3SFE514 Objectoriented Design

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3SFE514 Object-oriented Design

• Lecture 2 Class diagrams

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Class diagrams

• Class diagrams model the compile-time structure
  of a system
• They show classes and the relationships between
  them
• UML defines 4 types of relationship
• associations
• generalization (inheritance, extends)
• implementation (of an interface)
• dependencies

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Stock Control System

• All parts of the same kind have the same
  attribute values (name, number, cost)
• wasteful of resources
• hard to maintain, eg if the cost changes
• Better to create a new class for shared data
• call this a catalogue entry
• describes a type of part
• each individual part described by one entry

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The Catalogue Entry Class

• Contains data describing types of part
• public class CatalogueEntry
• private String name
• private long number
• private double cost
• public CatalogueEntry(String nm, long num, double
  cost)
• name nm
• // etc
• public String getName( ) return name
• // etc

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Linking Parts to Catalogue Entries

• Catalogue entry class defines shared data
• Each part holds a reference to an entry
• public class Part
• private CatalogueEntry entry
• public Part(CatalogueEntry e) entry e
• When parts are created, an entry is given
• CatalogueEntry screw new CatalogueEntry(screw,
  28834,0.02)
• Part s1 new Part(screw)
• Part s2 new Part(screw)

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Associations

• Model data relationships between classes
• at run time a part will hold a reference to a
  catalogue entry

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Association Properties

• How to model a data member of a class
• if it holds a simple data value, model as an
  attribute
• if it holds a reference to another object, model
  as an association
• Associations have
• a name (optional)
• two (or more) association ends

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Association Ends

• Each end of an association can have a number of
  properties
• rolename
• can be derived from variable names in source code
• visibility
• the access level of the corresponding data member
• multiplicity
• stating how many references the source object
  holds
• navigability (shown by an arrowhead)
• shows which direction the reference points in

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Multiplicities

• Associations derive from
• data members that hold a single reference
• usual multiplicity 0..1 (zero or one) (null
  reference)
• can specify exactly 1 (the UML default)
• a data structure that can hold many references
• usual multiplicity 0.. or (zero or more)
• can specify more precisely if required, eg 1..
  (one or more)
• Multiplicities have lower and upper bounds
• represents an unbounded multiplicity

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References between objects

• Associations model variables which hold
  references
• Different cases
• instance variables (including static)
• model as an association (default case)
• parameters
• association with the stereotype ltltparametergtgt
• local variables
• association with the stereotype ltltlocalgtgt

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Implementing Assemblies

• Assemblies could be implemented using a data
  structure to hold references to parts
• public class Assembly
• private Vector parts new Vector()
• public void add(Part p)
• parts.addElement(p)
• These links are instances of a new association

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The Contains Association

• Contains is the association name
• an assembly contains parts
• parts is the rolename
• An assembly can contain zero or more parts
• this is documented by the multiplicity

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Polymorphism

• Using inheritance to construct hierarchies of
  assemblies and subassemblies
• public abstract class Component ...
• public class Part extends Component ...
• public class Assembly extends Component
• private Vector components new Vector()
• public void addComponent(Component c)
  components.addElement(c)

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Generalization

• Javas extends relationship is represented by
  generalization in UML
• Also called specialization if viewed downwards

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Properties of Generalization

• Generalization is not an association
• usually unlabelled
• never has multiplicity annotations
• Explained by substitutability
• an instance of a subclass can be used wherever an
  instance of a superclass is expected
• similar to reference conversions in Java
• Superclass features are inherited

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A Polymorphic Association

• Assemblies contain Components
• instances of Contains can link assemblies to
  instances of both Component subclasses
• the Component class is abstract (no instances)

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Design models

• Diagrams which reflect the structure of the
  software artefacts under consideration
• contrast analysis models which reflect the
  structure of the application
• developer vs user perspective
• UML used for both analysis and design
• What's the relationship between code and design
  model?

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Code Generation

• The activity of producing eg Java code
  corresponding to a UML model
• Potentially a useful aid to implementation
• A straight-forward rule-based process
• UML class becomes Java class
• associations implemented by storing references
• Widely supported by tools

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Reverse Engineering

• The activity of producing a UML model from eg
  Java code
• Useful for dealing with undocumented or legacy
  systems
• Hard to automate abstraction
• can extract syntactic information from code
• harder to recapture the abstract design

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Round-trip Engineering

• Designs evolve as development progresses
• One-off generation of code or design of limited
  use
• Ideally, would be able to maintain both code and
  UML in parallel
• change to one reflected automatically in the
  other
• move in a 'round trip' from one to the other and
  back again