2/23-2/25 - Design Pattern

Question 1

Alexander’s Pattern Language

Answer 1

• Each pattern describes a problem which occurs over and over again and then describes the core of the solution to that problem in way that you can reuse that solution multiple times.
• A pattern language is an organized way of tackling an architectural problem using patterns

Question 2

Software Design Patterns

Answer 2

• Descriptions of communicating objects and classes that are customized to solve a general design problem in a particular context
• They are not full designs or individual classes or libraries
• Design patterns name common successful ways of building software

Question 3

Elements of a Design Pattern

Answer 3

1. Pattern Name
2. Problem Solved
3. Solution
4. Consequences

Question 4

Example: A Text Editor

Answer 4

Problem: Document Structure
• Primitive glyphs
• Lines
• Columns
• Pages
• Documents

Question 5

Composites

Answer 5

• This is the composite pattern
• Applies to hierarchical structure
• Leaves and internal nodes have same functionality
• Composite implements the same interface as the contained elements

Question 6

Pros of Composites

Answer 6

• Easy to add, remove new object types

* Clean interface, elegant code. No need to iterate over all object types to draw

Question 7

Cons of Composites

Answer 7

• Have to fight to add corner cases

* Hard to enforce that a composite only has certain elements (Ex. A line does not contain columns)

Question 8

Problem: Enhancing the User Interface

Answer 8

• We want to add borders, scrollbars, both
• How do we incorporate this into the physical structure?
• Object behavior can be extended using inheritance

Question 9

Using Inheritance to Extend Object Behavior

Answer 9

• Major drawback: inheritance structure is static
• Subclass elements of Glyph: BorderedGlyph, ScrollGlyph, etc.
• Leads to a lot of classes

Question 10

Decorators

Answer 10

• This is the decorator pattern
• A way of adding responsibilities to an object
• Problem: Supporting look and feel settings

Question 11

Using Decorators to Add Responsibilities to an Object

Answer 11

• Decorations can be chosen dynamically

* Transparent enclosure

Question 12

Problem: Supporting look and feel settings

Answer 12

• Different look and feel standards

Question 13

Factories

Answer 13

• This is the factory pattern
• A class which abstracts the creation of objects. Different instances provide alternative implementations if that family
• “current” factory is still a global variable. Can be changed even at runtime
• Problem: Supporting multiple window systems

Question 14

Problem: Supporting multiple window systems

Answer 14

• Big interfaces
• Different models of window operations
• Different functionality

Question 15

Bridge Pattern

Answer 15

• This is the bridge pattern

• We have two hierarchies
• Logical – the view of our application, tuned to our needs
• Implementation – interface to the outside world; abstract base class with multiple
Implementations

• Logical implementation views can evolve independently as long as the bridge is maintained
• Problem: Spell checking

Question 16

Logical Hierarchy (Bridge Pattern)

Answer 16

the view of our application, tuned to our needs

Question 17

Implementation Hierarchy (Bridge Pattern)

Answer 17

• Interface to the outside world

* Abstract base class with multiple implementations

Question 18

Problem: Spell checking (Bridge Pattern)

Answer 18

Solution: iterators

• Hide the structure of a container from clients
• Iterators work well if we don’t need to know the type of elements being iterated over

Question 19

Visitor Pattern

Answer 19

• The visitor pattern is more general
• Iterators provide traversal of containers
• Visitors allow traversal and type-specific actions on container elements
• Separate traversal from action. Have a do-it method for each element type
• Extremely powerful pattern. Is non-obvious
• Whenever you have a hierarchical structure of heterogeneous elements write support for visitors. Other traversals can be written as a subclass of visitors

Question 20

Strategy Pattern

Answer 20

• Strategy pattern isolates variations in algorithms
• Formatter is the strategy. Compositor is the context
• General principle: encapsulate variation
  
  • i.e. Define abstract classes for things that are likely to change

Question 21

Command Pattern

Answer 21

User essentially has a small programming language that is finite

• Abstraction makes this explicit
• Class structure can represent all possibilities explicitly