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A state diagram is a diagram used in computer science to describe the behavior of a system considering all the possible states of an object when an event occurs. This behavior is represented and analyzed in a series of events that occur in one or more possible states. Each diagram represents objects and tracks the various states of these objects throughout the system.
There exist different types of state diagrams that have different semantics and are slightly different. State diagrams graphically represent finite state machines. They are only used to understand object behavior throughout the whole system.
The elements that constitute a state diagram are rounded boxes representing the states and arrows showing transitions to the next state. The activity section depicts the activities the object performs while it is in that state. Every state diagram starts with an initial state, which is the state where the object is created. Right after the initial state, objects change their states, and the next state is determined by conditions based on activities. In some cases, state diagrams represent a super state, which is a condition created when many transitions lead to a particular state. The super state depicts that all states inside this diagram transition to a redundant state, making the diagram more complex.
A transition in a state diagram is a progression from one state to another and is triggered by an event that is internal or external to the entity modeled. An action is an operation that is invoked by an entity that is modeled. A very traditional form of state diagram for a finite machine is a directed graph.