A model is an abstraction of the real world. It allows for dealing with the complexity of the problem by focusing on the most important and essential features of a situation or application. In an object-oriented model, the focus is built around objects. These objects encapsulate both the data and the behavior within one common representation, thus providing a powerful environment for developing complex systems.

In contrast to the Systems Development Life Cycle (SDLC), which can be pictured as a waterfall, the object-oriented development life cycle is more like an onion. In the early stages (core) of development, the model is abstract, focused on the external qualities of the application system. As the model evolves, it becomes more detailed shifting the focus to how it will function and how it is built.

Object-oriented analysis models the static structure of a system, while dynamic modeling depicts the aspects of a system through perspective of state transitions. In the UML, state transitions are shown using state diagrams. A state diagram can show the various state transitions, or changes, an object can experience in its lifetime. The state diagram can also show the events that cause those transitions. When dealing with dynamic modeling, a state is a condition during the life of an object. During this condition, it satisfies some condition, performs an action, or waits for some event. When the object receives some event the state changes or undergoes a state transition. This event is something that takes place at a certain point in time. Examples include, a customer places an order; a person applies for a loan; a company hires a new employee; or a student registers for a class.

On either type of model, the vertical axis of the diagram represents time and the horizontal axis represents the various participating objects. As time increases the direction of travel is down the vertical axis. Along the top of the horizontal axis are participating objects. The arrangement should simply be so that the diagram is easy to read and understand. Each object has a vertical line traveling downward called the lifeline, which represents the objects' existence over a certain period of time. An object symbol, a box with the object's name underlined, is placed at the head of each lifeline. A thin rectangle, superimposed on the lifeline of an object, represents the activation of the object. The activation shows the time period during which the object performs an operation.

The textbook, Modern Systems Analysis and Design contains a good example on page 468 in the book and on this page sequence diagram for a class registration scenario with prerequisites. The scenario starts with the student entering the registration window. If the time is correct, the registration process can b

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