design patterns

Question 1

what are design patterns

Answer 1

reusable solutions to common problems that occur in software design

Question 2

what are the 4 elements of a design pattern

Answer 2

name; description of the pattern in 2 or less words
problem; when you should use the pattern
solution; elements that make up the design and interactions responsibilities and relationships
consequences; trade offs of using the pattern

Question 3

what are the 3 categories of design patterns

Answer 3

creational; techniques to create objects
structural; how classes are imposed in larger structures
behavioural; communication between objects

Question 4

what is the observer design pattern

Answer 4

behavioural design pattern that allows you to define a subscription mechanism to notify other objects about any changes made in the object their observing

Question 5

subject/publisher

Answer 5

the object that is changing state and informs observers

Question 6

observers/subscribers

Answer 6

dynamically attached to the subject and can subscribe to event notifications

Question 7

subject as an abstract class

Answer 7

abstract class defines add remove and notify observers
the concrete subject extends the abstract class
the concrete observer implements the observer interface and registers with concrete subjects to receive updates

Question 8

subject as an interface

Answer 8

can add remove and notify observers
the concrete subject implements this and keeps a list of the observers and notifies them
the concrete observer implements the observer interface and registers with concrete subjects to receive updates

Question 9

what is an adaptor design pattern

Answer 9

structural design pattern that converts the interface of a class into an interface that the client expects

Question 10

what are the 4 classes needed for the adaptor

Answer 10

client; who wants to use the interface
target; the interface that the client uses
adaptor; adapts the adaptee to the target interface
adaptee; the existing interface that needs changing

Question 11

class adaptor

Answer 11

adaps the adaptee by inheriting it and implementing the target interface

Question 12

object adaptor

Answer 12

relies on composition
the adaptor implements the target interface and contains the adaptee instance to which it forwards the request