Free Recall

• Get out a sheet of paper or open a text editor
• For 2 minutes write down whatever comes to mind about the last class
  • This could be topics you learned
  • Questions you had
  • Connections you made

A Starting Point

• Simplifying assumptions:
  • the program execution is sequential
  • the program executes on a single machine
• The program is hierarchically structured in terms of three levels:
  • main program
  • subordinate objects
  • external devices

Relevant Concepts

• Main program
  • an active procedure
  • controls the execution logic
  • invokes methods on subordinate objects
• Subordinate objects
  • are objects in the programming sense
  • offer public methods to the main program
  • do not interact with each other
  • have no public fields

Key Relations

• The relation between the main program and the subordinate objects is a reference relation
  • the entity above may invoke services provided by the entity below i.e., the procedure may call methods on the objects below
• The relation between objects and external devices is an encapsulation relation
  • an external device is encapsulated by a single object
  • access to the external device below is controlled by the object above

Notation

Design Principles

• 1. Separation of Concerns
• 2. Information Hiding
• 3. Data Encapsulation
• 4. Device Encapsulation
• 5. Balanced Levels of Abstraction
• 6. Protection Against Change

1. Separation of Concerns

• The principle of separation of concerns demands that:
  • unrelated concerns should be associated with distinct entities in the design
  • related concerns should be associated with a relevant entity in the design
• This principle impacts design decisions relating to modularity
• Object-oriented design enables the application of this principle
• Strict application of the principle is not always straightforward
• Changes to requirements may have a major impact on the design

Example: HTML, CSS, and Javascript

• HTML: HyperText Markup Language, used to specify the contents of a webpage
• CSS: Cascading Style Sheets, used to specify the style of a webpage
• Javascript, used to specify the behavior of the webpage
• Each piece has it own separate use and a change in one should* not affect the others

2. Information Hiding

• Also known as encapsulation
• Limit knowledge about design decisions as much as possible.
  • fundamental to encapsulation
• Postpone design decisions for as long as possible.
  • fundamental to top-down design
• This relates strongly to the scope of program changes… How we can minimize them?

Effective Java: Item 15

• Item 15: Minimize the accessibility of classes and members
• A well designed component hides all its implement details
• Cleanly separates its API from its implementation
• Gives programmers the ability to think about one part of the system at a time
• Allows precise optimization
• Cardinal Rule: Make each class or member as inaccessible as possible

Information Hiding: Example

• Let’s go to IntelliJ for an example
• You can download these files here

3. Data Encapsulation

• Bundling data with methods that operate on this data.
• Decoupled implementation details from operations on the data.
• This simplifies programming.

Illustration: Custom Dictionary

• Consider an object called MyDictionary:
  • initially contains an empty set W of words
  • at most N words can be stored
• addWord(w) – adds one word to the set W, if there is room for it
• removeWord(w) – removes one word from the set W
• containsWord(w) — returns true iff the word is in the set W
• Simple Implementation: array of strings

Illustration: Custom Dictionary Revisited

• Consider the following change in requirements:
  • at most N words can be stored
  • the number of words is very large
• This new implementation requires a tree structure.

4. Device Encapsulation

• Devices are a volatile element of most designs.
• Protect the system against device/protocol substitutions:
  • micro-controller reassignment of pins
  • communication interface (USB connection vs. Ethernet)
  • memory mapped I/O
• Layers of encapsulation:
  • application-specific virtualization
  • virtual device
  • device driver

Illustration: Timers

5. Balanced Levels of Abstraction

• In a hierarchically designed program or system:
  • when moving up in the structure the level of abstraction should increase
  • when moving down in the structure the level of abstraction should decrease
  • entities at the same level in the structure should exhibit comparable degree of abstraction

Illustration: Message Delivery

6. Protection Against Change

• The fundamental engineering concern of object-oriented design is to protect the design and implementation against impact of potential changes
  • modifications to delivered code are expensive
  • modifications can introduce errors
  • limiting the scope of potential changes reduces cost and mitigates risks
• Any proposed design needs to be analyzed with respect to the impact of changes
  • processing logic
  • processor changes
  • device substitution
  • elimination of performance bottle necks