Relationships Among Classes

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Lesson 4

• Relationships Among Classes

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Association

• Association is a relationship where two classes
  are weakly connected i.e. they are merely
  associates.
• The dependency of one class on another is
  bidirectional
• There is no hierarchy.
• The relationship is between equals.

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Association (continued)

• The association has a cardinality
• one-to-one
• one-to-many
• many-to-many
• An association relationship between two classes
  may be identified early in analysis.
• Later analysis may refine the relationship, and
  identify it to be another kind (described below).

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Inheritance

• Inheritance is a relationship among classes where
  a subclass inherits the structure and behavior of
  its superclass.
• Defines the is a or generalization/specializatio
  n hierarchy.
• Structure instance variables.
• Behavior instance methods.

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Inheritance (continued)

• A subclass may augment the existing structure and
  behavior of its superclass.
• A subclass may constrain or restrict the behavior
  inherited from its superclass.
• Single inheritance any class has at most one
  immediate superclass.

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Inheritance (continued)

• Concrete or leaf classes classes that will have
  instances.
• Most classes are concrete.
• Abstract classes classes with no instances.
• Expected that subclasses will add structure and
  behavior to the abstract class.
• The subclasses will have instances (i.e. are
  concrete).

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Inheritance (continued)

• Base class the root of the class hierarchy.
• Is the most general class.
• In Java, Object is the base class.
• In C, you can have forests of inheritance
  trees, thus several base classes.

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Inheritance (continued)

• In Java and C
• A superclass is referred to with the keyword
  super.
• An object refers to itself with the keyword this.

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Inheritance (continued)

• Polymorphism where the same message can be sent
  to two objects of different classes.
• Works because the classes are related in the
  inheritance tree i.e. there is a common
  superclass where the method is defined.
• The subclasses redefine the method, so that
  behavior is specialized in some way.

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Inheritance (continued)

• A subclass augments the behavior of its
  superclass by redefining the superclasss
  methods.
• The subclasss method will invoke the
  superclasss method either before or after doing
  some other action.
• E.g.

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Inheritance (continued)

• Note the polymorphism we can send the set
  message to instances of either class.
• Note the code reuse.
• A subclass augments the structure of its
  superclass by declaring additional instance
  variables.
• E.g.

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Inheritance (continued)

• A superclasss instance variables are directly
  accessible to the subclass if they are declared
  protected.
• If declared private, these variables can only be
  accessed by sending messages to super.
• It is usually poor practice to declare instance
  variables public.

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Inheritance (continued)

• Multiple Inheritance where a subclass may have
  more than one immediate superclass.
• Java does not support multiple inheritance, but
  C supports it.

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Aggregation

• Denotes a whole/part hierarchy among classes.
• Is directly parallel to aggregation relationships
  between objects.
• One or more objects form part of some container
  object.
• The part objects are created and destroyed when
  the container object is created and destroyed.

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Aggregation (continued)

• In Java, only containment by reference is used,
  since objects are created at runtime.
• In C, containment by value is allowed, since
  objects can be created at compile time as well as
  run time.

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Using

• When there is a peer-to-peer link between two
  objects, a using relationship exists between
  the two corresponding classes.
• One class is said to use the other class (i.e.
  the client uses the supplier).

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Instantiation

• A parameterize class (also known as a generic
  class) is one that serves as a template for other
  classes a template that may be parameterized by
  other classes, objects and /or operations. A
  parameterized class must be instantiated (that
  is, its parameters must be filled in) before
  objects can be created. C and Eiffel both
  support generic class mechanisms.
• e.g. Queueltintgt intQueue
• QueueltDisplayItem gt itemQueue

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Instantiation

• In C, a parameterized or generic class can
  serve as a template for other classes.
• When an object is to be instantiated, the generic
  class is given some parameters to complete the
  class definition.
• E.g. Queueltintgt intQueue
• Java does not support generic classes.

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Metaclass

• A metaclass is the class of a class.
• You can treat the class as an object, and send
  messages to it.
• Best supported in Smalltalk and CLOS.
• Indirectly supported in Java using class methods
  and class variables.

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This presentation is based on the following book
Object Oriented Analysis and Design by G.Booch
and partially compiled by Leonard Manzara.