An object in a Sequence diagram is represented by a vertical dashed line, called the objectlifeline. The name of the object is displayed above the lifeline, contained within a rectangle. The object is an instance of a class in the system.
The lifeline represents the existence of the object at a particular time. If the object is created or destroyed during the period of time shown on the diagram, then its lifeline starts or stops at the appropriate point.
Once you draw an object lifeline, you can extend it upwards or downwards by selecting the line and dragging on its top or bottom handle. However, lifelines will automatically be extended if a Message/Stimulus symbol is drawn near the end of the lifeline.
Specifying class that object instantiates
An object lifeline on a Sequence diagram instantiates a class in the project. When you add an object to the Sequence diagram, you are prompted to add the name of the class that the object instantiates. The class name is displayed next to the object name in the rectangle at the top of the object lifeline.
Showing and hiding class names of objects
To show or hide the class names for the entire diagram
1 Open the diagram properties dialog by selecting Edit > Diagram Properties or right-clicking the diagram workspace and selecting Diagram Properties.
2 Select or clear Show class names. Note that if the class name has been turned off for a particular object lifeline (see steps immediately below), then that class name will remain hidden.
To show or hide the class name for a single object lifeline
1 Double‑click the object lifeline.
2 Toggle on (or off) the Hide class name property within the Symbol tab of the object's definition.
Modeling Creation of an Object
Object creation is denoted in UML by a message line drawn into the left or right edge of an object’s name box, at the top of the object lifeline.
In order to draw this, it is generally necessary to select the target object, and move it downward on the Sequence diagram’s drawing workspace, so that a message line drawn from a another (sending) object can be drawn into the object’s name rectangle.
The fact that the object is being created is captured in a read-only Create Action boolean property in the definition of the created object, within the Symbol tab.
Modeling destruction of an object
Object destruction is denoted by a large X at the bottom of the object lifeline. To denote that an object is destructed on the Sequence diagram:
1 Select the object and open its definition.
2 On the object definition’s Symbol tab, select Destroy action. A large “X” appears on the diagram at the bottom of the object's lifeline.
Displaying stereotyped icon of an object
You may choose to show the object's stereotyped icon at the top of the object lifeline, instead of the rectangle that normally holds the object name. To do this, the class that the object instantiates must have a stereotype that, when chosen, causes the class to be displayed differently. Examples are classes declared to have stereotype of Actor, Boundary, Entity, Control, Form, Frameset, and so on.
To display the object according to its class’s stereotype
Right-click the object lifeline and select Display According to Stereotype. This choice shows up only if the object's class has an applicable stereotype assigned.
To change an object displayed by its stereotyped icon back to its normal rectangular form
Right-click the object lifeline and select Display As Object.
