This is an individual, closed-book exam. There are 6 problems. You have 75 minutes to complete this exam.

If a problem asks for an example or a fragment of code, please, answer with the simplest example or fragment of code you can think of.

If a problem asks for a Yes/No or similar answer, please make sure that your solution provides this answer.

If you feel that you need to make additional assumptions for the solution of a problem, please state the assumptions you make.

1.

What is the name of the pattern intended to prevent concurrent access to data?

Sketch the code/structure of this pattern.

2.

The Singleton Pattern relies on two key Java constructs or features to ensure that: (1) at most only one object of a class is ever created by clients, and (2) a reference to the unique instance of the singleton class is always accessible to clients.

Name and briefly explain these features.

Sketch the code/structure of this pattern.

3.

A command of a Command Pattern, say SetCurrentClock, is intended to set the current time within an application program.

Discuss the key elements of a class that implements the SetCurrentClock command and sketch its code. Hint: you can assume the existence of any time manipulation method that you need. Use an expressive enough name for any such method and/or describe with a comment or separately what the method does.

Sketch the code/structure of the SetCurrentClock class.

4.

The Factory Method Pattern is used to construct families of related objects. This pattern relies on a key property of Object Oriented Programming Languages and Java in particular.

Describe this property and how it enables the pattern to construct different objects. Hint: In your answer, I am looking for a clear, complete and concise explanation. I do not want to read code.

5.

The Visitor Pattern relies on both overloading and overriding of methods. Briefly define these concepts and use a minimal abstract example to clarify your statements or definitions. I am not asking for a description or an example of an actual visitor.

What are the conventional names and signatures of the overloaded and overridden methods in the Visitor Pattern.

6.

For each question check zero or more choices according to the directions.

(a)

A not-yet-loaded Java class is loaded when (check the most accurate choice):

       

A constructor is invoked.

       

A static member is referenced.

       

Both of the above choices.

       

All of the above choices and in additional situations.

(b)

The equals method in class java.lang.Object (check all the correct choices):

       

Takes two arguments to compare.

       

Can be overridden in subclasses.

       

Tests whether two references refer to the same object.

       

Tests whether two references refer to objects with the same value.

(c)

Enumeration in package java.util identifies (check all the correct choices):

       

An interface.

       

An abstract class.

       

A (concrete) class.

       

None of the above choices.

(d)

The Prototype Pattern creates new objects by (check the most accurate choice):

       

Shallow copying.

       

Deep copying.

       

An appropriate combination of both the above choices.

       

Invoking either method clone or a ``copy constructor.''

(e)

The ``extrinsic state'' of a Flyweight object is (check all the correct choices):

       

Information shared by all the Flyweight objects.

       

Information shared by some Flyweight objects.

       

Information shared by no Flyweight objects.

       

Information stored by clients of Flyweight objects.