Designing for Visibility

1
Designing for Visibility

• Chapter 19
• Applying UML and Patterns
• Craig Larman

2
Objectives

• Identify four kinds of visibility
• Design to establish visibility
• Illustrate kinds of visibility in the UML notation

3
Introduction

• Q. What is visibility?
• A. Visibility is the ability of one object to
  see or have reference to another.

4
Visibility Between Objects

• Q. When is visibility necessary?
• A. To send a message from one object to another,
  the receiver object must be visible to the
  sender, so the sender has to have a pointer or
  reference to the receiver.

5
Visibility Between Objects

• Example
• Q. If A sends messages to B, which must be
  visible to which?
• A. B is visible to A means A can send a message
  to B.
• Some say that "B is an acquaintance of A".

6
Visibility Between Objects
7
Visibility

• Visibility is related to the scope
• Is one resource (such as an instance) within the
  scope of another?
• The motivation to consider visibility
• For an object A to send a message to an object B,
  B must be visible to A.

8
Four Kinds of Visibility

• How visibility can be achieved from object A
  to object B
• Attribute visibility - B is an attribute of A
• Parameter visibility - B is a parameter of a
  method of A
• Local visibility - B is a local object in a
  method of A
• Global visibility - B is in some way globally
  visible

9
Attribute Visibility

• Attribute visibility from A to B exists when B is
  an attribute of A
• Relatively permanent visibility because it
  persists as long as A and B exist
• Common form of visibility
• public class Register
• private ProductCatalog Catalog

10
Attribute Visibility
11
Parameter Visibility

• Parameter visibility from A to B exists when B is
  passed as a parameter to a method of A.
• Relatively temporary visibility because it
  persists only within the scope of the method
• The 2nd most common form of visibility in the OO
  systems

12
Parameter Visibility
13
Parameter to attribute Visibility

• It is common to transform parameter visibility
  into attribute visibility.

14
Local Visibility

• Local visibility from A to B exists when B is
  declared as a local object within a method of A.
• Relatively temporary visibility since it persists
  only within the scope of the method.

15
Local Visibility

• There are two common means by which
• local visibility is achieved
• Create a new local instance and assign it to a
  local variable.
• Assign the returning object from a method
  invocation to a local variable. A variation of
  this method does not explicitly declare a
  variable, but one implicitly exists as the result
  of a returning object from a method invocation
• Ex anObject.getAnotherObject.doSomething()

16
Global Visibility

• Global visibility from A to B exists when B is
  global to A.
• Relatively permanent visibility since it persists
  as long as A and B exist.
• The least common form of visibility in OO
  Systems.

17
Global Visibility

• Ways to achieve global visibility
• Assign an instance to a global variable.
• Use the Singleton pattern

18
Singleton Pattern (Gang of Four)

• Problem
• Exactly one instance of a class is needed.
  Objects need a single point of access.
• Solution
• Define a class method that returns the singleton
  object, instantiating it if it does not exist.
• Example
• A print queuemany programs must access one queue

19
Illustrating Visibility in the UML
20
Visibility in the UML

• Public
• Any outside classifier with visibility to the
  given classifier can use the feature specified
  by pre-pending the symbol
• Protected
• Any descendant of the classifier can use the
  feature specified by pre-pending the symbol
• Private
• Only the classifier itself can use the feature
  specified by pre-pending the symbol -

21
Terms Classifier

• A classifier is a mechanism that describes
  structural and behavioral features.
• Modeling elements that can have instances are
  called classifiers.
• Classifiers include classes, interfaces,
  datatypes, signals, components, nodes, use cases,
  and subsystems.
• A classifier has structural feature (in the form
  of attributes), as well as behavioral features
  (in the form of operations).

22
Terms Feature

• A feature is a property, such as operations or
  attributes that is encapsulated within entity
  such as an interface, a class, or a datatype.

23
Questions Answers

• Q. Which would you use if you wanted a
  relatively permanent connection?
• A. attribute, or global
• Q. Which would you use if you didn't want a
  permanent connection?
• A. parameter, or local
• Q. How would you create a local visibility?
• A. create a new instance - use result of a
  method call

24
Questions Answers

• Q. how would you achieve a global visibility?
• A. use a global variable in C, static (or
  class) variable (in C or Java) - use the
  Singleton pattern (a static method that returns
  the object)