1. David Evans http://www.cs.virginia.edu/~evans
Lecture 1:
Engineering Software, Constructing Poetry and Dancing about Architecture
CS201j: Engineering Software?
University of Virginia
Computer Science
David Evans

2. Menu What is Engineering?
Can we engineer software?
Small, Fun Programs vs. Big, Important Programs
Managing Complexity
Course Mechanics
PS1, Java Introduction
29 August 2002
CS 201j Fall 2002

3. What is Engineering?
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CS 201j Fall 2002

4. Webster’s Definitions
en·gi·neer·ing ( n j -nîr ng) n.
1a. The application of scientific and mathematical principles to practical ends such as the design, manufacture, and operation of efficient and economical structures, machines, processes, and systems.
b. The profession of or the work performed by an engineer.
2. Skillful maneuvering or direction: geopolitical engineering; social engineering.
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CS 201j Fall 2002

5. Design Under Constraint
“Engineering is design under constraint… Engineering is synthetic - it strives to create what can be, but it is constrained by nature, by cost, by concerns of safety, reliability, environmental impact, manufacturability, maintainability and many other such 'ilities.' ...”
William Wulf
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CS 201j Fall 2002

6. Computing Power 1969-2002 (in Apollo Control Computer Units)
Moore’s Law: computing power doubles every 18 months!

7. Constraints Software Engineers Face
Not like those for “real” engineers:
Cost, weight, physics, etc.
Lab machines have ~ 1 million times what was needed to get to the Moon
Complexity of what we can understand
Most important constraint is cost of human effort to get reliability, safety, maintainability
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CS 201j Fall 2002

8. How is engineering software different from engineering bridges?
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CS 201j Fall 2002

9. Bridges Software Continuous Discrete Calculus
Testing/analysis is easy: if the bridge holds for 1M kg, it also probably holds 0.99Mkg
Discrete
Logic, Discrete Mathematics
Testing/analysis is difficult
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CS 201j Fall 2002

10. Bridges Software Made of physical stuff Made of virtual stuff
Some costs are obvious
Changes after construction are hard
Made of virtual stuff
All costs are non-obvious
Changes should be easy (but they’re not)
for (int i = 0; i < rows; i++) {
for (int j = 0; j < columns; j++) {
nextStates [i][j] =
getCellAt (i, j).getNextState (); }
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CS 201j Fall 2002

11. Obvious when it falls down Falls down quietly (usually)
Bridges
Software
Obvious when it falls down
Bridge makers get sued
Architects need licenses
Falls down quietly (usually)
Software vendors blame user, charge for upgrades
Anyone can make software
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CS 201j Fall 2002

12. Requirements are (usually) obvious and easy to describe
Bridges
Software
Requirements are (usually) obvious and easy to describe
A good design is apparent to everyone immediately
Requirements are mysterious and hard to describe
A good design is only apparent to “experts” but has impact later on
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CS 201j Fall 2002

13. Small, Fun Programs vs. Big, Important Programs
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CS 201j Fall 2002

14. Big, Important Programs
Small, Fun Programs
Big, Important Programs
CS101, CS200, etc.
Happy if it works once
Test by running once
If it doesn’t work on some input, no big deal
Simulated in CS201j
Must work on all possible inputs
Need validation strategies
If it doesn’t work on some input millions are lost, people die
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CS 201j Fall 2002

15. Big, Important Programs
Small, Fun Programs
Big, Important Programs
Written by a few people over a short period of time
Manage complexity mostly by memory
Used by a few people over a short period of time
Written by many people over many years
Can’t rely on memory to manage complexity
Used by many people over many years
Needs to be maintained as requirements change
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CS 201j Fall 2002

16. How Big are Big Programs?
Largest program in CS200/CS101:
~500 lines
F-22 Steath Fighter Avionics Software
1.5M lines of code
5EEE (phone switching software)
18M lines
Windows XP
~50M lines
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CS 201j Fall 2002

17. How can we manage Complexity?
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18. Modularity Divide complex systems into many components
Develop components independently
Assemble them to solve the problem
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CS 201j Fall 2002

19. Abstraction and Specification
Ignore details
Separate what from how
Need a specification – description of what a component should do
Components can be built based on what they should do, not how they should do it
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CS 201j Fall 2002

20. How can we make systems dependable?
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CS 201j Fall 2002

21. Analysis Use reasoning and tools to check a design is sound
Use reasoning and tools to check an implementation is sound
Test systematically
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CS 201j Fall 2002

22. Redundancy
Express things in more than one way and check they are consistent
Extreme example:
Space Shuttle
5 on-board computers
4 duplicates running same software
1 running completely separate implementation (to same specifications)
Common example: variable declarations
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CS 201j Fall 2002

23. Design
There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies.
Tony Hoare
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CS 201j Fall 2002

24. Course Overview
This course is about constructing dependable software systems
Managing complexity: modularity, abstraction, specification
Achieving dependability: analysis, redundancy
Good design is key
How to divide problems into modules
How to hide details
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CS 201j Fall 2002

25. Course Mechanics
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CS 201j Fall 2002

26. Staff Coach: David Evans Assistant Coaches Sol Chea Serge Egelman
Joined UVA in Nov 1999
Call me “Dave” or “Coach”
Research areas: security, programming swarms
Assistant Coaches
Sol Chea
Serge Egelman
Tiffany Nichols
Mike Peck
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CS 201j Fall 2002

27. Help Available Me TAs Web site: http://www.cs.virginia.edu/cs201j
Office Hours: Wednesdays 2:30-3:30pm
Always available by , if I don’t reply in 24 hours, send again and complain
TAs
Will post staffed lab hours
Web site:
Everything goes on the web
But mainly: your classmates
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CS 201j Fall 2002

28. CS201J Experimental Course First time offered
National Science Foundation sponsorship
Focus on using lightweight analysis tools
First time offered
I will probably make lots of mistakes
But…you get to take it in a small class
Counts as CS201, but doesn’t cover everything in CS201
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CS 201j Fall 2002

29. Honor Code Classroom Pledge is Horrible!
The whole point of being at a University is so you can:
Learn from your classmates
Learn better by teaching your classmates
Sign and return the CS201j Pledge on Friday
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CS 201j Fall 2002

30. Java
Warning: will quickly introduce lots of new concepts – we will spend a lot more time on many of these later.
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CS 201j Fall 2002

31. Java
Syntax: a lot like C++ (designed to be easy for C and C++ programmers to learn)
Semantics (what programs mean): a lot like Scheme
This class does not focus on details of Java language
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CS 201j Fall 2002

32. Programming Systems C++ Program Scheme Program C++ Compiler
Scheme Interpreter
Object Files
Machine
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CS 201j Fall 2002

33. Java VM Portability Security Simplicity Why use a virtual machine?
Java Program
Why use a virtual machine?
Portability
If you can implement a Java VM on your machine, then you can run all Java programs
Security
A VM can limit what programs can do to the real machine
Simplicity
VM instructions can be simpler than machine instructions
Java Compiler
Class Files
Java Virtual Machine
Machine
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CS 201j Fall 2002

34. Programming in Java Program is divided into classes A class:
Defines a new datatype
Defines methods and state associated with that datatype
We call a value of a class datatype an object
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CS 201j Fall 2002

35. Problem Set 1 Lots of new concepts, but only a few lines of code
Implement a class that simulates Conway’s Game of Life:
If a cell is alive and it has 2 or 3 live neighbors, it stays alive
Otherwise it dies (overcrowding or isolation)
If dead cell has exactly 3 live neighbors, it becomes alive
What abstractions should we use?
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CS 201j Fall 2002

36. Game of Life Abstractions
Grid of cells
Cell
Keeps track of its state
Methods to calculate its next state
But, this depends on states of its neighbors
State of a cell
Dead or alive
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CS 201j Fall 2002

37. Example Class public class Cell {
// OVERVIEW: A Cell is an object that
// represents a cellular automaton.
// A cell has a current state and
// location, and a method for
// determining its next state.
private CellState state; …
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CS 201j Fall 2002

38. Some Cell Methods boolean isAlive()
// EFFECTS: Returns true if the cell is
// alive, false otherwise.
{ return state.isAlive(); }
public CellState getNextState ()
// EFFECTS: Returns next state value for // this.
{ return state; }
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CS 201j Fall 2002

39. Objects Cell c1 = new Cell (); local variable c1 abstract Cell object
instance variable
state:
CellState object
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CS 201j Fall 2002

40. Method Calls <object>.<method> (<parameters>)
c getNextState ()
local variable
abstract Cell object c1
this
Evaluate c1 to obtain the object it refers to.
Set this to point to that object
Evaluate the body of the method
public CellState getNextState ()
// EFFECTS: Returns next state value for
// this.
{ return this.state; }
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CS 201j Fall 2002
(this is optional here)

41. Inheritance
We can create a new class that inherits methods and state from another class:
public class ExtremeLifeCell extends Cell { … }
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CS 201j Fall 2002

42. Overriding
In the new class, we can replace implementations of methods:
public CellState getNextState ()
// EFFECTS: Returns the next state for this cell.
// The next state will be alive if this cell or any of its neighbors
// is currently alive. {
if (countAliveNeighbors () > 0) {
return CellState.createAlive ();
} else {
return getState (); }
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CS 201j Fall 2002

43. Dynamic Dispatch Cell c; … c.getNextState ();
c could be any subtype of Cell
If c is an ConwayLifeCell, then the ConwayLifeCell
getNextState method is called
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CS 201j Fall 2002

44. PS1 Create a ConwayLifeCell class that is a subtype of the Cell class
Override the getNextState method to implement the Game of Life rules
Because of dynamic dispatch, when you run the simulator with your new class, it will call your getNextState method
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CS 201j Fall 2002

45. Classes, Objects, Dispatch
Lots of complicate issues to deal with:
When is it safe to override a method?
How do we hide implementation details?
When should we use inheritance?
We will address these later in the course
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CS 201j Fall 2002

46. Charge This class is about: Before 5pm Friday: Before class Tuesday:
Managing complexity: modularity, abstraction, specification
Engineering dependability: analysis, redundancy, design
Before 5pm Friday:
registration survey
Before class Tuesday:
Read and sign CS 201j Pledge
Problem Set 1 Due
Remember to take pictures!
29 August 2002
CS 201j Fall 2002