OO State Transition Diagrams (PPT 18)

Question 1

What is a state?

Answer 1

A condition or situation in the life of an object during which it satisfies some condition, performs some activity or waits for some event

Question 2

How can we find out a state of an object?

Answer 2

We can sometimes do this by looking at the value of an objects attributes

Question 3

When can an object change state?

Answer 3

It can only change state when an event occurs

Question 4

What are State Transitions?

Answer 4

They show movement from one state to another as a result of an event. Represented by direct lines between states, with the event that causes the change written as the name

Question 5

What are the three types of event?

Answer 5

Call or signal event
-The receipt of a message from one object in the system

Change event
-A condition becomes true within the object

Time event
-An amount of time has elapsed

Question 6

What is a State Transition Diagram?

Answer 6

A diagram that captures the lifecycle of an object

• States the object can assume
• Events/messages it can receive
• Response to events

Question 7

How many state diagrams should you have?

Answer 7

In theory, one for each object but in reality, just model those with interesting states

Question 8

What are some other state diagram notations?

Answer 8

Reflexive notation
-Transitions may lead to the same state they started in

Conditions
-Specify conditions under which transition will take place or not

Actions
-What an object does in response to an event, shown as part of the transition

Activities
-Show the activity which an object performs for the duration of a certain state

Initial/Final States

• Initial state shows starting point of life cycle (circle)
• Final state shows end of life cycle (Circle with circle around it)

Question 9

What is the difference between Activities vs Actions?

Answer 9

Activities are ongoing while a state exists and can be interrupted by other events.