• Table of Contents

State Machine Diagram

As we delve further into requirement analysis, we will explore some modeling techniques that can be used to better understand system behavior. First on our list is the state machine diagram:

• A powerful graphical representation of the behavior of a system, component or object over time that model the dynamic nature of a system or entity.
• They depict the different states of an entity and how it responds to various events by changing from one state to another.
• An entity's behavior is not only determined by its inputs but also by its previous state. It can be used to model an entity's past, current, and future states.
• As such, it is useful for describing state-dependent behavior for objects, which can respond differently to the same event depending on their current state.

A state diagram is a directed graph where the nodes are states and the edges are transitions — occurrences of events.

Figure 8.1 Simple state machine diagram.

State

At any given point in time, the system or object is in a certain state. A state is a constraint or a situation in the life cycle of an object, in which a constraint holds, the object executes an activity or waits for an event.

States can trigger actions — an atomic execution — and complete without interruption. In total, there are four triggers for actions: On Entry, Do, On Event, On Exit.

Exactly one initial state is mandatory, while a final state is optional. The initial state represents the starting point of the entity's behavior, while the final state represents the end point of its behavior.

There can only be on initial state, but there can multiple final states. For example, consider a Tic-Tac-Toe game.

Figure 8.2 State diagram for a Tic-Tac-Toe game.

Transition