State Diagrams

State Diagrams

The state diagram, sometimes referred to as the state diagram, illustrates the behavior of class object that acts differently in response to external stimuli, namely a series of events in a system. It describes how an object changes its states in its lifetime or across several use case executions and transition between states. In the figure below you can see two examples of diagrams. State is a specific configuration of the values ​​of data attributes. What we observe about the state is a difference in behavior when there is a transition from one state to another. That is to say, if the same message is sent to an object twice, it can behave differently each time, depending on the state in which it is when this message arrives.


Transition from one state to another, i.e. change of state, is caused by an event, which usually is a message reception. A guard is a condition that must be fulfilled before the transition can be done. Guards are suited to represent transitions to different states under the influence of the same event. An action indicates the processing to be performed during the transition. Each state can determine an activity to carry out when the object is in this state.

An instance of the Puck class may be either in play or out of play. If the instance is in play, it either is moving or is not moving. The observed state of a puck in play has two substates: Moving and Not Moving. In play is a super state. Moving and Not Moving are its substates. The sub-state inherits all transitions, entering and leaving its super-state.


A concurrent state diagram can illustrate a group of states that represent the behavior of an object from the perspective of two or more independent possibilities. Concurrent states will be discussed in one of our next articles, during implementation of the ball and paddle game called “Brickles” in the Java runtime environment. In the future such diagrams can be considered from testing perspective as a non-concurrent state diagram but first we will define states that are defined from all combinations of the states from the different concurrent parts, and then define the corresponding transitions.


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