Elizabeth Bigelow

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UML Part II -- Structural Modeling

• Elizabeth Bigelow
• School of Computer Science
• Carnegie Mellon University

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Whats a class?

• Description of a set of objects that share the
  same attributes, operations, relationships, and
  semantics. A class implements one or more
  interfaces.
• Well structured classes have clear boundaries and
  form part of a balanced distribution of
  responsibilities across the system.

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Whats an object?

• An entity
• which has state
• characterized by the actions that it suffers and
  that it requires of other objects
• is an instance of some class
• denoted by a name
• restricted visibility of and by other objects
• may be viewed either by its specification or by
  its implementation

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Systems Designed in O-O

• Exhibit characteristics different from those
  designed in functional manner
• Large o-o systems tend to be built in layers of
  abstraction, each layer denoting a collection of
  objects and classes of objects with restricted
  visibility to other layers
• Collections are called subsystems
• Elements of a subsystem tend to be flat rather
  than hierarchical and deeply nested

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Canonical Structure of Large O-O System
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Flow of Control

• Global flow of control is different from
  functionally decomposed system
• Often cannot identify a central thread of
  control!
• Subprogram calls are often very deeply nested

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Methods

• Denotes the response by an object to a message
  from another object
• Classes of operations
• constructor (alters state of an object)
• selector (evaluates current state)
• iterator (permits all parts of an object to be
  visited)

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Classes of Objects

• Actor, Agent or Server
• depends on how relates to surrounding objects
• actor only acts on other objects
• server suffers operations but may not operate on
  other objects
• agent performs task on behalf of another object

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Very Important Property

• Each object is a unique instance of some class.
• Alternatively, a class denotes a set of similar
  but unique objects.
• Example class of Window from which we
• create instances or objects. Operations defined
• for class, but only have effect on object.

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Class

• Specifies objects internal data and
  representation defines operations it can perform
• Objects are created by instantiating a class.

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Terms and Concepts

• Names
• Attributes
• Operations
• Responsibilities

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Class Inheritance

• New classes can be defined in terms of existing
  classes using class inheritance.
• When a subclass inherits from a parent class, it
  includes the definitions of all the data and
  operations that the parent class defines.

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Inheritance
Parent Class
Operation
Subclass
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Example of Inheritance
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Class versus Type

• Objects class defines how the object is
  implemented (internal state and implementation of
  operations)
• Objects type only refers to its interface--the
  set of requests to which it can respond
• Can an object have many types?
• Can objects of different classes have the same
  type?

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Aggregation

• Aggregation implies that one object owns or is
  responsible for another. (having or being part of
  another object) gt identical lifetimes!
• Acquaintance (or association) implies that an
  object merely knows of another object. May
  request operations of each other but are not
  responsible for one another. (looser coupling)

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Aggregation Types

• Aggregations can be fixed,variable or recursive
  (read diamond as is made up of
• .

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Model Goals

• Communication
• Expression of graphical constraints
• Abstraction
• Reasoning

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Constraints

• Functional relationships between entities of an
  object model (objects, classes, attributes, links
  and associations)
• Restriction of values that entity can assume

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