State diagrams describe the behaviour of a system. They describe all the possible states that a particular object can get into and how the object’s state changes as a result of events that reach the object. In most OO techniques, state diagrams are drawn for a single class to show the lifetime behaviour of a single object. There are many forms of state diagrams, each with slightly different semantics. The UML style is based on David Harel’s statechart.

A State diagram, also know as Statechart diagram and a Satechart diagram, shows the behaviour of one object and how it changes its state in response to events, together with its responses and actions. They shows the allowable states for objects of a specific class and the permitted transitions between states. They also shows all that possible ways in which objects can respond to events form either other objects, or from outside the system.

What is a state?

There are two ways to think about object state:

The state of an object is a specific collection of attribute values for the object.

The state of an object describes the behaviour of the object relative to external stimuli.

An object’s state can be thought of as an abstraction of a particular set of attribute values and links to other objects. The state is derivable from the attributes and links and is of sufficient interest in its own right as to be worth modelling separately.

e.g. a shipment could have status conditions (i.e. states) for waiting to be shipped, in transit and delivered

To identify states think about the various conditions of an object that need to be reported to the outside world. For example, a shipment should have status conditions (i.e. states) for waiting to be shipped, in transit and delivered. Transitions can be identified from the sequence diagrams, every input message to an object can be considered a transition trigger. Each transition should be associated with a state some transitions will carry out an action and return to the same state.

Notation

Event

Triggers a change in State

Can be a Designated Condition that MUST be TRUE

Used to label a Transition

Transitions

Represents a response to an event

May cause a change of state or is recursive within the same state

May invoke operations or transmit events

A transition links one state with another

A transition can support an event/action block

Start :

Fit the Light Bulb -> ‘Off’ State

Turn Light Bulb On -> ‘On’ State

Turn Light Bulb Off -> ‘Off’ State

While in the ‘On’ State ….

…… Bulb Burns Out -> End

Example from pervious exam

In a washing machine’s program there is an object representing the washing cycle. The cycle has a number of states. When the machine is switched on it is in the filling state. Once 10 litres of water have gone into the drum, the heating state begins. When the temperature reaches the “wash temperature” the state becomes washing and soap is added. After 30 minutes of washing the drain and spin state of the cycle is entered. The drain and spin state lasts for 5 minutes. After the 5 minutes is up the washing cycle ends. For safety reasons, an uneven load during the washing state ends the cycle.

(12 marks)

Possible solution

Will now look at an Example of a detailed state machine diagram. Imagine an Object that controls a Heating, Ventilation and Air-Conditioning system (HVAC). If the Temperature gets too hot – we will turn ON the Air-conditioning and the Fan until it reaches a required temperature – and then they will be switched off. If the Temperature gets too cold – we will turn ON the Heater and the Fan until it reaches a required temperature – and then they will be switched off.

In detail with specific States...

If the Temperature gets too hot – we will turn ON the Air-conditioning and the Fan until it reaches a required temperature – and then they will be switched off

When the outside temperature gets above an UPPER bound ( and air conditioning is installed) - a fan is switched on AND then the air-conditioning is turned on.

(go to Cooling State)

When the outside temperature gets below the UPPER bound the air-conditioning is turned off AND the fan is switched off.

(go to Idle State)

If the Temperature gets too cold – we will turn ON the Heater and the Fan until it reaches a required temperature – and then they will be switched off

When the outside temperature gets below a LOWER bound ( and heater is installed) - a fan is switched on AND then the heating is turned on.

(go to Heating State)

When the outside temperature gets above the LOWER bound the heating is turned off AND the fan is switched off.

(go to Idle State)

First go at the State Machine diagram Showing main elements...

Identifying the Elements of a Statechart Diagram

Action blocks name the event and say what action is to be take. They are used to label transitions but can also be used within states. When attached to state it means that the state responds to an event with an action that does not cause a transition. This is shown by putting Event Name/Action Name in the state box. The following syntax is used (all parts are optional)

Event [guard]  / action
		or more fully
Event(arg1, arg2) [Guard] / Action(arg1, ...),Action2(arg1, …)

An event describes stimuli external to an object. An event is used to denote at what point an action is triggered. If a transition has no event within its label, it means that the transition occurs when the activity associated with the state has occurred.

A guard Is a condition that returns either True or False. A guarded transition will only occur if condition is true. If no event is specified the transition will occur as soon as the guard condition is satisfied. Correct syntax is to place the guard condition between square brackets

An external event will usually cause a series of internal events to occur, and sometimes these may trigger external events too. For example, a customer depositing a cheque into a savings account will trigger an internal event to place an uncleared credit against the account, and trigger an external event to request transfer of the money by the banking system from the issuer of the cheque.

To identify the events that affect an entity – primary source of events for objects is the set of use cases. Use cases are groupings of events that provide meaningful functionality for a system and a use case is realised by a group of collaborating objects Examples of events affecting a car sharer – registration, entering a journey, matching of a journey with another car sharer, entering of an agreement, cancellation of an agreement, payment date arriving etc.

Actions specify behaviour in response to an Event (Transition). Operations are performed in response to messages/events and can occur on a transition between the two states or within some known state.

Our example:

When the outside temperature gets above an UPPER bound ( and air conditioning is installed) - a fan is switched on AND then the air-conditioning is turned on.

(go to Cooling State)

When the outside temperature gets below the UPPER bound the air-conditioning is turned off AND the fan is switched off.

(go to Idle State)

When the outside temperature gets below a LOWER bound ( and heater is installed) - a fan is switched on AND then the heating is turned on.

(go to Heating State)

When the outside temperature gets above the LOWER bound the heating is turned off AND the fan is switched off.

(go to Idle State)

A state transition represents a change of state at runtime.

An action corresponds to a function from the functional model and specifies a response to an event. Action can be used for the transition and activity for the state (both are processes typically implemented by some method – but they are treated differently). Actions are associated with transitions and are considered to be processes that occur quickly and are not interruptible. Activities are associated with states and can take longer

On Entry:

actions triggered as soon as the state is entered

On Exit:

actions triggered as soon as the state is exited

Do:

these ongoing actions take place during the lifetime of a state

On Event:

these actions take place in response to an event

Entry/Exit Actions

Entry/exit actions are defined on states as an alternative to being attached to transitions. Typically entry/exit actions are used when all transitions into a state perform the same action, this results in a more concise description. Entry/Exit actions are preceded by entry/ or exit/. Internal actions do not perform entry or exit actions and an entry or exit action is automatically performed whenever the state is entered or exited respectively. Entry and Exit transitions are shown within the lower state segment with the pseudo event names entry and exit. This is a shorthand which replaces the need to attach the associated event/action block to all transitions either entering or leaving a state.

An entry or exit action is automatically performed whenever the state is entered or exited respectively. When the outside temperature gets above an UPPER bound ( and air conditioning is installed) - a fan is switched on AND then the “air-conditioning is turned on” -> (go to Cooling State)

When the outside temperature gets below the UPPER bound the “air-conditioning is turned off” AND the fan is switched off. -> (go to Idle State)

When the outside temperature gets below a LOWER bound ( and heater is installed) - a fan is switched on AND then the “heating is turned on”

(go to Heating State)

When the outside temperature gets above the LOWER bound the “heating is turned off “ AND the fan is switched off.

(go to Idle State)

Complete diagram for the HVAC system:

Possible/Suggested Steps in Creating a Statechart Diagram for an Object:

1. Draw the initial and final state for the object.

2. Draw the stable states of the object.

3. Specify the partial ordering of stable states over the lifetime of the object.

4. Specify the events that trigger the transitions between the states of the object. Specify transition actions (if any).

5. Specify the actions within a state (if any).

Step 1 - Start With the Intial and Final States

Step 2 - Determine Stable Object States

Step 3 - Specify the Partial Ordering of States

Step 4 - Specify the Transition Events and Actions

Step 5 - Specify the Actions Within a State

A Composite state is a complex state that is broken down into substates, each of which inherit the transitions of the parent. A composite state acts as a super container which holds substates and nested sub-state. The substates can be drawn either within the state on the diagram, or in a separate diagram. The following diagram represents a generic composite state which has been included in the original diagram.

We can specialise a complex state by partitioning it into substates each of which inherit the transitions of the parent. This is the same as specialisation of classes in an inheritance hierarchy- the different substates inherit the properties of the Superstate. A sequential state acts as a super container which holds substates and nested sub-states. Complex state models can be simplified using techniques similar to those of generalisation in class modelling

State Machine example for an Advertising Campaign object:

Example of a Composite State with Substates:

If a composite state is active, only one of its substates is active at any point in time. If you want to achieve concurrent states, a composite state can be divided into two or more regions, whereby one state of each region is always active at any point in time. This type of Concurrent Substate is also known as an Orthogonal State. Each region of an orthogonal state can have an initial state. A transition to the boundary of the orthogonal state then activates the initial states of all regions. Each region can also have a final state.

The ad campaign from the previous section could be “Running” & “Monitoring” at the same time. Running & Monitoring are regions of the “active” composite state:

It's possible for a composite state to consist of multiple, concurrent substates. Partition a diagram into regions with dashed lines, each is a concurrent substate. Each is independent and can occur at the same time-may finish at different times. When the enclosing state is entered, there is one thread of control for each substate. Splitting or merging the concurrent threads can be shown by a synchronisation bar. Concurrent States can be exited in various ways; in these 2 diagrams the state will only be exited when both concurrent regions exit:

State Modelling Focuses on event driven behaviour over time of objects, belonging to a single class. State diagrams are often under-used and under-valued on real-life projects.

Examples from previous exams:

Note how the state refers to a specific class

Both Examples to be done for next week’s Tutorial (to be decided by nodule leader)

Also : Sequence Diagram example to be done for next week