Objects take on the responsibility for things like managing data, moving data around in the system, responding to inquiries, and protecting the system. Messages between objects are represented by arrows that point from sender object to the receiver object.Įverything in an object-oriented system is accomplished by objects. Each object also has its timeline represented by a dashed line below the object. Often, an anonymous object (:Account) may be used to represent any object in the class. Object names can be specific (e.g., myAccount) or they can be general (e.g., myAccount :Account). These indicate the existence of the object. A lifeline is represented by dashed vertical line drawn below each object. LifelineĮntities of participants in a collaboration (scenario) are written horizontally across the top of the diagram. The three ways of naming an object are shown in Figure below. An object can be named in one of three ways: the object name, the object name and its class, or just the class name (anonymous object). But for "simple" exception handling, I think a break is a better way to represent it, then alt.In the UML, an object in a sequence diagram is drawn as a rectangle containing the name of the object, underlined. ConclusionĪs always, the best method depends on the scenario. This is ofcourse a very simple example, and you can have a lot more interaction in the break interaction operator, like logging or transaction handling. I think it even makes things better, because you can see where a sequence stops when an exception occurs. What I like about this way of displaying exception handling is that it makes the diagram much less cluttered. In this example the exception isn't caught until it enters the UI, in this case the ATM. Using a break interaction operator, it would look something like this: Let's take a look when the situation when something happens, what causes the ATM to decline de withdrawal. When the guard condition is true, the current interaction run is abandoned and the clause in the break interaction operand runs. The break interaction operator is similar to the break mechanism in other programming languages. This looks nice, for a small diagram, but once the diagram starts to grow and you get a lot of nested alt interaction operations, you might start to loose the overview. Using an alt interaction operator, it would look something like this: The else clause of the alternative combined fragment runs when no other option is selected. If there is no guard, the operand always runs when it is selected. However, as for any operand, the selected operand in the alternative structure runs only if the guard condition tests true. Only one of the offered alternatives runs on any pass through the interaction. The server confirms the withdrawal is approved and, the service passes this on to the ATM's UI and the ATM dispenses the money.Īn alternative interaction operator represents the logic equivalent of an if-then-else statement. The customer enters the data, the ATM calls a service, which calls the bank's server. I'll describe both methods using a classical example: withdraw money from an ATM when the balance is too low. They both use interaction operators, alt and break. 1Īfter searching for quite a while on how to model exception handling in a UML Sequence Diagram I found two ways to represent exception handling in a UML Sequence Diagram. There are several proposed notations for exception handling. Some clumsy approaches to model try-catch blocks are by utilizing combined fragments - alt (alternatives) and breaks, while adding stereotypes for reply messages representing thrown exceptions. UML provides neither notation to model exception handling in sequence diagrams nor any reasoning why it is absent.
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