20-Jan-2003

1
Advanced Topics in Object ModelingDemeter

• 20-Jan-2003
• Lecture 7

2
Outline

• Previous Business
• HW1 due tomorrow at 400 PM
• Overlapping HTML links
• TA Office hours in Fuller A22
• Reading Questions
• Lecture Contents

3
Reading Questions

• TBA

4
Now ready for dynamism

• Structured class diagrams are only so useful
• How do objects from these classes communicate?
• Static relationships ? dynamic relationships

1
1
Controller
updates
updates


Model
View
5
Object Communication

• Message Metaphor
• Object m sends a message to object n
• Messages ? method calls/operations
• synchronous invocation
• blocks for return value
• side effects to parameters discouraged

6
Object Sequence Diagram
cController
mModel
vView
request
alter model
update view
pull data
time

• Messages as arrows
• Rectangles as active
• Objects must have reference to target
• Messages to self typically omitted from diagram

7
Coupling

• The physical interaction between two classes.
• Too much coupling leads to
• maintenance problems
• lower reuse
• less flexible code
• Are there guidelines for interaction? Yes!

8
Law of Demeter
Dont Talk toStrangers

• Within a method, messages can only be sent to
• Its enclosing object
• A parameter of the method
• For pragmatic reasons includes global objects
• Attributes of the enclosing object
• Includes elements of a collection
• An object returned by a method of the enclosing
  object
• An object created within the method

9
Law of Demeter visualized
o Class1
O1 Object
i Item
globalObj
bBook
x
1
2
m (O1,O2)
O1.x ()
2a
3
4
5
Class1
items Item
5 any object created in m
myOp Book
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Specific anti-example

• public int m (Obj o)
• return o.getRoot().getValue()

• What if Obj decides to rename getRoot()? Or throw
  a new exception? Or change the return type? Or
  what if null suddenly becomes a valid return
  value?
• Most common abuse of interaction

11
Object Sequence Diagram 2
cController
mModel
vView
request
1
1
Controller
Update model
updates
updates
update view


Model
View
time

• Controller really in control
• Model and View totally subservient
• To Obey Demeter, Controller maintains modelView
  aggregation

12
Tuesday Assignments

• Homework due tomorrow
• design books will be accepted ONLY at the
  beginning of class
• electronic submission of assignment must be
  received by 400 PM
• any late assignments will be ungraded

13
References

• Law of Demeter