The object lifeline is depicted as a dotted vertical line associated with a single object in the sequence diagram. The scale on the time axis is not indicated, since the sequence diagram only models the temporal ordering of early-later interactions. That is, the messages located in the sequence diagram above are initiated earlier than those below. Messages are displayed as horizontal arrows with the name of the message, and their order is determined by the time of occurrence. Object interactions are implemented through messages that are sent from one object to another. The initial moment of time corresponds to the uppermost part of the diagram. The second dimension of the sequence diagram is the vertical time axis, which is directed from top to bottom. In this case, the entire entry is emphasized, which is a sign of the object. Inside the rectangle, the object name and class name are separated by a colon. Graphically, each object is depicted as a rectangle and is located at the top of its line of life. Thus, all the objects in the sequence diagram form a certain order, determined by the order or degree of activity of the objects when interacting with each other. To the right is another object that interacts directly with the first. The extreme left of the diagram shows the object that initiated the interaction. The first from left to right in the form of vertical lines, each of which depicts the life line of an individual object participating in the interaction. In UML, a sequence diagram has two dimensions. The key point for sequence diagrams is the dynamics of the interaction of objects in time. The sequence diagram depicts only those objects that are directly involved in the interaction. To simulate the interaction of objects in time in the UML language, sequence diagrams are used. The temporal aspect of behavior can be essential in the simulation of synchronous processes that describe the interaction of objects. When reviewing state and activity diagrams, it was noted that, although these diagrams are used to specify the dynamics of system behavior, time is not explicitly present in them.
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