Object Oriented Mobile App Development Exam 2

Which company pioneered the “personal” computer in the early 1970s?

Xerox

Smalltalk has only two reserved words. What are they?

Self and Super

Smalltalk has three non-class globals. What are they?

true, false, nil

Instead of using commands (reserved words like if-then-else, do, while, for), Smalltalk uses what syntax to handle branching and looping?

Message sending / blocks (closures)

Although Smalltalk is often described as object-oriented, what feature makes it close to a functional paradigm?

Blocks / closures as first-class objects

What is image-based persistence?

The whole system state is saved as an image and resumed later. All coding is done inside the live environment

Modern commercial implementation of Smalltalk?

Cincom VisualWorks

Modern open-source implementation of Smalltalk

Pharo

Describe implementation of not without if-then

Boolean class with two subclasses for True and False that have a not method. The not method for each returns false if true and true if false.

Describe implementation of or without if-then.

False and true each have their own or method. In false or returns what was passed in. in true it returns true.

Describe implementation of and without if-then.

False and true each have their own method. The and method for false returns false. The and method for true returns whatever was passed in.

Alan Kay’s analogy for objects?

Biological cells

Sending a message is ________?

Making a choice

“Do not ask, tell” means?

Tell objects what to do instead of asking for data and deciding externally.

Two general categories defined by a class?

state and behavior

what does it mean that “A subclass expresses a delta”

A subclass represents only the differences from its superclass.

Two steps in sending a message?

1.   Look up the method

2.   Execute the method

When using self/this, where does method lookup start?

In the receiver’s class

When using super, where does method lookup start?

In the superclass

Tests verify that what worked before _____.

Still works

Tests reduce the fear _____.

Of change

A test that is not _______ does not exist!

Automated

A test: In this , this produces this __.

Context, input, output

Three possible outcomes of a test?

Success, Fail, Error

Method run before each test?

setUp

Method run after each test?

tearDown

At a minimum, you should write a test for ___.

Every bug

What is test-driven development?

Write tests before writing implementation code.

Testing like documentation?

Tests show how code should behave.

Testing better than memory because?

Tests are executable and repeatable.

How do tests help software evolve?

They catch regressions during changes.

Design patterns are _________ solutions to design problems.

Reusable

How do design patterns assist communication?

Shared vocabulary among developers

Five elements of a design pattern?

Name, intent, problem description, solution description, consequences

Three categories of design patterns?

Creational, structural, behavioral

What is the template method pattern?

A superclass defines an algorithm structure while subclasses fill in steps/hooks.

Difference between a data structure and an object?

Data structures expose data; objects hold data and behavior.

Presence of if-then-else or switch-case should be examined for replacement with what?

self-send (hook) to be implemented in subclasses

Intent of singleton pattern?

Ensure only one instance exists.

Solution for singleton pattern?

Private constructor + static/shared instance accessor

A composite allows us to compose objects into tree structures to represent what?

Part-whole hierarchies

A composite allows clients to treat _______ objects and _________ objects uniformly.

Individual, composite

Example of composite pattern?

File systems

Example of composite pattern in Flutter?

Widget trees

Advantage of state pattern?

Eliminates large conditionals by delegating behavior to state objects. Each state implements only its operations, and it implements its transitions.

How many concrete classes needed for rock-paper-scissors?

4

What methods implemented in each concrete class?

A vs method, playAgainstRock, playAgainstPaper, and playAgainstScissors

Meaning of the double in double dispatch?

There are two messages passed.

How are conditionals handled?

Through polymorphic method calls instead of if-then-else.

Without explicit initialization, what is in temporary variables in C++?

undefined values

Without explicit initialization in Smalltalk, Python, or Dart?

nil / None / null

If a function can return null, what burden is on clients?

They must check for null.

What is a null object?

An object with neutral/do-nothing behavior replacing null.

What is lazy initialization?

Delaying object creation until first use.

Duration of a variable?

When a variable is deallocated

Scope of a variable?

Where a variable is visible

What should you do instead of placing a literal in a large method?

Instance variable with setter or separate hook methods

What is the flyweight pattern?

Shared object, but also allow for classes to have their own modified version

Using flyweight for Date objects?

Reuse shared Date instances instead of creating duplicates.

What is coupling?

Degree of interdependence between software modules

What is cohesion?

How closely related a class’s responsibilities are

What message sends are preferred by the Law of Demeter?

Sends to:

• An argument passed to you

• Instance variables

• An object you create

• Self, super, and your class

Who are the clients of a class?

Code/classes that use it

Purpose of inheritance?

Reuse and specialization

In subclassing, classes share ______________ and ___________.

Interface and implementation

In subtyping, classes share ______________ but not ___________.

Interface, implementation

What is duck typing?

If an object behaves correctly, its actual type does not matter (“if it walks like a duck and quacks like a duck, it must be a duck”).