--- title: "Money transfer sequence diagram questions" subject: "Object Oriented Analysis and Design" chapter: 2 key: "semester_6/object_oriented_analysis_and_design:money-transfer-sequence-diagram" source: https://www.bctengineer.com/q/semester_6/object_oriented_analysis_and_design/money-transfer-sequence-diagram site: BCT Engineering --- # Money transfer sequence diagram questions Object Oriented Analysis and Design · Chapter 2: Object Oriented Analysis - Asked in: 2069 Bhadra, 3×3 marks - Video: [Sequence Diagram in UML | Software Engineering](https://www.youtube.com/watch?v=VnRQ5CNC4Fs) — Gate Smashers - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/object_oriented_analysis_and_design/money-transfer-sequence-diagram - Whole chapter: https://www.bctengineer.com/md/semester_6/object_oriented_analysis_and_design/chapter-2 ## Question Based on the below diagram for model action of money transfer, answer the following questions: (a) How many numbers of classes are involved in this transfer activity? What are they? (b) What are the events followed for transfer complete? (c) Why this check( ) function is required for this transfer? [Figure: sequence diagram with lifelines a1:Account, a2:Account and John:Person. a1:Account receives the message xfer(m:Money, a2), then calls check() on itself with a return 'evaluation'; then, if [OK], calls minus(m) on itself and sends plus(m) to a2:Account.] ## Short answer Two classes (Account and Person); events: John sends xfer(m,a2), a1 calls check(), if OK a1.minus(m) and sends plus(m) to a2; check() ensures sufficient balance and account validity, preventing invalid transfers. ## Answer #### (a) Number of classes involved Two classes are involved: **Account** (the lifelines `a1:Account` and `a2:Account` are two objects of the same class) and **Person** (`John:Person`, the actor/initiator). So there are three objects but two classes. #### (b) Events followed for the transfer to complete 1. `John` sends `xfer(m:Money, a2)` to `a1:Account`. 2. `a1` calls `check()` on itself to evaluate whether the balance is sufficient; it returns `evaluation`. 3. If the result is **[OK]**, `a1` calls `minus(m)` on itself to reduce its balance by `m`. 4. `a1` sends `plus(m)` to `a2:Account`, which increases its balance by `m`. 5. The transfer is complete; control returns to John. ```text John:Person a1:Account a2:Account | xfer(m,a2) | | |------------>| | | |--check()-. | | |<-evaluation | | | [OK] | | |--minus(m)-. | | |------plus(m)>| ``` #### (c) Why check() is required `check()` verifies the business rule that the source account has enough balance (and is valid/active) before any money is moved. Its result (`evaluation`) is the guard **[OK]** for the following messages. Without it, `a1` could be overdrawn, or money could be transferred out of an invalid or blocked account, leaving the accounts in an invalid state. Because both `minus(m)` and `plus(m)` are sent only when the guard is [OK], a failed check stops the whole transfer before any balance changes. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How many classes and objects are in the transfer diagram? A. Three classes, two objects B. Two classes (Account, Person) and three objects (a1, a2, John) C. Three classes and three objects D. One class Answer: B. a1 and a2 are two objects of the same class Account. (Key point: Classes involved) 2. What is the correct order of events? A. plus(m) on a2, check(), minus(m) B. xfer(m, a2) to a1, check(), then if [OK] minus(m) on a1 and plus(m) to a2 C. minus(m), check(), xfer D. check() on a2 first Answer: B. John sends xfer; a1 checks itself, debits, then credits a2. (Key point: Event order) 3. Why is check() required? A. To verify the source account has enough balance and is valid, so the account is not overdrawn and the system stays consistent B. To print a receipt C. To credit a2 D. To delete a1 Answer: A. minus and plus are guarded by [OK], so either both happen or neither. 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