UML Designer User's Guide

Adding the initial method call

The first method call on a sequence diagram must originate from the system activation at the left edge (only an active object can send a message to another object). This initial method call represents a call from outside the diagram; in our diagram, it represents input from an outside actor initiating the cataloging operation. We have not yet defined an actual method for Library that would accept external input, so for now we can use the standard yourself method as a placeholder. This method call does not itself have any significance; it is used simply to activate the Library object.

To add this method call to the diagram, follow these steps:

  1. Select Method call figure icon Method call from the tool bar.
  2. Click mouse button 1 on the system activation to indicate the source of the message (in this case, an object outside the diagram).
  3. Click mouse button 1 on the Library object's lifeline to indicate the receiver of the message.
  4. When prompted to choose a method, select Object in the Classes list, Object in the Protocols list, and #yourself from the Messages list. This indicates that the yourself method (inherited from Object and defined in the <Object> protocol) is the target of the call.
  5. Select OK. The method call to yourself, and its return, appear as a method call figure. In addition, the Library object's lifeline now shows activation between the message send and return.


    Sequence diagram illustration (2)

  6. Select the activation for the yourself method call on the Library object's lifeline.
  7. Use mouse button 1 to drag the bottom selection handle downward, lengthening the activation. (The activation of the Library object lasts for the duration of the cataloging operation.)

    As you add more method calls, you can continue to lengthen the activation as needed. You can also move a method call and its associated activation up or down by dragging the center of the activation.


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