2. Inheritance and Polymorphism
Dr. M.M. van Baal
Verdugo Hills High

3. What does Inheritance mean in English?
Ask students to give their answers.

4. What about Polymorphism?
Umm…
Break down into poly and morph.

5. Before the Java definitions…
Take out a piece of paper and draw an automobile.

6. My next automobile. (Anybody draw something like this?):

7. My next automobile. (According to my girlfriend):

8. They look a little different
Would you agree they both have…
4 tires
An engine
Windows
Doors
Seats
Can start their engines
Refuel by putting gas in the tank
Therefore they must both be part of the same class: Automobile
Student’s drawings probably also have the tires, doors, etc.

9. Therefore…
Since they are both the same class, they must be able to do the following the same way too
Accelerate
Make turns
Weave through traffic
Attract attention (both of the police and the opposite sex)
Do you agree?

10. Not quite…
There are a lot of similarities, both are indeed automobiles
But one is a sports car and one is a minivan
Are those different classes?

11. More like different subtypes
Think about how sports cars were invented
Did they start all over from scratch?
Someone took the automobile and made a more specific type

12. How does this work in java?
We take the class: public class Automobile
Members (= fields = instance variables):
public String engine
public int doors
public int seats
Methods (= to “do” things):
startEngine()
refuel()
accelerate()

13. And extend its definition
We will add and replace what we need
public class SportsCar extends Automobile
makeTurns()
weaveThroughTraffic()
attractAttention()
accelerate()
Notice we’re replacing accelerate() that is defined in Automobile. That’s because the SportsCar needs to expel a very loud purr from the engine as it accelerates.
We will keep (= inherit) everything else:
startEngine()
refuel()
We get this for free and don’t have to write the code again

14. Inheritance in terms of Java
Put it in your own words!

15. So, Polymorphism?
1 Command, implemented differently by different classes (here: different cars).
Example pumpGas( ) method:
Sports car (at back of car).
Minivan (on right side of car).
Luxury car (on left side of car).
Pickup truck (behind rear license plate).

16. An auto is an auto
In other words, whatever Automobile shows up, I can pump gas into it.
Java will ask the Automobile (regardless of what type it is) to pumpGas() and it will do the right thing.

17. Another polymorphic example.
Consider the class: public class Animals.
Animals has an eat() method.
There are several classes that inherit from Animals:
public class Bear extends Animals
public class Chicken extends Animals
public class Tarantula extends Animals
public class Cockroach extends Animals

18. eat() method: You can call the eat() method in Animals.
However, every animal eats differently => each animal defines the eat() method in Animals differently.
How do they eat():
Bear
Chicken
Tarantula
Cockroach
Did you know a cockroach can live for about a week without its head until it finally dies of hunger? Just info: nothing to do with Java!!!!

19. Inheritance in GridWorld
class Bug extends Actor
Inherits pustSelfInGrid()
Replaces act()
Adds canMove()
class BoxBug extends Bug
Inherits canMove()

20. Polymorphism in action
For each Actor (whether Actor, Bug, etc) in the grid, act() is called in each step:
for (Actor a : actors) {
// only act if another actor hasn't removed a
if (a.getGrid() == gr)
a.act(); }