Modelling classes

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Modelling classes

• Drawing a Class Diagram

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Class diagram

• First pick the classes
• Choose relevant nouns, which have attributes and
  operations.
• Find the attributes
• Any relevant information about the class objects.
• Next find the operations
• Any behaviour of the class any behaviour that
  changes the attribute values of the class.
• Next find the associations
• Classes are associated if they
• Invoke each others operations
• Have protected access to each others attributes.

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Use Case to Class

• The actors are not always persistent classes
• The primary tasks in the use case diagram must be
  the responsibility of a class.
• Is it a persistent class?
• Which class holds the attributes that the
  operation adds / amends/ updates / deletes?

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Class operations

• Is it public or private?
• Public, if other classes use this operation
• Private, if it only called by the owning class.
• Does it imply an association?
• Yes, if other classes invoke it, or it invokes
  operations in other classes.

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Association

• When the operation close a claim is invoked
• It invokes the getage operation in claimant
• Which invokes the operation CalculateAge in
  claimant

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Sub and Super Classes

• A super-class is an abstraction of several
  classes that have mostly common attributes and
  operations.
• A sub-class instance IS A super-class instance.
• Some super-classes have no instances themselves -
  they are merely abstract classes.

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Generalisation

• A super-class is a GENERALISATION of a sub-class.
• A sub-class is a SPECIALISATION of a super-class.
• Operations and attributes in the super-class are
  automatically INHERITED by objects in the
  sub-class.
• Operations and attributes in the sub-class can
  OVERRIDE those in the super-class.

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Generalisation

• Inheritance
• Information hiding
• Mono and polymorphism
• Overriding needed for
• extension
• restriction
• convenience
• Optimisation

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Generalisation examples

• A generic person class will have
• attributes of name, address, e-mail address,
  phone no, etc.
• Operations of
• Phone
• Mail
• E-mail
• An employee is a person
• A driver isan employee
• An assessor isan employee

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And more

• A radio alarm clock isa clock
• A watch isa clock
• Is a timer a clock?

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General rules

• An object of a specialised class can be
  substituted for an object of a more general class
  in any context which expects a member of the more
  general class, but not the other way around.
• There must be no conceptual gulf between what
  objects of the two classes do on receipt of the
  same message.

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Generalisations

• Are denoted as solid paths with a large hollow
  triangle pointing at the more general element.
• Must not be circular -i.e. an element cannot have
  a generalisation relationship to itself.

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Sample Generalisations

• A School of Computing student is a student.
• A school of computing student is an
  account-holder.
• A lecture is a member of academic staff is a
  member of staff of the school of Computing.
• A school of computing lecturer is an account
  holder.
• A frog is a water-based animal.
• A frog is a land-based animal.
• A fish is a water-based animal.
• A rabbit is a land-based animal.
• An elephant is a land-based animal.

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Specialisations

• More specific elements.
• Specialize more general elements.
• Receive all the characteristics (attributes,
  operations, methods and associations) of the more
  general elements via the mechanism of
  inheritance.
• May add their own characteristics.
• May override inherited methods.
• May be substituted for their more general
  elements.

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Starting a Class Diagram

• Choose your candidate objects from the system
  description.
• For each candidate
• Is there more than one object in this class?
• Is this object a system user / location?
• Can you describe the type of information you need
  to know about each member of this group of
  objects?

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Starting Class Diagrams

• Rather than trying to define the entire class
• concentrate on the data that is required for
  business classes.
• set up a data model, that can be converted into a
  relational database, only without the keys.
• later, loosen the structure, to give
  object-oriented advantages.
• add operations on the data.

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What Is an Attribute?

• An attribute is the type of information you need
  to know about each object in a class.
• An attribute is an attribute when-
• it has a finite length
• it has a single value for each object
• An attribute is not an attribute when-
• it in turn has several attributes
• it has multiple values

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Operations

• The operations in a class include
• Constructor operations create new objects in the
  class.
• Selector operations get information about and
  from an object in a class.
• Mutator operations set information about and to
  an object in a class.
• Destructor operations destroy objects of the
  class.
• An operation may also send a message to a class
  that is associated directly with the class to
  which the operation belongs. It can only send a
  message it cannot operate on the other class!

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Operations

• Are named services that may be requested of
  instances of a classifier.
• Are implemented by methods.
• May have parentheses containing a comma-separated
  parameter list that indicates the formal
  parameters passed to a method.
• May have a return list consisting of a
  comma-separated list of formal parameters passed
  back from a method.

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Parameters

• May have a kind specified. This value may be
  in, out or inout.
• May have a colon followed by a type expression
  that indicates the types of values a parameter
  may have.
• May have an equal sign followed by a default
  value that is used to set the value for
  unspecified parameters when the method is invoked.

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Return parameter

• Must have a name or identifier string that
  represents the name of the parameter.
• May have a colon followed by a type expression
  that indicates the type of values a parameter may
  have.

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What is an association?

• An association is a relationship between object
  classes.
• An association is used to implement a link
  between objects to send messages or
  instructions from one class to another.
• An object from one class can invoke a method on
  an object from another class, thereby accessing
  it through its public interface.

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Associations

• Associations correspond to verbs
• Express the relationship between classes
• Class A and class B are associated if an object
  of class A
• Sends a message to an object of class B
• Creates an object of class B
• Has an attribute whose values are objects of
  class B or collections of objects of class B
• Receives a message with an object of class B as
  an argument

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Multiplicity

• Specifies how many objects from the class are
  involved in each association.
• Each class in the association has multiplicity.
• It can be.
• An exact number e.g. 1.
• A range of numbers e.g. 1..10.
• An arbitrary, unspecified number, using .

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Associations

• Associate objects from two classes.
• The multiplicity of the association is decided at
  both ends of the association.
• To decide on multiplicity,
• Put yourself in the position of a single object
  from the origin class.
• Ask yourself how many objects you are related to
  in the destination class.
• This decides the multiplicity at the destination
  end of the association.

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Types of Association

• Simple binary associations.
• Involve two classes, where each may get
  information from the other.
• Uni-directional associations.
• Involve two classes, where one can get
  information from the other, but not vice-versa.
• E.G. Customer and order. Depending on the
  implementation, this could be uni- or
  bi-directional.

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Associations

• May be reflexive i.e. associate a class with
  itself.
• May have a name that represents the name of the
  association.
• May have a name-direction arrow. (Small solid
  triangle attached to the name, pointing in the
  direction of application of the name). If
  omitted, names are read right left and
  top-bottom.
• May be association classes.
• May have an aggregation indicator.

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Association Classes

• Are denoted as class symbols attached by dashed
  lines to associations.
• Are associations with class properties or classes
  with association properties.
• Define a set of characteristics that belong to
  the relationship. The characteristics are not
  owned by any of the classes they relate.

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Aggregation

• Indicated by a diamond.
• A hollow diamond indicates weak or shared
  aggregation.
• A solid diamond indicates composition
  associated class objects must belong to only one
  component and are deleted if the composite is
  deleted.
• Must not be associated to both ends of an
  association.

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Multiple Class Associations

• Associations among three or more classes are
  shown as a diamond with paths from each corner /
  side. Such an association.
• Must not involve aggregation or qualifiers.
• May have a single class appear more than once or
  on multiple paths.

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Aggregation

• Indicated by a diamond.
• A hollow diamond indicates weak or shared
  aggregation.
• A solid diamond indicates composition
  associated class objects must belong to only one
  component and are deleted if the composite is
  deleted.
• Must not be associated to both ends of an
  association.

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