1. COMP 110 More about inheritance
Michele Weigle - COMP 14 - Spr 04
COMP 110 More about inheritance
Catie Welsh
April 13, 2011

2. Michele Weigle - COMP 14 - Spr 04
Announcements
Program 4 early code check due Friday, April 15th in class
Bring a print out of your code with you – whatever you have so far
Also include a copy of any pseudocode you’ve written out
To earn the 25 points, I’d like to see a significant amount of code written that does 1 of the following
Game board code written – numbers randomly assigned, 2 of each #
Mouse clicking code written so that your shapes are centered in the correct square of the grid
You can NOT use late days for this! I must have a copy of your code by 1pm on Friday!

3. Michele Weigle - COMP 14 - Spr 04
Questions?
Any other questions?

4. Michele Weigle - COMP 14 - Spr 04
Today in COMP 110
Lab 8
Some review
More inheritance

5. Silly exercise time
What’s the point?
Well…

6. The difference between return and printing
Returning a value from a method does NOT print it out to the screen
The returned value is given to the caller of the method
It is up to the method caller to do something with this value

7. The difference between return and printing
public class NumberUtils {
private static Random rnd = new Random();
public static int roll20SidedDie()
return rnd.nextInt(20) + 1; }
public class Tester
public static void main(String[] args)
NumberUtils.roll20SidedDie(); // what does this do?
Generates a random die roll, but it ignores the value returned from the roll20SidedDie method
Have we printed anything to the screen?

8. The difference between return and printing
public class NumberUtils {
private static Random rnd = new Random();
public static int roll20SidedDie()
return rnd.nextInt(20) + 1; }
public class Tester
public static void main(String[] args)
int num = NumberUtils.roll20SidedDie(); // what does this do?
Puts the random number returned by roll20SidedDie into the variable num
Have we printed anything to the screen?

9. The difference between return and printing
public class NumberUtils {
private static Random rnd = new Random();
public static int roll20SidedDie()
return rnd.nextInt(20) + 1; }
public class Tester
public static void main(String[] args)
int num = NumberUtils.roll20SidedDie(); // what does this do?
System.out.println(num);
Puts the random number returned by roll20SidedDie into the variable num
Then prints the value of num to the screen

10. Review: Inheritance
What is a base class?
What is a derived class?

11. Review: Inheritance
We have discussed before how classes of objects can have relationships
Person
Transportation
Student
Employee
Car
Airplane
Animal
Undergrad
Grad
Faculty
Staff
Horse
Elephant
Camel
Masters
Doctoral
Nondegree

12. Put these classes into an inheritance heirarchy
Crocodile
Human
Reptile
Animal
Mammal
Whale
Animal
Reptile
Mammal
Crocodile
Human
Whale

13. Review: 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!

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

15. Review: How does this work in Java?
public class Person { private String name; public Person() name = “No name yet”; } public void setName(String newName) name = newName; public String getName() return name;
Person
- name
+ setName(String newName) : void
+ getName() : String

16. Review: How does this work in Java?
public class Student extends Person { private int id; public Student() super(); id = 0; } public Student(String stdName, int idNumber) setName(stdName); setID(idNumber); public void setID(int idNumber) id = idNumber; public int getID() return id;
Person
- name
+ setName(String newName) : void
+ getName() : String
Student
- id
+ setID(int idNumber) : void
+ getID() : int

17. Overriding methods
What if the class Person had a method called printInfo?
public class Person {
// a bunch of other stuff
// ...
public void printInfo()
System.out.println(name); }

18. Overriding methods
What if the class Student also had a method called printInfo?
public class Student extends Person {
// a bunch of other stuff
// ...
public void printInfo()
System.out.println("Name: " + getName());
System.out.println("ID: " + getID()); }

19. Overriding methods: example
Both Person and Student have a printInfo() method
Student std = new Student("John Smith", 37183);
std.printInfo(); // calls Student’s printInfo method,
// not Person’s
Output would be:
Name: John Smith
ID: 37183

20. Overriding methods: why is it useful?
You often want derived classes to perform custom behavior
For example, drawing shapes
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…); }

21. Type compatibilities Given this inheritance heirarchy… Person Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

22. Is this code legal? Yes! Person p = new Person(); Person Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

23. Is this code legal? Yes! HighJumper h = new HighJumper(); Person
Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

24. Is this code legal? Yes! An Athlete is a Person, so this is okay
Person p = new Athlete();
Yes! An Athlete is a Person, so this is okay
Person
Athlete
HighJumper
ExtremeAthlete
Skydiver
XGamesSkater

25. 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

26. 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

27. What’s going on? We are creeping up to the idea of polymorphism
Enables the substitution of one object for another as long as the objects have the same interface
More details later

28. The class Object Every class in Java is derived from the class Object
Every class in Java is an Object
Object
Animal
Person
Reptile
Mammal
Student
Employee
Crocodile
Human
Whale

29. Friday
Program 4 help –
In-class code check