OO Relationships (PPT 11 - 13)

Question 1

What is a relationship?

Answer 1

Collaboration between objects

Question 2

What is an association relationship?

Answer 2

Association represents a link/memory between objects. Messages may flow in either direction across the association

Question 3

How do we name an association?

Answer 3

It is stated in terms of the objects being associated. The combination of objects and association name should read like a sentence

Question 4

How many associations can an object have?

Answer 4

It can have as many as it wants and can also have as many associations between objects as you want

Question 5

What is a reflexive association?

Answer 5

Associations which involve only one class of object

Question 6

What are role names?

Answer 6

Used to indicate the role performed by each end of the association

Question 7

What is multiplicity?

Answer 7

The number of instances of objects which may be involved at either end of the association

Question 8

What is a composition relationship?

Answer 8

It is a stronger form of association, relates a whole object to its parts. Represented by a line with a filled in diamond on the end. Idea is one object is either physically part of another or is exclusively owned by another one

Question 9

What is a aggregation relationship?

Answer 9

It is a slightly weaker form of composition. Means a part may outlive the whole it is part of. The owner only has temporary ownership, not exclusive ownership

Question 10

What is a dependency?

Answer 10

Where objects come into temporary contact with each other during the running of the program

Question 11

How do we know if we need a relationship?

Answer 11

• If one class creates an instance of another one
• If one class received an instance of another on as input to a method/function
• If one class has an instance of another one as one of its fields/attributes

Question 12

What is a uni-directional association?

Answer 12

Messages only travel in one direction. Helps protect coupling and reuse

Question 13

What is a generalisation?

Answer 13

Relationship between a general concept and its more specific forms. Allows parent child hierarchies

Question 14

What is a superclass?

Answer 14

It is a class which represents a general concept. Defines characteristics and behaviours expected of any type of this class, e.g. shape

Question 15

What does a subclass inherit?

Answer 15

It automatically inherits all characteristics and behaviour from it parent

Question 16

What is the difference between Generalisation and Specialisation?

Answer 16

Generalisation

• From the bottom up
• Uses superclass

Specialisation

• Top down
• Usually apparent in the application domain

Question 17

What are the golden rules of inheritance?

Answer 17

• Must model as a is-a relationship

- Child concept remains the same for its lifetime

Question 18

What is polymorphism?

Answer 18

Any instance of a child class can be used anywhere an instance of the parent is expected

Question 19

What is overriding?

Answer 19

A child class may refine or replace a feature inherited from a parent class by defining a feature with the same name

Question 20

What is an abstract class?

Answer 20

It is a class which is an abstract concept, such as a shape

Question 21

What is an interface?

Answer 21

A construct to specify a common piece of behaviour required by a system. No implementation is provided, another class has to implement the operations

Question 22

What are the differences between abstract and interface classes?

Answer 22

Abstract class are what an object is while interface is what an object does
Identifying
-Abstract: usual inheritance rules apply
-Interface: when disparate concepts wish to provide the same behaviour

Structure

• Abstract:define attributes, operations, relationships
• Interface: define operations only

Question 23

What are some dangers with Inheritance?

Answer 23

• Inheritance may create fragile hierarchies (changes to a parent can ripple throughout the code)
• Inheritance is said to provide weak encapsulation (parent class has direct access to innards of child class)