Association Classes and Qualified Associations

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Association Classes and Qualified Associations

• Association Classes
• Reference Course Notes p. C-21 to C-23.
• Association Classes a means of associating data
  values with a link (associations).
• Association classes are particularly useful for a
  many to many multiplicity
• Example 1 Student Enrolments

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Association Classes and Qualified Associations

• We can introduce an association class that
  contains attribute (data) values that corresponds
  to the EnrolsIn link, naming the association
  class Registration in the following example (The
  association class could also have been called
  Enrolment).

There is one instance of Registration for each
Student-Subject registration/enrolment.
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Association Classes and Qualified Associations

• The following is legal

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Association Classes and Qualified Associations

• However the following is not

Fred Student
CS1 Subject
Registration mark 57
Registration mark 68
note also the class, Registration, could have
been modeled as an intermediate class and then
there would be no restriction (see lecture 6)
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Association Classes and Qualified Associations

• Example 2 Work Positions
• Another example of the possible use of an
  association class fig 6.46
• A person is employed by a company. They can only
  be employed by one company.
• They have a salary which we need to remember
• Where can we store this bit of data?

(a) store salary with Person
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Association Classes and Qualified Associations

• Example 2 Work Positions
• Another example of the possible use of an
  association class fig 6.46
• A person is employed by a company. They can only
  be employed by one company.
• They have a salary which we need to remember
• Where can we store this bit of data?

(a) or as an association class (i.e. as a
property of the position occupied) see p C-22 for
a discussion
A salary is not an intrinsic property of a
person, the position the person fills is paid.
The salary is a property of the association
(position) rather than of the person or company.
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Association Classes and Qualified Associations

• Qualified Associations Reference Course Notes
  p. C-23 to C-25.
• Consider the following class diagram

If we were modelling a real world situation then
we would expect each student to be
uniquely identified by an identification number
(i.e. id). The above model doesnt indicate that.
This is quite legal.
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Association Classes and Qualified Associations

• Qualified Associations Reference Course Notes
  p. C-23 to C-25.
• If we wish to state (in the model) that the
  attribute id has to be unique across all student
  objects then we could use the idea of a qualified
  association between university and student.
• The qualifier is an attribute of the association
  between two classes
• In this case id is an attribute of the
  association between University and Student. The
  University is responsible for allocating unique
  student numbers so the qualifier goes on it. e.g.

Every student at the University has a unique
student number. An id number may be associated
with 0 or 1 student (0 is needed as an id may
not have been assigned to a student yet e.g.
printout of id numbers in advance of being used).
A student is enrolled at one university. A one
to many relationship is implied with qualified
associations e.g. a university can have many
students each with their own unique id number.
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Association Classes and Qualified Associations

• Qualified Associations Reference Course Notes
  p. C-23 to C-25.
• The University class is responsible for
  allocating the student numbers (and ensuring that
  they are unique).
• To best understand what is happening here
  consider the following informal object
  type diagram

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Interfaces

• "An Interface is a specialised class declaration
  that can declare constants and method
  declarations, but not method implementations".
  (Java API)
• Interface is NOT referring to the user interface
  e.g. GUI
• An interface is a description of behavior.
  Classes that implement the interface must provide
  the services (methods) described by the interface
  (Budd p401)
• Only method definitions NOT implementations may
  be provided (e.g. no method body)
• A class may implement multiple interfaces
• While a method must be implemented in the class,
  they may be skeletons e.g. empty body
• The stereotype interface is used above the
  interface name in the class diagram to specify
  and interface
• A dotted line (dependency) with an arrow pointing
  to the interface is used to denote the use of an
  interface
• An optional stereotype use can be added to the
  dependency line. Largely it is redundant

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Interfaces

• Example 1 Providing a common interface
• The Switch interface could be used on
  ElectricalDevices and other unrelated classes,
  giving a common interface (behavior).
• Uses
• A class designer may choose to write the Switch
  interface because they believe that this is a
  common behavior that will be useful in a number
  of classes.
• A class designer or programmer may choose to
  implement the Switch interface because it gives a
  class a common look and feel to other classes
  that implement the interface.
• A programmer inheriting from a class that
  implements Switch (or its base class (superclass)
  ... implements Switch) knows that these methods
  are available.

A class diagram showing the Switch interface
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Interfaces
The Switch interface in detail
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Interfaces

• Example 2 Multiple Interfaces
• The following example uses multiple interfaces
  (Switch and Controller).
• The class GasHeater inherits from the superclass
  GasDevice, e.g. a GasHeater is-a type of
  GasDevice. However a GasHeater can also be an
  ElectricalDevice. e.g. GasHeater inheriting from
  the superclasses GasDevice and ElectricalDevice.
  Rather than using multiple inheritance (Java,
  VB.NET don't support multiple inheritance, C
  does) However GasHeater can inherit from the most
  appropriate superclasses GasDevice and implement
  interfaces that are appropriate Switch and
  Controller, thereby giving it behaviors in common
  with ElectricalDevices.

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Interfaces
The Controller interface in detail
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Syntax for detailed class descriptions

• Static Methods and Fields
• Static fields/methods apply to the whole class
  rather than an instance
• Static methods/fields are also called Shared
  methods/fields (VB.NET) or Class methods/fields
  (Delphi)
• Static fields or methods are underlined in a
  class diagram.
• Static Fields
• Only one instance of a static field exists for a
  class, irrespective of the number of instances
• Use
• e.g. a field to count the number of instances of
  a class

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Syntax for detailed class descriptions

• Static Methods
• Static methods can be called without referring
  to an instance of a class
• A constructor is a special type of static
  method.
• Examples of static methods (in Java) are
  System.out.println, Integer.parseInt
• Static methods may refer to static fields of the
  class or call static methods of the class
• Static methods may not refer to non-static
  fields of the class or call non- static methods
  of the class
• Use
• general purpose methods that don't need to
  access (non- static) fields e.g. Math.sqrt
• methods that only access static fields in the
  class e.g. getCount() the number of instances
  of a class

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Syntax for detailed class descriptions

• Fields and Methods
• (Conventions for specifying attributes and
  operations in the class diagram)
• Fields (Attributes)
• Format
• access field-name data-type valueaccess
  field-namemultiplicity data-type values
• The field name is compulsory, the other parts are
  optional.Static fields are underlined.
• Access types (or visibility)
• - (Private)
• (Protected)
• (Public)

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Syntax for detailed class descriptions

• Fields and Methods
• Conventions for specifying attributes and
  operations in the class diagram
• Some standard data types are
• Integer
• Double (floating point)
• String
• Char
• Boolean
• Date
• Currency

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Syntax for detailed class descriptions

• Multiplicity
• Attributes can exhibit multiplicity (the default
  is exactly one) e.g.
• title String
• The following means that some instances of the
  class Subject need not store a date of the
  examination because there is no exam
• exam0..1 Date
• The following  means that the names of all staff
  delivering a module can be stored (i.e. more than
  one)
• staff1.. String

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Syntax for detailed class descriptions

• Enumerated Types
• Enumerated types may be specified as follows

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Syntax for detailed class descriptions

• Example

- private protected public
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Syntax for detailed class descriptions

• Methods (operations)
• Format
• access method-name(param1 data-type, ...)
  return-type
• No return type is shown for void methods
  (subroutines/procedures).Static methods are
  underlined.Abstract methods and classes are
  shown in italics

Student
 
Student()Student(firstName, lastName)setName(
firstName String, lastName String)setTitle(new
Title String)getTitle() Stringenrol(Student)-
getEnterScore() DoublegetCount() Integer
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Syntax for detailed class descriptions

• Putting it all together!

Student
 -number Integername Stringtitle
Stringenrolled Boolean False-feesOwing
Currency 0enterScore Doublecampus String
"Bendigo"favouriteColour Colourexam0..1
Datestaff Stringcount Integer 0
Student()Student(firstName, lastName)setName(
firstName String, lastName String)setTitle(new
Title String)getTitle() Stringenrol(Student)-
getEnterScore() DoublegetCount() Integer
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Syntax for detailed class descriptions

• Design level vs Implementation Level Class
  Diagrams
• Class diagrams can be shown with differing
  amounts of detail. Analysis/Design level detail
  may omit some details (e.g. parameter type). This
  may be appropriate as the details may not be
  known or may not be relevant at the current stage
  Class diagrams that are appropriate for
  implementation show all details relevant to a
  programmer e.g. all parameters, data types,
  return types, visibility (e.g. public, private)
  etc.