The document discusses complex interactions in software design, focusing on state machines and events, including guard conditions, iterative message sends, and defensive programming. It details the essentials of state machines, including transitions, entry and exit events, internal transitions, and event types. Additionally, it highlights structuring models through packages and managing software complexity with a three-tiered architecture.

1. Arab Open University
2nd
Semester, 2006-2007
M301
Unit 4.5
States and Activities
reem.attas@arabou.org.sa

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Topic Road Map
 Capturing more complex interactions
Design issues
An introduction to state machines
Variation of events and actions
Packages

3. Conditional Message Sends
in Sequence Diagrams

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If Statement
Guard

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If .. Else Statement
Mutually
Exclusive
Branch

6. Iterative Message Sends in
Sequence Diagrams

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For Loop

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Topic Road Map
Capturing more complex interactions
 Design issues
An introduction to state machines
Variation of events and actions
Packages

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Defensive Programming
Pre-conditions should always be checked.
Disadvantage: conditions are repeatedly
checked in different places.

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Strategies for Implementing Use Cases
One Central Class.
Actor Class.
Use Cases as Classes.

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Topic Road Map
Capturing more complex interactions
Design issues
 An introduction to state machines
Variation of events and actions
Packages

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The Essentials of State Machines
 An event is an occurrence of a phenomenon at a
certain moment in time and in a certain place.
 Each state represents a particular period for
which a given set of conditions applies.
 Change takes place through transitions from one
state to another. Transitions are said to be fired
or triggered by events.

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Two Simple States for a Room Object
Initial
State
Event
Transition

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… The Essentials of State Machines
 An event when it occurs, has consequences.
One consequence is a change of state. Other
consequences are recorded in the action part of
the label on the transition.
 That is, instead of labeling a transition just with a
simple name, we also record the actions
(consequences) that result from the occurrence
of the event.

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Check In Sequence Diagram

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Adding Detail to a State Machine for a
Room Object
Event
Action
;
aGuest.setRoom (self)

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A Simple State Machine for the Hotel
Class
Self
Transition

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Self Transition
 Originates in one state and returns to the same
state.
 If the same event is used on more than one
transition, you must make sure that no more
than one guard can evaluate to true. At most,
you want just one transition to fire in such
situations.

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Entry and Exit Events
An entry event happens every time an
object enters a state.
An exit event happens every time an
object leaves a state.

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… Entry and Exit Events

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Internal Transition
A special transition that does not involve a
change of state.
In contrast to a self-transition, an internal
transition does not cause the execution of
any entry or exit actions.

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… Internal Transition
 An internal transition may be used to handle an interrupt
where you want some action to take place that does not
affect the current state.
 Internal transitions are written inside the state to which
they refer.

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Final State
 A state chart diagram shows no more than one initial
state but there can be zero, one or more final states.
 Initial and final states signify the creation and destruction
of an object.

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Topic Road Map
Capturing more complex interactions
Design issues
An introduction to state machines
 Variation of events and actions
Packages

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Other Kinds of Events
External Vs. Internal Events.
Synchronous Vs. Asynchronous Actions.
UML defined 4 types of events:
 Call Events.
 Signal Events.
 Change Events.
 Time Events.

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Time Event

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Looking Inside States
Simple State: a state that contains no
substructure.
Composite State: a state that has other
states nested within it, in order to
represent some internal behavior.

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A Simple Telephone State Chart
Diagram

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A State Chart Diagram of the
Composite State Active
Default
Sub-
state

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No Default Sub-state

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Topic Road Map
Capturing more complex interactions
Design issues
An introduction to state machines
Variation of events and actions
 Packages

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Package
A general purpose mechanism for
organizing model elements into groups.
Ex. A package allows you to gather
together classes that share some common
feature or behavior, such as order
processing or stock control.

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Why to Partition a S/W?
Manage size and complexity.
Information hiding.
Logical decomposition.

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Typical MIS

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Nested Packages

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Packages Present Architecture
 Three-tiered architecture advantages:
 Parallel development.
 Reusability.
 Distribution.

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Subsystems
A subsystem has two sets of elements:
 Specification: Uses cases.
 Realization: Classes.

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… Subsystems
Or
<<subsystem>>
Operations

39. TMA4 – Q5

40. Thank You!