1. State Diagrams
What state are we in?

2. Introduction What are the two main parts of a computer program?
Data
Algorithms
We have previously emphasized
how real-world information can be turned into data.
How processing functions can be described with activity diagrams
We will now describe how the data might change when processed

3. Object Oriented Programming
Object-Oriented programming seeks to solve a problem using data models taken from the problem domain.
Problem domain: the concepts and elements of the problem that is being solved.
An Object is simply a "thing" in the problem domain
When designing software, a programmer will listen to the problem description and then identify the objects in the domain.

4. Object Oriented Programming
A programmer is asked to write a program that will manage a persons DVD collection. The software should be able to search by actor or movie title or year or rating. What are the objects?
A programmer is asked to write a flight simulator. The simulator shows a cockpit display to the user as the plane approaches a landing strip. What are the objects?
A programmer is asked to write a web site identical to Facebook. What are the objects?

5. More about Objects An Object is a model of some thing.
An object has state (attribute data)
An object has behavior (actions it can take)
Example: Radio
What is the state?
What is the behavior?

6. More about Objects We might say that the state includes:
Station (some number)
Power (on/off)
Band (AM/FM)
Volume (some number)
We might say that the behaviors include:
Select the station
Turn on/off
Switch band
Volume up/down

7. Objects Change Over Time
Consider a program that allows users to control a virtual radio. When the software first starts, what are the values of the attributes?
Station, Power, Band, Volume
The user then does the following with the program:
The user turns on the radio
The user selects channel 98.9
The user turns up the volume
What are the values of the attributes now?
The point is, the values of the attributes change over time.

8. The State of an Object
The State of an Object is a snapshot of the attribute values at one moment in time.
Recall the radio example. The start state is something like:
{Station:89, Power:Off, Band:FM, Volume:0}
A series of state changes then follow:
{Station:89, Power:On, Band:FM, Volume:0}
{Station:98.9, Power:Off, Band:FM, Volume:0}
{Station:89, Power:Off, Band:FM, Volume:1}

9. State Diagrams
An activity diagram depicts the actions and their order when processing data
State diagrams depict how objects might change when processed.
Consider withdrawing money from an ATM:
State might involve: Checking, Approving, Rejecting.
Activities might involve: Insert Card, Enter PIN, Check balance, select withdraw, enter amount, take money, take card.

10. UML State Diagrams Part of the Unified Modeling Language
Represents how objects are affected by processing activities
the notation
a state
a transition from one state to another
start state
final state

11. Consider representing a student as a software object.
Example: Elementary School
Kindergarten
1st Grade
2nd Grade
3rd Grade
4th Grade
5thGrade
6th Grade
Consider representing a student as a software object.

12. What causes Transitions
an EVENT
satisfies matriculation standards
Kindergarten
1st Grade
fails standards
potential events in software
• a user’s action
• triggered by another object
• a timed event

13. Traffic Light What might the states of a traffic light be?
red / green / yellow / others?
What events cause a transition?
elapsed time
time of day
arrival of car
power outage

14. north/south car arrives
+ 2 sec. delay
RED
YELLOW
GREEN
FLASHING
at 6 a.m.
at 2 a.m.
east/west
car arrives
after 10 sec.
after 5 sec.

15. More Notation Activities can be included within states.
Activities can be triggered when entering/exiting/in.
Machine Operating Safely
entry / start motor
exit / stop motor
do / display warning light
state name
activities
entry -- activity upon incoming transition
exit -- activity upon outgoing transition
do -- continuous activity within state

16. Example - Calculator 1st Operand click digit (0 - 9)
entry / oprnd1  0
do / display oprnd1
click C
click C
click digit (0 - 9)
click digit (0 - 9)
1st Operand More Digits
entry / oprnd1  oprnd1*10 + digit
do / display oprnd1
click digit (0 - 9)
click operator (+, -, *, /)
2nd Operand
entry / op  operator; oprnd2 0
do / display oprnd2
click C
Calculate Result
entry / result  oprnd1 op oprnd2;
oprnd1  0
do / display result
click =
click digit (0 - 9)
click digit (0 - 9)
2nd Operand More Digits
entry / oprnd2  oprnd2*10 + digit
do / display oprnd2

17. Example Get a Job! Initial Stage Person of Interest
applicant inquiry
Initial Stage
entry / create file; send application;
respond to inquiry
employee recommendation
Person of Interest
entry / create file;
send application
Awaiting Application
entry / respond to inquiry
applicant inquiry
applicant inquiry
Still Awaiting
entry / return application
receive
incomplete app
receive
incomplete app
Application Complete
entry / send app to supervisors
app complete
all
supers
reject
Example Get a Job!
super requests
interview
Interview Pending
entry / schedule interview
exit / conduct interview
super requests interview
renegotiate
No Hire
entry / send rejection
all supers reject
Offer Pending
entry / send offer
super says hire
Hired
offer
accepted
offer rejected