Polymorphism

Exercises

1. Starting from the classes Building, HighBuilding and Skyscraper from the last exercise (reusing classes), define a class Visits that stores an array of 10 buildings (representing a street). Define a method that enters all the buildings in the street using the method enter for each of the buildings, one after another. Fill the array with mixed objects from the classes Building, HighBuilding and Skyscraper. Make sure, that the output of your program visualizes the fact that different method implementations are used depending on the type of the actual object. If necessary, modify the building classes with further print statements.
2. Define a Class Bank and a class Account.  (Account may be defined as an inner class, but we did not cover this in the lecture. So you can define it as a normal class)
• An Account contains the name of the owner of the account, the balance of his account, and his PIN.
• The bank manages an array of ten accounts.
• The Bank class provides a method createAccount, that creates a new account with an initial amount of money and returns a PIN, a method getBalance that takes a PIN and the name of the owner and returns the balance of the account.
• Define some further useful methods as you like.
3. The next exercises are concerned with a simple implementation of money cards like telephone cards or credit cards. For this purpose define two interfaces:
• an interface CardUse for card use with methods read to read the state and reduceBy with a parameter amount to change the state of the card. The user has to identify himself by a PIN for these operations;
• an interface CardChange for the administration of the card by authorized people, that need a method reset to reset the card state, a method fill to fill the card with an amount of money and a method changePIN to change the PIN for the card.

Define a third interface CardAll that includes all card operation (Use inheritance).

4. Change the interface CardAll into an abstract class that implements the balance of the card and a basic solution for the methods fill and reduceBy leaving the rest of the methods abstract; In this implementation the PIN may be ignored; Choose the correct access specifier to make the balance accessible to subclasses but not to the public;
5. Introduce a method getBankCard into the Bank class from exercise 2 that takes the name and the PIN of a person as a parameter and returns an object of type CardUse with the following properties:
   • It can be used only 1000 times,
   • it contains the name of the owner as an internal state;
   • a call of the method reduceBy with parameter amount reduces the associated account by amount.

   To accomplish this task, define a class BankCard that implements the interface CardUse. Test your card. (Again, you may define BankCard as an inner class).

		Claudia Niederée, nov 1998