1. Inheritance Notes: Using Object Oriented Design
When Classes start a family…

2. HAS-A vs. IS-A Relationships
Average Free Response Question
The StudentRecord Class HAS-A array
Money class
Money object HAS-A couple of ints

3. HAS-A vs. IS-A Relationships
New: IS-A Relationship
Car IS-A Vehicle
Triangle IS-A Polygon
ProperFraction IS-A Fraction
Many more to come…
Car
Vehicle
Truck
Corvette

4. Inheritance vocabulary
Give an example of a polygon:
Parent class – child class – grandchild class
Super class – class - subclass
Polygon
Quadrilateral
Triangle
Rectangle

5. The Daddy of them all: Object
All classes have the IS-A relationship with Object
Inherited methods:
toString()
equals()
hashCode()
Many more that are not
in the AP Subset
Fraction
Object
Money
ProperFraction

6. Creating a child class: extends
public class Car extends Vehicle
Car is the subclass
It inherits all of Vehicle’s methods and variables
It can “overwrite” the methods by using the same signature
Ex: speedUp() method
Car
Vehicle
Truck
Corvette

7. A new keyword:
super

8. Call Daddy’s methods: super
Create a new Vehicle:
Vehicle v1 = new Vehicle();
Create a new Car:
Car c1 = new Car();
Polymorphism: If Vehicle and Car both have a speedUp() method, how do I call Vehicle’s speedUp () in Car’s methods?
super.speedUp()
Car
Vehicle
Truck
Corvette
speedUp()
speedUp()

9. Call Daddy’s methods: super
public class Vehicle{
private int speed;
public void speedUp(){
speed++;
S.O.P(“I’m fast”); }
What does this do?
In the runner:
c1.speedUp();
public class Car extends Vehicle{
public void speedUp(){
S.O.P(“I’m really fast!”);
super.speedUp(); }
Car
Vehicle
Truck
Corvette

10. Call Daddy’s constructor: super
How does Car call Vehicle’s constructor?
This is confusing because Vehicle’s constructor method is actually named _________________.
super(parameter1, parameter2, …)
Vehicle()
Car
Vehicle
Truck
Corvette

11. Never create me: abstract class
abstract public class Polygon
An object of the Vehicle class can never be instantiated (created)
However, Vehicle can have child classes that CAN be instantiated
Polygon
Rectangle
Triangle
Square

12. Please create me: abstract method
abstract double getArea();
An abstract method forces the creator of a child class to write the body of the method before the class can be compiled.
Polygon
Rectangle
Triangle
Square

13. Rules about constructors
The superclass constructor can be called with the line super(param1, …)
Daddy comes FIRST!!! The super constructor call MUST be the first line in the child constructor
Car
Vehicle
Truck
Corvette
Car c1 = new Car();

14. More rules about constructors
EMPHASIZE THIS! The magic of Java constructors!!!
Daddy, help me!!!!
Even if the word super is not present on the first line of the constructor, the child class constructor automatically calls its parent constructor before it does anything else.
See sample questions…

15. Sample Question Answer? class Morf { private int numMorfs;
public Morf(int m) {numMorfs = m;}
public int getMorfs() {return numMorfs;} }
class Dorf extends Morf
private int numDorfs;
public Dorf(int m, int d) {super(m); numDorfs = d;}
public int getDorfs() {return numDorfs;}
What will display when the following lines execute in a client file?
Dorf dorf = new Dorf(10, 25);
System.out.println("Morf count: " + dorf.getMorfs());
System.out.println("Dorf count: " + dorf.getDorfs());
Answer?
See files MorfDorfQuestion.java and MorfDorfQuestion2.java

16. Another Sample Question
class Morf {
private int numMorfs;
public Morf(int m) {numMorfs = m;}
public int getMorfs() {return numMorfs};
public String toString(){ return “Morfs: “ + numMorfs;} }
class Dorf extends Morf
private int numDorfs;
public Dorf(int m, int d) {super(m); numDorfs = d;}
public int getDorfs() {return numDorfs;}
What will display when the following lines execute in a client file?
Dorf dorf = new Dorf(10, 25);
System.out.println(dorf.toString());
System.out.println(dorf);
Answer?
See files MorfDorfQuestion.java and MorfDorfQuestion2.java

17. Don’t touch me: private variables
private int myNum
Private variables are only available to the class itself.
They are NOT accessible to subclasses or client files.
How are parent class private variables accessed?

18. Don’t touch me: private methods
private void simplify()
private methods can only be called within the class, not by children or clients.

19. Only family can touch me: protected methods
protected int reproduce()
Like private-Lite
Only the class and child classes can call protected methods

20. Finally
Let’s look at all of these topics in the following three files:
Polygons.java
PolygonsRunner.java
Triangle.java
Copy the files, paste them to your APCode folder, and add them to your workspace for a day of fun.

21. Question 1 Explanations: Answer? a.) didn’t implement moveBy method
public abstract class Point {      private int x, y;
     public Point(int x, int y)
{ this.x = x; this.y = y; }      public int getX() { return x; }      public int getY() { return y; }      public String toString()
{ return "(" + getX + "," + getY() + ")"; }
     public abstract Point moveBy(int dx, int dy);      public abstract double distanceFrom(Point p); }
public abstract class SpecialPoint extends Point {      public SpecialPoint(int x, int y)
{ < Code not shown > }      public double distanceFrom(Point p)
{ < Code not shown > } }
public class SpecialerPoint extends SpecialPoint {      < Code not shown > }
Which of the following is the minimal set of public constructors and/or methods required in the SpecialerPoint class, so that the statements
SpecialerPoint pos = new SpecialerPoint(0, 0); System.out.println(pos.moveBy(10, 10).distanceFrom(pos));
compile with no errors? 
a. public Point moveBy(int dx, int dy)
b. public SpecialerPoint(int x, int y)
public Point moveBy(int dx, int dy)
c. public double distanceFrom(Point pos)
d. public double distanceFrom(SpecialerPoint pos)
public SpecialerPoint moveBy(int dx, int dy)
e. public SpecialerPoint()
public SpecialerPoint(int x, int y)
Explanations:
a.) didn’t implement moveBy method
b.) implements moveBy method (other abstract method implemented in SpecialPoint)
c.) no constructor
d.) no constructor
e.) could be correct —> not minimum set of constructors/methods
Question from Susan Ke

22. Unused Slides

23. Neutered classes: final
final public class Motorcycle
By putting final in the class definition, it can never have subclasses
Similarly, by putting final in front of a method name, its child classes can not overwrite the method