Paul Doyle

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Paul Doyle April 2008 DT211 Year 3
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Package Design Diagram Techniques
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What is a Package

• Classes are not sufficient to group code
• Some classes collaborate, implying dependencies
• Some don't know each other
• A package is a group of classes
• Classes in a package are often compiled together
  into a library, but unit of compilation is mostly
  individualclass
• A package is a unit for testing
• A package can be a releasable component

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Dependencies in Packages

• Dependencies between elements are an important
  aspect of any system design result in coupling
  between elements and determine the scope of the
  impact of the changes
• Reasons for existence of dependencies
• Elements in packages send messages to elements in
  other packages
• Class in a package may have another class in a
  different package as part of its data
• Class in a package may mention another class in a
  different package as a parameter to an operation

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What is a Package

• Security
• Utils
• Search
• Account

Utils
Security
Search
Account
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Package Notation

• Different ways to show the same thing
• Classes within packages

Security
Security
Credentials
IdentifyVerify

Credentials
IdentifyVerify
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Package Notation

• Packages can contain other packages
• They can be deeply nested

acme
Search
Indexing
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Package Notation

• Nested packages imply inheritance
• Not always implemented in reality

acme
Search
Indexing
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Package Notation

• Element visibility
• MD5Crypt is a private class in security

Users
Security
visible
Credentials
User
visible
IdentifyVerify
- MD5Crypt
NOT visible
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Package Dependencies

• Package A depends on Package B
• Class in one package used class in another
• Element in A uses element in B

A
B
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Packages in Software

• Dont need them in small programs
• Larger projects can assign owners to packages
• Can help understand impact of changes
• Provides levels of independence
• Helps provide a high level view of systems

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Package Example
Search
Indexing
Utils
GUI
Account
Security
Users
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Package Example
The package user imports security, so classes in
users may use public classes in security without
having to specifiy the package name
ltltImportgtgt
Security
Users
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Package Example
The users package imports only the credentials
elements from the security package
Security
Credentials
ltltImportgtgt
Users
IdentifyVerify
- MD5Crypt
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Package Example
Private visability causes a class not to be seen
even through Its package is imported
Security
Credentials
ltltImportgtgt
Users
IdentifyVerify
- MD5Crypt
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Package Example

• Package B imports C and access D
• B can see public elements in C and D
• A Imports B so A can see public elements in B
• A Can also see public elements in C because C is
  publicly imported into B
• A cannot see anything in D because D is privately
  imported into B

ltltImportgtgt
C
A
ltltImportgtgt
B
D
ltltAccessgtgt
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Package Example

• Highly dependant indicates poor structure and
  brittle software

A
B
D
C