Questions

• Reading?
• Quiz?
• Homework?

Subclasses and super classes

• Super classes are more general (BankAccount)
• Sub classes are more specific (CheckingAccount, SavingsAccount)
• A subclass can do everything its super class can do, plus more
• Think of two possible classes where one would be the super class and the other the sub class

Activity - Inheritance Heirarchy

Record your participation in Piazza clicker question Lesson23 Q1

Arrange these classes in heirarchy from most general (super) to most specific (sub)

tiger, animal, feline, mammal

1. tiger, animal, feline, mammal
2. tiger, feline, mammal, animal
3. animal, mammal, feline, tiger
4. animal, tiger, feline, mammal

Activity - Adding to the heirarchy

Record your participation in Piazza clicker question Lesson23 Q2

• Draw a diagram of the heirarchy (animal, mammal, feline, tiger).
• Add lion, canine, reptile, wolf to the heiarchy diagram
• What do all animals have in common? What behaviors or properties?

Substitution Principle

• If a method parameter is a superclass, the method can take any subclass
• A method that requires an Animal can take any subclass of Animal
• Consider this method

  public void doSomethingToAnimal(Animal myAnimal)
  {
       ...
  }

• Any of these method calls would be valid

  • Wolf myWolf = new Wolf(...);
    doSomethingToAnimal(myWolf);

  • Tiger myTiger = new Tiger(...);
    doSomethingToAnimal(myTiger);

  • Mammal myMammal = new Mammal(...);
    doSomethingToAnimal(myMammal);

• Why?

Activity - Substitution Principle

Record your participation in Piazza clicker question Lesson23 Q3

Look at this vehicle heirarchy from your book

Given this method

public void doSomething(Car theCar)
{
    ...
}

Objects of which classes could be passed to the doSomething method? Check all correct answers

1. Vehicle
2. Motorcycle
3. Car
4. Truck
5. Sedan
6. SUV

Activity - Subclasses

Record your participation in Piazza clicker question Lesson23 Q4

Given this Definition of Vehicle from Wikipedia:

A vehicle is a mobile machine that transports people or cargo.

Which of these objects could be subclasses of Vehicle? Click all that apply.

1. snow mobile
2. stretch limo
3. ship
   
4. horse

Activity - Writing a subclass

Record your participation in Piazza clicker question Lesson23 Q5

Let's make a subclass, Toy, of the Product class we wrote earlier in the semester. A Toy has an appropriateAge variable and methods to get and set the new variable. The getDescription of the Toy returns the name concatenated with the appropriate age.

• Download the project toy_subclass.zip
• Look at the Product class
• Now create a Toy class that is a subclass of Product
  • What keyword do you use?

  • public class Toy extends Product

• Declare the instance variable appropriateAge
• Implement the constructor
  • The constructor takes the description, price, and appropriate age as parameters.
  • Provide Javadoc for the constructor
  • Call super class constructor to handle the instance variables defined in Product

    super(..., ...)

  • Initalize remaining instance variable(s)
• Implement getAppropriateAge
• Provide Javadoc
• We will override getDescription later

Concept - Subclass inherits ...

• The methods of the super class.
• The instance variable of the super class
  • But these can only be accessed through the public interface - the public methods.

Concept - Methods in a subclass

Three types

• The subclass can inherit methods from the super class.
• It can create new methods.
• The subclass can override a method in the super class to do what is needed in the subclass

Activity - Override getDescription

Record your participation in Piazza clicker question Lesson23 Q6

• getDescription needs to return the description plus the appropriate age.
• We will override getDescription to return something like this

  Duplo Rocketship  Age: 2

Activity - Testing the Toy class

Record your participation in Piazza clicker question Lesson23 Q7

• In the main method of ToyTester, create a Toy called rocket with a name of "Duplo Rocket", a price of 18.95 and an appropriate age of 2
• Add System.out.println( rocket.getPrice()). What happens? Why?

• Add: rocket.increasePrice(10);
       System.out.println( rocket.getPrice());

  
  What happens? Why?
• Add System.out.println( rocket.getDescription()) What happens?
• Add System.out.println( rocket.getAppropriateAge()). What happens?
• Now declare a Product object reference variable myProduct and assign rocket to it.

  Product myProduct = rocket;

• Will it compile? Why?
• Add System.out.println( myProduct.getAppropriateAge()). What happens?