State Diagram

State Diagrams: State Diagrams State diagrams are created during the analysis and design phase to describe the behaviour of nontrivial objects. State diagrams are good for describing the behaviour of one object across several use cases and are used to identify object attributes and to refine the behaviour description of an object.

There are three major components of a state diagram:

State:

A state is a condition in which an object can be at some point during its lifetime, for some finite period of. State diagrams describe all the possible states a particular object can get into and how the objects state changes as a result of external events that reach the object.

States are represented by the values of the attributes of an object.

A state represents a stage in the behaviour pattern of an object, and in a state diagram it is possible to have initial states and final states.

An initial state, also called a creation state, is the one that an object is in when it is first created, whereas a final state is one in which no transitions lead out of.

. In a state diagram:

• A state is represented by a rounded rectangle.

 • A start state is represented by a solid circle.

• A final state is represented by a solid circle with another open circle around it.

Transition

A transition is a progression from one state to another and will be triggered by an event that is either internal or external to the object.

Transitions are the result of the invocation of a method that causes an important change in state.

 A transition is a change of an object from one state (the source state) to another (the target state) triggered by events, conditions, or time. Transitions are represented by an arrow connecting two states.

Transitions can also be labeled with guards (a Boolean expression which evaluates to true or false) inside square brackets, such as [trade accepted]. A guarded transition occurs only if the guard resolves to true. Only one transition can be taken out of a given state. If more than one guard condition is true, only one transition will fire. The choice of transition to fire is nondeterministic if no priority rule is given

The arrows in state diagram represent transitions, progressions from one state to another.

Event: Is something that occurs at a point of time.

Events are internal or external factors influencing the system.

Event causes the transitions

State diagrams are used to model states and also events operating on the system.