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The <<extends>>use case was intended to indicate conditional relationships and some-

thing akin to inheritance, but so much confusion exists in the object-modeling commu-

nity about the distinction between <<uses>>and <<extends>>that many developers have

simply set aside <<extends>>, feeling that its meaning is not sufficiently well under-

stood. Personally, I use <<uses>>when I would otherwise copy and paste the entire use

case in place, and I use <<extends>>when I only usethe use case under certain defin-

able conditions.

Interaction Diagrams Although the diagram of the use case itself might be of lim-

ited value, you can associate diagrams with the use case that can dramatically improve

the documentation and understanding of the interactions. For example, you know that the

Withdraw Cash scenario represents the interactions among the following domain objects:

customer, checking account, and the user interface. You can document this interaction

with an interaction diagram (also called a collaboration diagram), as shown in

Figure 11.11.

346 Day 11

FIGURE11.11

UML interaction

diagram.

Customer User-Interface(ATM) Checking Account

1: Request Withdrawal

2: Show options

3: Indicate amount and account

4: Check Balances, status, etc.

5: Return Authorization

6: Debit $300

7: Dispense cash

8: Request receipt

9: Print Receipt

The interaction diagram in Figure 11.11 captures details of the scenario that might not be

evident by reading the text. The objects that are interacting are domainobjects, and the

entire ATM/UI is treated as a single object, with only the specific bank account called

out in any detail.