Design Patterns

Question 1

What is the purpose of Design Patterns?

Answer 1

Create an architecture that enables the re-use of code blocks to create customised objects.

Question 2

What are the 5 design pattern principles?

Answer 2

Encapsulation of dynamic code; Program to an interface or abstract class; Favour object composition over class inheritance; Objects should be open for extension, closed for modification; Depend on abstractions, not on concrete classes.

Question 3

What are the 3 design pattern categories?

Answer 3

Factorial (Object Creation)
Structural (Object composition)
Behavioural (Object interaction)

Question 4

CDP: When should we use the singleton pattern?

Answer 4

When we only need one instance of the class.

Question 5

CDP: How do we create a Singleton Object?

Answer 5

Create a static reference to the class; Create a private constructor to instantiate the reference; Create a static public method (getInstance) to access the object.

Question 6

CDP: What are the 2 types of initialisation of a Singleton Object?

Answer 6

Eager Instantiation : Create object when class is loaded.
Lazy Instantiation : Create object only when you need.

Question 7

CDP: Why might multithreaded programs cause issues with Singleton instantiation? How can we resolve this?

Answer 7

Because we need to use thread management to ensure only 1 thread instantiates the singleton. We can try to resolve this through synchronizing the method. However, this means threads will wait idly outside the locked object.

Question 8

CDP: What is the most effective way of implementing thread management in the Singleton Class?

Answer 8

Create a condition in the instantiation method which only instantiates the reference if the object is currently null.

Question 9

CDP: Define the Builder Pattern…

Answer 9

Separates the construction process from the objects representation. This enables isolation and encapsulation of production, meaning many representations can be created.

Question 10

CDP: What are the 3 qualities that BDP enables?

Answer 10

• Separate construction and representation code.
• Vary the products internal representation.
• Finer control over the production process.

Question 11

CDP: What are the 4 components of the BDP?

Answer 11

Builder : Specifies the interface for construction of the complex object.
ConcreteBuilder : Defines the parts of the builder interface.
Director : Object that takes responsibility of the object construction, but delegates actual construction to ConcreteBuilder class.
Product : Complex object created by the builder.

Question 12

CDP: What are the 3 advantages of BDP?

Answer 12

Encapsulate construction of complex object.
Allows multi-step construction = Customisation.
Hides the internal representation of the product from the client.

Question 13

CDP: When do we use FDP?

Answer 13

When we need to create varying types of the same super object.

Question 14

CDP: What are the 4 components of FDP?

Answer 14

Creator; ConcreteCreator; ConcreteProduct; Product.

Question 15

CDP: What is the role of the Concrete Classes in FDP?

Answer 15

They are responsible for defining the varying implementations of objects.

Question 16

CDP: What is an abstract FDP?

Answer 16

Architecture that uses re-usable code to create multiple objects.

Question 17

CDP: What are the 2 differences between FDP and AFDP?

Answer 17

FDP creates one object whereas AFDP creates multiple.
FDP uses inheritance whereas AFDP uses composition.

Question 18

What type of pattern is the Command Design Pattern?

Answer 18

Behavioural Pattern.

Question 19

Define the Command Design Pattern?

Answer 19

Architecture in which requests are encapsulated in objects. Mostly used in GUI.

Question 20

What are the 5 components of the Command Design Pattern?

Answer 20

Invoked
Command interface
ConcreteCommand
Receiver
Client

Question 21

Define Decorator Pattern…

Answer 21

A structural design pattern. Behaviours of the object are added after compilation, at run time. This prevents class explosion.

Question 22

What is Class Explosion?

Answer 22

The overuse of different classes to implement object behaviours or attributes. It leads to expensive overheads.

Question 23

What is a solution to Class Explosion? What might be an issue with this solution?

Answer 23

Create instance variables of all the potential behaviours of an object, and set boolean values in the superclass to distinguish which behaviours are needed for that specific object.

An issue might be the need to recompile whenever a behaviour is added.

Question 24

What are the 2 principles for the Decorator Pattern?

Answer 24

Open - Close
Favour object composition over class inheritence.

Question 25

What is the difference between decorator objects and the concrete object?

Answer 25

Decorator objects are behaviours added to the concrete object.

Question 26

How does the delegation model operate when calling methods in the decorator pattern?

Answer 26

The delegation model enables methods in the decorators to be called, and delegate all the way back to the concrete object.