System Design Patterns

Question 1

WHat can be represented by a class?

Answer 1

• real-world, external entities
• information entities
• abstract ‘metahpors’ and ideas

Question 2

What are the 3 main design principles to aim for?

Answer 2

• Distribtuion of behaviours (decentralisation)
• Even granularit (components to have similar levels of complexity)
• Loose coupling (objects can be reused in another context)

Question 3

Briefly describe CRC Card prototyping

Answer 3

+ low tech strategy for prototyping useful object concepts

+ flexible; allows the concept set to change

Question 4

How do we write a CRC card? 3 steps

Answer 4

• Name concept accroding to main purpose
• Identify (up to 7) responsibilities: knowing, doing, enforcing (physical size constraints force decomposition)
• Identify collaborators (classes on which this one depend)

Question 5

What do we put on the reverse side of a CRC card?

Answer 5

• categorise the card
• attributes (no remote get/set methods)
• constraints (business rules for attributes)

Question 6

What are 3 ways to split a CRC card?

Answer 6

• 2 peer concepts; if responsibilties handle disjoint subset of attributes or collaborators.
• Parent-child concepts; if some responsibilties handle a subset of attributes or collaborators
• Subcontractor concepts; no obvious splitting move

Question 7

Why focus on responsibilty for Responsibilty-Driven Design?

Answer 7

want to identfy objects which bear some responsibility for the system

• better than passive records which ancapsulate data
• better than abstract data types with just set/get methods

Question 8

What are 3 methods of Transformation to reorganise communication-paths between subsystems?

Answer 8

• Delegation
• Encapsulation
• Refactoring

Question 9

What is a design pattern?

Answer 9

reusable element of object oriented design

ready made solution to common design problems

Question 10

Describe 3 typical uses of design pattern

Answer 10

• look out for problems in design; apply pattern
• design from scratch using patterns to get high-quality design
• retrofit extensions/adaptionsusing patterns

Question 11

Describe the Bridge design pattern

Answer 11

reducing a number of specialised variants of a concept by encapsulating one varying dimension inside another

Question 12

Describe the Mediator design pattern

Answer 12

introduce objects which handle communication between other objects
- removes need to couple objects directly

Question 13

Describe the Chain of Responsibilty design pattern

Answer 13

give multiple objects the chance to handle a request

pass request along the chain

Question 14

Describe the Command design pattern

Answer 14

encapsulate a funciton as an object with a single execture method, to add functions to a system incrementally

Question 15

Describe the State design pattern

Answer 15

allows an object to alter its behaviour when its internal state changes

Question 16

Describe the Template Method design pattern

Answer 16

generalise control algorithms to common skeleton code with dynamically bound subroutines

Question 17

WHat problem does the abstract factory design pattern solve?

Answer 17

dependency among components

Question 18

What are the benefits of having the mediator pattern?

Answer 18

good for uncoupling, simplifying communication paths

Question 19

What design patterns indicate loose coupling?

Answer 19

Mediator, composite

Question 20

What patterns indicate generic behaviour?

Answer 20

Command, template method, state, factory method

Question 21

What patterns indiciate late design fixes?

Answer 21

Observer, Facade, Adapter

Question 22

What are the benefits of having the command pattern?

Answer 22

indiciates modular system with extensible behaviour

Question 23

what are the benefits of have the template method pattern?

Answer 23

indicates reusable basis for algorithmic design

Question 24

What are the benefits of having the chain of responsibilty pattern?

Answer 24

indicates devolved responsibilty (single purpose objects)