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J.T.W. tutorial 13: still more questions about references


§ 13 Tutorial 13

Question 13.1: Study, compile and run the following code:

class Car
begin
   
   /**
    * Car's model name
    */
   property String    model;
   
   /**
    * Car's value in dollars
    */
   property int value;
   
   /**
    * Car's serial number
    */
   property int serialNumber;
   
   /**
    * Global serial number counter
    */
   private classVar int serialCounter = 1000;
   constructor Car(String aModel, int aValue)
   begin
      model = aModel;
      value = aValue;
      serialNumber = serialCounter;
      serialCounter = serialCounter + 1;
   end
   public method String toString()
   begin
      return "I am a car, model=" + model + ", value=" + value +
             ", serial number=" + serialNumber;
   end
end
class Owner
begin
   
   /**
    * Owner's full name
    */
   property String name;
   
   /**
    *  Owner's money in dollars
    */
   property int money;
   
   /**
    * Owner's car
    */
   property Car ownersCar;
   constructor Owner(String aName, int aMoney, Car aCar)
   begin
      name = aName;
      money = aMoney;
      ownersCar = aCar;
   end
   public method String toString()
   begin
      return "I am a car owner, name=" + name + ", money=" + money +
             ", car=" + ownersCar;
   end
   method void describe()
   begin
      System.out.println(toString());
   end
end
class CarTest
begin
   beginMain
      var Car   ford   = new Car("Ford Escort",1000);
      var Car   nissan = new Car("Nissan Nivara",2000);
      var Owner joe    = new Owner("Joe Bloggs",500,ford);
      // Mary has no car to start with.
      var Owner mary   = new Owner("Mary Smith",600,null);
      joe.describe();
   endMain
end

Question 13.2: What is the purpose of the class variable serialCounter?

Question 13.3: Write a method sellCar that increases the owner's money by half the value of their car and the owner's car reference gets set to null, for no car. If the owner owns no car (null) simply do nothing.

Question 13.4: Write a method in the Owner class called purchase so that:

Car newCar = new Car("Mini Cooper",1000);
joe.purchase(newCar);

results in Joe's money going down by newCar.value and Joe's car being set to newCar. Call the sellCar method before Joe purchases his new car

Question 13.5: Write a function in the Owner class called netWorth so that:

System.out.println("Joe's net worth = " + joe.netWorth());

prints out Joes' money plus the value of his car, if he has a car. You will need to use an if (...) then ... statement to test whether or not a reference is pointing to a valid object or null for no object like so:

if (ownersCar == null)
then begin
   // do not access ownersCar.value as ownersCar points to no object
end
else begin
   // do access ownersCar.value
end

Question 13.6: Write a method in the Owner class called smashCar so that:

mary.smashCar();

halves the value of Mary's car.

Question 13.7: Write a method in the Owner class called stealCarFrom so that:

mary.stealCarFrom(joe);

results in Mary selling his current car (if he has one) for its market value and Mary acquiring ownership of Joe's car. Also make Joe invoke his sellCar method to relinquish ownership of his car if he has one.

Question 13.8: Write a function in the Owner class called swapMoney so that:

Owner.swapMoney(joe,mary);

swaps the money of Joe and Mary.

Question 13.9: Write a function in the Owner class called swapCars so that:

Owner.swapCars(joe,mary);

swaps the cars of Joe and Mary.

Question 13.10: Write a function in the Car class called swapSerialNumbers so that:

Car.swapSerialNumbers(ford,nissan);

swaps the serial numbers of ford and nissan.

Question 13.11: Write a function in the Owner class called sellCarTo so that

joe.sellCarTo(mary);

results in Joe's money going up by the value of his car and Mary's money going down by the value of his car, and the ownership of Mary's car gets transferred to Joe.

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