CS 509 Design of Software Systems

1
CS 509Design of Software Systems

• Lecture 6
• Monday, March 1, 2004

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Class Format for Today

• Term Project Administration
• Questions
• Review of Chapter 10
• In-class Exercises
• Discuss contents of new text book, choose lecture
  topics
• Review requirements for HIS

3
Term Project Administration

• Phase 3 CIS Design (and journals) due
• Return Quiz 3
• Hand out Phase 4 assignment
• Change to grading policy P4 5, P5 10
• Phase 4 discussion
• Only have 1 week for analysis
• Would it be more or less productive to have
  larger groups for phase 4?
• 2 groups of 4 instead of 4 groups of 2?

4
Questions?

• About what was covered last time
• From the reading
• About the Term Project
• Anything else?

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Chapter 10

• Mapping Models to Code

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Overview of Activities

• What are the purposes of the activities in this
  chapter?
• Optimizing the class model
• Mapping associations to collections
• Mapping operation contracts to exceptions
• Mapping the class model to a storage schema

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Optimizations

• Address performance requirements of system model
• Optimizing associations
• Reducing multiplicity
• Adding redundancy
• Why do each of the above?
• Adding derived attributes

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Transformations

• Model transformations
• Refactorings
• Forward engineering
• Reverse engineering

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Model Transformations

• Applied to an object model, produces another
  object model
• Purpose
• To simplify or optimize original model
• To bring into closer compliance with requirements
• Actions
• Add, remove or rename classes, operations,
  associations and/or attributes

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Refactoring

• Transformation of source code
• Improve readability or modifiability without
  changing behavior
• Focus on a specific field or method of class
• Done in small incremental steps interwoven with
  testing
• Examples (What do they do?)
• Pull Up Field, Pull Up Constructor, Pull Up Method

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Forward Engineering

• Applied to a set of model elements
• Results in a set of source code statements
• Class declaration, expression, DB schema
• Maintain strong correspondence between object
  design model code
• Reduce number of errors introduced during
  implementation

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Reverse Engineering

• Applied to source code
• Results in a set of model elements
• Recreate the model for an existing system
• Why?
• Does not necessarily create original model
• Why not?

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Transformation Principles

• Goal is to improve design
• Making changes is potentially hazardous
• How to avoid introducing new errors
• Address a single criteria or design goal
• Local transformations, not architectural changes
• Applied in isolation to other changes (how?)
• Followed by testing to validate

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Optimizing for Performance

• Increase in performance may result in more
  complexity
• Want a balance between efficiency clarity
• Minimize repeated association traversals
• Reduce many associations to one
• Eliminate unneeded classes by moving attributes

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More Optimizations

• Collapsing objects into attributes
• How do you know where this applies?
• When should this activity be done?
• Delaying expensive computations
• Proxy design pattern may be useful
• Caching results of expensive computations
• Data can be stored temporarily (time vs. space)
• Need to be careful about when to update

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Mapping Associations

• Associations in UML have no direct counterpart in
  Java
• Can map to reference or collection
• Unidirectional vs. bi-directional
• One-to-one, one-to-many, many-to-many
• Qualified associations
• Association classes

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From Contracts to Exceptions

• Contracts are mapped to exceptions using a
  corresponding check in the code (an if statement)
  that throws an exception if a contract condition
  is not satisfied
• When is it necessary to check contract
  conditions?
• How do inheritance and encapsulation apply?

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Persistent (Relational) Storage

• Mapping object models to persistent storage
  required defining a schema
• Tables and keys
• Primary key, candidate key, foreign key
• Map each class to a table of the same name
• Map attributes to columns in table
• Choose primary key (how?)

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Association Relationships

• Buried associations with multiplicity one
• Why is this called a buried association?
• How does it work?
• Association (or Bridge) Table
• What is a bridge table?
• What is it used for?
• How does it work?

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Mapping Inheritance

• Vertical mapping
• Typo in book on page 418 (sub vs. super)
• Horizontal mapping
• What are the differences? The trade-offs?
• Which one has a role column?
• What is it for?

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In-class Exercises

• Discuss contents of new text book
• Choose lecture topics for
• 3/8, 3/15, 3/22
• Review requirements for HIS
• Work on defining attributes for categories

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For Next Time

• Phase 4 due (HIS RAD) and Journals
• Reading TBD
• Think about course content for CS562