Identifying design patterns

In groups, determine a possible pattern that could be used to solve each of the following problems:

• Sorting collections of records in different orders. 
• Modelling a file system. 
• Updating a UI component when the state of a program changes. 
• Parsing and evaluating arithmetic expressions. 
• Adjusting the brightness of a screen based on a light sensor. 

Then pick one and start to think about potential entities and draw up a rough UML diagram.

Identifying design patterns

In groups, determine a possible pattern that could be used to solve each of the following problems:

• Sorting collections of records in different orders. Strategy
• Modelling a file system. Composite
• Updating a UI component when the state of a program changes. State
• Parsing and evaluating arithmetic expressions. Composite
• Adjusting the brightness of a screen based on a light sensor. Observer

Then pick one and start to think about potential entities and draw up a rough UML diagram.

Code and Design Smells

In groups, discuss the following examples. Identify the code smells and any underlying design problems associated with them.

• Divergent Change: One class is commonly changed in different ways for different reasons.
• Solution: Strategy Pattern

a) Mark, Bill and Jeff are working on a PetShop application. The PetShop has functionality to feed, clean and exercise different types of animals. Mark notices that each time he adds a new species of animal to his system, he also has to rewrite all the methods in the PetShop so it can take care of the new animal.

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• Code smell: Long parameter list
• Solution: Builder pattern, introduce parameter object, method calls inside construction

• Feature envy: one class is more interested in the details of another class than its own (potential Law of Demeter violations)
• Solution: Move method/Method forwarding

How do these code smells cause problems when developing code?

• Maintainability: Code can get complex and fragile
• Readability: Code can be hard to understand and inconsistent
• Scalability: Rigid/not flexible, hard to extend (think open-closed principle)

Is a code smell always emblematic of a design problem?

• Design tradeoffs: fixing some code smells might introduce new ones
• Some code smells are low risk, and are okay for small programs

Template Pattern

Template Pattern

What type of pattern?