why is it useful?
You often want derived classes to perform custom behavior
public class Shape {
public void draw(Graphics g) { }
public class Circle extends Shape {
public void draw(Graphics g) {
g.drawOval(…arguments…); }
public class Rectangle extends Shape {
public void draw(Graphics g) {
g.drawRect(…arguments…); }

10. The Keyword super
Last time we saw that the keyword super could be used to invoke the constructor of the base class
public class Person {
private String name;
public Person() {
name = "No name yet"; }
public class Student extends Person {
private int id;
public Student() {
super();
id = -1; }
Student student = new Student(); //name is set to No name yet, and id is set to -1

11. The Keyword super
The use of super must always be the first action taken in the constructor
Not required to be specified, but if it is, it must be first
Java will automatically call the default constructor of the base class if super is not specified
public Student() {
super();
id = -1; }
public Student() {
id = -1; }
public Student() {
id = -1;
super(); } OK OK
ERROR

12. The Keyword super You cannot use repeated supers
If specified, specify only once
public Student() {
super();
id = -1; }
public Student() {
super();
super("No name yet");
id = -1; } OK
ERROR

13. The Keyword super What if the base class is a derived class?
public class Person {
private String name;
public Person() {
name = "No name yet"; }
public class Student extends Person {
private int id;
public Student() {
super();
id = -1; }
public class Undergraduate extends Student {
private int year;
public Undergraduate() {
super(); //calls constructor of Student, not Person
year= 0; }

14. Method Overriding
public class Person {
// ...
public void printInfo() {
System.out.println("Name: " + name); }
public class Student extends Person {
// ...
public void printInfo() {
System.out.println("Name: " + getName());
System.out.println("ID: " + getID()); }
Overridden
Is there a way to call the printInfo() method of the class Person from within the class Student?
Yes

15. Calling Overridden Methods
public class Person {
// ...
public void printInfo() {
System.out.println("Name: " + name); }
public class Student extends Person {
// ...
public void printInfo() {
super.printInfo();
System.out.println("ID: " + getID()); } }
The keyword super can also be used to call overridden method of a base class

16. Inheritance and Overloading
public class Person {
private String name;
public void setData(String newName) {
name = newName; }
public class Student extends Person {
private int id;
public void setData(String newName, int newID) {
setData(newName);
id = newID; }
Overloaded
Methods with a different number or type of parameters are overloaded, not overridden

17. Type Compatibilities Given this inheritance heirarchy… Person Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

18. Is This Code Legal? Person p = new Person(); Yes! Person Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

19. Is This Code Legal? HighJumper h = new HighJumper(); Yes! Person
Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

20. Is This Code Legal? Person p = new Athlete();
Yes! An Athlete is a Person, so this is okay
Person
Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

21. Is This Code Legal? Skydiver s = new Person();
No! A Person is not necessarily a Skydiver, so this is illegal
Person
Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

22. Is This Code Legal?
Athlete ath = new Athlete(); XGamesSkater xgs = ath;
No! An Athlete is not necessarily an XGamesSkater, so this is illegal
Person
Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

23. When to Use Inheritance
Use inheritance when an is-a relationship is present
A Student is a Person, Student inherits from Person
Do not confuse an is-a relationship with a has-a relationship
A Student has a date of enrollment, so an object of class Date, should be an instance variable of the Student class

24. The Class Object
Every class in Java is derived from the class Object (implied)
Every class in Java is an Object
Object
Animal
Person
Reptile
Mammal
Student
Employee
Crocodile
Human
Whale

25. Dynamic Binding
Person person = new Person("John Smith");
person.printlnfo(); //calls printInfo of Person class
Student student = new Student("John Smith", );
student.printlnfo(); //calls printInfo of Student class
Person p = new Student(“Tom Jones", );
p.printInfo(); //which printInfo method is called?
The printInfo method of the Student class is called because p is really a student!

26. The Operator instanceof
We can test whether an object is of a certain class type using the instanceof operator:
public void objectType(Person p) {
if(p instanceof Student) {
System.out.println("p is an instance of the class Student"); }
else if(p instanceof GradStudent) {
System.out.println("p is an instance of the class GradStudent"); …
Syntax:
object instanceof Class_Name

27. The Class Object
The Java class Object provides methods that are inherited by every class
For example
equals, toString
These methods should be overridden with methods appropriate for the classes you create

28. Printing Objects to Screen
The method println is overloaded to accept objects as an argument
System.out.println(int i)
System.out.println(double d)
System.out.println(Object o)
When printing an object with System.out.println(Object o), the object’s toString() method is called and the result is printed
public void println(Object o) {
System.out.println(o.toString()); //dynamic binding }

29. Overriding toString public class Person { private String name;
public Person() {
name = "No name yet"; }
public void setName(String newName) {
name = newName;
//converts a Person object to a String, overrides toString in the class Object
public String toString() {
return "Name: " + name + "\n";
public static void main(String[] args) {
Person person = new Person();
person.setName("John Smith");
System.out.println(person); //prints "Name: John Smith" to screen (without quotes)

30. Overriding toString public class Student extends Person {
private int id;
public Student() {
super();
id = -1; }
public void setID(String newName) {
name = newName;
//converts a Person object to a String, overrides toString in the class Person
public String toString() {
return "Name: " + name + "\nID: " + id + "\n";
public static void main(String[] args) {
Student student = new Student();
student.setName("John Smith");
student.setID( );
System.out.println(student); //prints "Name: John Smith"
// "ID: " to screen (without quotes)

31. Programming Demo Programming with inheritance
Sports
Define the following classes
Person
Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

32. Sports Each class has a method called jump Person Athlete HighJumper
Simply prints a message to screen
Person
System.out.println("Whee!");
Athlete
System.out.println("I jump well!");
HighJumper
System.out.println("I jump REALLY well!");
ExtremeAthlete
System.out.println("EXTREMEEEEEEEEEE JUMP!");
SkyDiver
System.out.println("EXTREMEEEEEEEEEE fall!");
XGamesSkater
System.out.println("I'm doing some sort of 360 or such. Tally-ho.");

33. Sports Write a class called Sports that demonstrates TypeCompatibility
Dynamic binding
Instanceof operator

34. Friday
No recitation, enjoy the Holidays