State Diagrams

State diagrams (also called State Chart diagrams) are used to help the developer better understand any complex/unusual functionalities or business flows of specialized areas of the system. In short, State diagrams depict the dynamic behavior of the entire system, or a sub-system, or even a single object in a system. This is done with the help of Behavioral elements.

It is important to note that having a State diagram for your system is not a compulsion, but must be defined only on a need basis.

Defining a State diagram

Just as we define classes for a Class diagram, it is necessary to define the elements of a State diagram. Let us first see what the elements of a State diagram are.

Elements of a State diagram

A State diagram consists of the following behavioral elements:

Element and its Description	Symbol
Initial State: This shows the starting point or first activity of the flow. Denoted by a solid circle. This is also called as a "pseudo state," where the state has no variables describing it further and no activities.
State: Represents the state of object at an instant of time. In a state diagram, there will be multiple of such symbols, one for each state of the Object we are discussing. Denoted by a rectangle with rounded corners and compartments (such as a class with rounded corners to denote an Object). We will describe this symbol in detail a little later.
Transition: An arrow indicating the Object to transition from one state to the other. The actual trigger event and action causing the transition are written beside the arrow, separated by a slash. Transitions that occur because the state completed an activity are called "triggerless" transitions. If an event has to occur after the completion of some event or action, the event or action is called the guard condition. The transition takes place after the guard condition occurs. This guard condition/event/action is depicted by square brackets around the description of the event/action (in other words, in the form of a Boolean expression).
History States: A flow may require that the object go into a trance, or wait state, and on the occurrence of a certain event, go back to the state it was in when it went into a wait state—its last active state. This is shown in a State diagram with the help of a letter H enclosed within a circle.
Event and Action: A trigger that causes a transition to occur is called as an event or action. Every transition need not occur due to the occurrence of an event or action directly related to the state that transitioned from one state to another. As described above, an event/action is written above a transition that it causes.
Signal: When an event causes a message/trigger to be sent to a state, that causes the transition; then, that message sent by the event is called a signal. Represented as a class with the <<Signal>> icon above the action/event.
Final State: The end of the state diagram is shown by a bull's eye symbol, also called a final state. A final state is another example of a pseudo state because it does not have any variable or action described.

Note: Changes in the system that occur, such as a background thread while the main process is running, are called "sub states." Even though it affects the main state, a sub state is not shown as a part of the main state. Hence, it is depicted as contained within the main state flow.

As you saw above, a state is represented by a rectangle with rounded edges. Within a state, its Name, variables, and Activities can be listed as shown in Figure 6.1.

Figure 6.1: the structure of the state element

States

A state is denoted by a round-cornered rectangle with the name of the state written inside it.

Initial and Final States
The initial state is denoted by a filled black circle and may be labeled with a name. The final state is denoted by a circle with a dot inside and may also be labeled with a name.

Transitions
Transitions from one state to the next are denoted by lines with arrowheads. A transition may have a trigger, a guard and an effect, as below.

"Trigger" is the cause of the transition, which could be a signal, an event, a change in some condition, or the passage of time. "Guard" is a condition which must be true in order for the trigger to cause the transition. "Effect" is an action which will be invoked directly on the object that owns the state machine as a result of the transition.

State Actions
In the transition example above, an effect was associated with the transition. If the target state had many transitions arriving at it, and each transition had the same effect associated with it, it would be better to associate the effect with the target state rather than the transitions. This can be done by defining an entry action for the state. The diagram below shows a state with an entry action and an exit action.

It is also possible to define actions that occur on events, or actions that always occur. It is possible to define any number of actions of each type.

Self-Transitions
A state can have a transition that returns to itself, as in the following diagram. This is most useful when an effect is associated with the transition.

Compound States
A state machine diagram may include sub-machine diagrams, as in the example below.

The alternative way to show the same information is as follows.

The notation in the above version indicates that the details of the Check PIN sub-machine are shown in a separate diagram.

Entry Point
Sometimes you won’t want to enter a sub-machine at the normal initial state. For example, in the following sub-machine it would be normal to begin in the "Initializing" state, but if for some reason it wasn’t necessary to perform the initialization, it would be possible to begin in the "Ready" state by transitioning to the named entry point.

The following diagram shows the state machine one level up.

Exit Point
In a similar manner to entry points, it is possible to have named alternative exit points. The following diagram gives an example where the state executed after the main processing state depends on which route is used to transition out of the state.

Choice Pseudo-State
A choice pseudo-state is shown as a diamond with one transition arriving and two or more transitions leaving. The following diagram shows that whichever state is arrived at, after the choice pseudo-state, is dependent on the message format selected during execution of the previous state.

Junction Pseudo-State
Junction pseudo-states are used to chain together multiple transitions. A single junction can have one or more incoming, and one or more outgoing, transitions; a guard can be applied to each transition. Junctions are semantic-free. A junction which splits an incoming transition into multiple outgoing transitions realizes a static conditional branch, as opposed to a choice pseudo-state which realizes a dynamic conditional branch.

Terminate Pseudo-State
Entering a terminate pseudo-state indicates that the lifeline of the state machine has ended. A terminate pseudo-state is notated as a cross.

History States
A history state is used to remember the previous state of a state machine when it was interrupted. The following diagram illustrates the use of history states. The example is a state machine belonging to a washing machine.

In this state machine, when a washing machine is running, it will progress from "Washing" through "Rinsing" to "Spinning". If there is a power cut, the washing machine will stop running and will go to the "Power Off" state. Then when the power is restored, the Running state is entered at the "History State" symbol meaning that it should resume where it last left-off.

Concurrent Regions
A state may be divided into regions containing sub-states that exist and execute concurrently. The example below shows that within the state "Applying Brakes", the front and rear brakes will be operating simultaneously and independently. Notice the use of fork and join pseudo-states, rather than choice and merge pseudo-states. These symbols are used to synchronize the concurrent threads.