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Sequence Diagrams. Introduction. A Sequence diagram depicts the sequence of actions that occur in a system. The invocation of methods in each object , and the order in which the invocation occurs is captured in a Sequence diagram.

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  • A Sequence diagram depicts the sequence of actions that occur in a system.
  • The invocation of methods in each object, and the order in which the invocation occurs is captured in a Sequence diagram.
  • This makes the Sequence diagram a very useful tool to easily represent the dynamic behavior of a system.
  • A Sequence diagram is two-dimensional in nature. On the horizontal axis, it shows the life of the object that it represents, while on the vertical axis, it shows the sequence of the creation or invocation of these objects.
message arrows for communications
Message Arrows for Communications
  • The message arrows represent the communications between two objects in a sequence diagram. It goes from the lifeline of one object to that of another object
    • Synchronousmessage where the sending object suspends action and waits for the response to the message
    • Asynchronousmessage where the sending object continues with its operations without waiting for the response
    • A return of control from the synchronous message
    • A creationof a new entity

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example 1 withdraw saving
Example 1: Withdraw saving


He enters his card into an ATM (automated teller machine).

The ATM machine prompts “ Enter PIN”.

The customer enters his PIN.

The ATM (internally) retrieves the bank account number from the card.

The ATM encrypts the PIN and the account number and sends it over to the bank.

The bank verifies the encrypted account and PIN number.

If the PIN number is correct, the ATM displays, “Enter Amount”.

Draws money from the bank account and pays out the amount.


withdraw saving
Withdraw saving
  • Use Case: Withdraw Saving
  • Actors: Bank, Client
  • Flow of Events:
  • Bank client insert ATM card into ATM machine
  • ATM machine request PIN code
  • Bank client enter PIN code
  • Verify PIN code entered with saving account
  • ATM machine request amount if the PIN is valid
  • Bank client enter the required amount
  • Process the transaction in the client account
  • ATM machine dispense cash
  • ATM machine print receipt when the transaction completed
  • Alternative Flow of Events
  • Invalid PIN code entered. Indicate error message. Return step 3.
identifying classes
Identifying Classes
  • Bank Client
  • ATM Card
  • ATM Machine
  • Savings Account
  • Cash
  • Message





Insert ATM card

Request PIN

Enter PIN code

Verify PIN code

PIN valid

Request amount

Enter amount

Process transaction

Transaction successful

Dispense cash

Print receipt

example 2 buying tickets
Example 2: Buying Tickets

An interaction between a customer, ticket seller, database and printer.

example 3 logging in
Example 3: Logging in

Let's start with the simple example above: a user logging onto the system.  The Logon use case can be specified by the following step:

1. Logon dialog is shown

2. User enters user name and password

3. User clicks on OK or presses the enter key

4. The user name and password are checked and approved

5. The user is allowed into the system

Alternative: Logon Failed - if at step 4 the user name and password are not approved, allow the user to try again