Final Exam Review

Final Exam Review, CS2390 Summer 1997

Typically, a final exam in 2390 is 4-6 questions like each of the below. Most of the questions below are from actual final exams in 2390. I recommend working together through the CaMILE links (FinExam Review Collaboration Space) to figure out the solutions. I will check up on the discussions, but I won't provide answers where no answers are posted.

Design You have been given the enviable task of designing the new "Jurassic Park: The Lost World" videogame. The studio is considering doing this new game in Smalltalk (Squeak, specifically) to be immediately portable to all platforms at once. They have a few questions for you, their ace Object-Oriented Consultant (Who Makes Really Big Bucks).

• "This game is going to be big, REALLY Big! With lots of scenes, tons of characters. We're going to need ten, twenty, maybe a hundred programmers! What makes Smalltalk better than, say, Assembly Language for handling teams?"

• "Smalltalk uses this, uh, virtual machine thingie. What is that? Is that a problem?"

• "We've started on the class hierarchy. Here's our story. First, the Hero enters the scene, carrying a really big gun that makes a huge noise. He's there to save the Guy who can't shoot any guns and needs to be saved. He's also there to save the Kid who has a toy gun that makes silly little noises and can't kill anything. Of course, all three characters can be killed off. Here's our class hierarchy so far. Is it right? If not, can you fix it?"

  

Post comments, discussion, answers in CaMILE at Design of Dino Game Collaboration Space

Languages

• Smalltalk does not have multiple inheritance. Say that you wanted to model a new multi-function printer/scanner/copier. How would you model this in C++ with multiple inheritance? How do you model it in Smalltalk without multiple inheritance?
• Smalltalk does not have a virtual keyword. What does virtual mean? How do you get around this lack in Smalltalk?
• Smalltalk does not have private, protected, or public. When might you want to use these keywords in Smalltalk?

Post comments, discussion, answers in CaMILE at Languages Collaboration Space

• How much time does a customer spend in the store with a given layout?
• Do customers' carts crash in Aisle 12?
• What is the average purchase at each register?
• What is the average wait time at the register?

Post comments, discussion, answers in CaMILE at Discrete vs. Continuous Collaboration Space

User Interfaces in VisualWorks Imagine that you have a class named Student with instance variables ssn containing a social security number, classes containing an ordered collection of strings with the names of the students' classes, and major containing a string naming the student's major. You are now working on the openOn: aStudent method for your view of Student.

1. You want to create a TextView that lets you view and change the student's major. What methods do you need in the Student in order to do this?

2. Fill in the blanks in the below piece of code for creating the TextView:

   view := TextView on: BLANK1
   	aspect: #BLANK2
   	change: #BLANK3
   	menu: nil.
   

3. Now you want to create a list selection box for reviewing classes and getting more information on them. Decide what new methods and instance variables (if any) you need in the Student, and fill in the blanks below:

   view := SelectionInListView on: BLANK4 
   	aspect: #BLANK5
   	change: #BLANK6
   	list: #BLANK7
   	menu: nil
   	initialSelection: #BLANK8.
   

Post comments, discussion, answers in CaMILE at User Interfaces in VisualWorks Collaboration Space

Fixing a C++ Class StructureConsider the below C++ class definitions: (You can assume that a class library definition for String has been #include'd. Also, this is valid C++, even without the public/private/protected keywords.)

class EmpRec
{ String _lastname
String _firstname;
String _street; 
String _city; 
String _state;
String _zip;};

class CrossingGuard : public EmpRec{ 
String _school; 
String _crossStreet1;
String _crossStreet2;};

class PoliceOfficer : public EmpRec{ 
int _precinct; 
float _salary;};

class MeterMaid : public PoliceOfficer{ float _hourly_wage;};

There are a couple of major flaws with this structure:

• Is a CrossingGuard (or a PoliceOfficer) an EmpRec?
• Is a MeterMaid a specialization of a PoliceOfficer? What's more, MeterMaids in this example have both a salary AND an hourly wage. (In this scenario, MeterMaids should just have an hourly wage.)
• CrossingGuards are tracked in the City database, but they are volunteers, not employees.

Below:

• Define a new class structure that (a) still provides CrossingGuard, PoliceOfficer, and MeterMaid classes with the right data members and (b) corrects these flaws. Draw the structure using the Coad & Nicola notation. (You should create more classes than currently exist in the above definition.)
• Write the C++ class definitions for the new set of classes.

Post comments, discussion, answers in CaMILE at Fixing a C++ Class Structure Collaboration Space

Simulations You are building a simulation of a shipping port where boats are entering and leaving, docking at a given dock, unloading and re-loading cargo, and trucks are taking the cargo in and out of the port. The following components will be part of your simulation:

• The boat ID, time that a boat arrives at the port and how much time it will take to unload its cargo.
• The number of docks available and whether it is being used or not.
• The time that a truck arrives at the port, whether it is carrying cargo, and the boat ID that it expects to meet.

(12) (a) Of the following questions, which call for a discrete simulation and which call for a continuous simulation? And WHY?

• How long do boats have to wait for an open dock?
• How long do trucks have to wait for their boat?
• How close to one another do the boats come when traveling in and around the port?
• What is the average time that a dock is kept busy?

(8) (b) For these questions, which of the following should be modelled as resources and which as simulation objects? And WHY?

• Trucks
• Cargo
• Docks
• Boats

Post comments, discussion, answers in CaMILE at Simulations Collaboration Space

Language ChoicesFor each of C++, Java, and Smalltalk, tell me:

• An application that is well-suited to this language (e.g., Device Drivers to C++) (No, you can't use that example),
• What needs of the application make it well-suited to the language (e.g., need for low-level, fast code),
• What characteristics of the language match the needs of the application (e.g., C++ lets you manipulate memory efficiently).

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Short Answer

A. What does the virtual keyword do in C++? Why don't we have one in Smalltalk or Java?
B. Characterize Smalltalk's inheritance mechanisms in terms of C++'s public, private, and protected. Is anything private in Smalltalk? How about public? Protected?

C. What is a bytecode compiler and a virtual machine (as used in Smalltalk and Java implementations)? What is the advantage of this approach? What is the disadvantage?
D. What is polymorphism? (Hint: printstring and inspect are both polymorphic.)
E. What is the sequence of messages between a model and its view (use Count as an example) when the model executes a self changed: #value message? (Hint: Model sends one to view, view sends one to model)

Post comments, discussion, answers in CaMILE at Short Answer Collaboration Space

LinkedList: You are working on a project using Smalltalk where you need linked lists, but you've decided that you don't like the implementation of linked lists provided in the Smalltalk image. You decide to implement your own class MyLinkedList. You need the following functions in your implementation:

• isEmpty returns true if the receiver list is empty
• isEqual: anotherList returns true if both lists have the same elements
• first returns the first item on the list
• rest returns the list minus the first item
• addToFirst: anItem places anItem at the front of the list
• cons: anotherList returns the receiver list concatenated with the argument list.
  

• (a) Write the class definition for MyLinkedList

Object subclass: #MyLinkedList
        instanceVariableNames: YOU FILL IN HERE
        classVariableNames: 
        poolDictionaries: 
        category: 'My-Data-Structures'

• (b) Write the method definitions for each of the desired functions.

Post comments, discussion, answers in CaMILE at LinkedList Collaboration Space

Count in C++ Below is the definition in C++ of the class Count.

template<class Count_Type>
class Count { 
public:
//Constructors/destructors
        Count() { } 
        virtual ~Count() { } 
//Implementors
        virtual void decrement() =0;
        virtual void increment() =0;
        void reset() {  value = reset_value; } 
//Accessors
        Count_Type get_value() {  return value; } 
        void set_value(Count_Type new_value) {  value = new_value; } 
        Count_Type get_reset_value() {  return reset_value; } 
        void set_reset_value(Count_Type new_value) {  value = new_value; } 
protected:
	//Data members
        Count_Type value;
        Count_Type reset_value;} ;

(a) In terms of inheritance, does this definition differ from the definition of the class Count in Smalltalk? Can subclasses of Count in Smalltalk access anything more or less than derived class of Count in C++? Can external classes that use Count in Smalltalk access anything more or less than external classes that use Count in C++?

(b) Why are the data members declared protected in the class above? Why not declare them private?

(c) Why are ~Count(), increment(), and decrement() declared virtual? What does that mean?

(d) Suppose that we don't want to allow the data members of Count to be arbitrarily manipulated from outside of this class or classes derived from Count. What would you change in the above definition? Why?

Post comments, discussion, answers in CaMILE at Count in C++ Collaboration Space