Chapter 9- key concepts and confusing terms

Accessor Operation. 

An operation that sets or returns the value of an attribute. Methods that implement accessor operations are called accessor methods. 

Activity Diagram. 

A diagram that models the logical flow of activities that takes place between the system and the outside world or within the system among its components.

Alternate Messages.

Two or more messages, only one of which may be executed. In a sequence diagram, alternate messages start on the same point of the object’s timeline and, often, result in a fork in the timeline of the receiving object. 

Anonymous Message.

A message that is broadcast and is not targeted towards a specific object. An anonymous message is usually an “event” message; that is, a message that is broadcasted as a signal or time event.

Arguments

See Parameters.

Asynchronous.

An interaction in which the action and the response to it are not in the same timeframe (or are out-of-phase). An asynchronous event is one in which the action that triggers the event and the event itself are not within a common timeframe. The object that is the source of action immediately regains control.

Call Event.

 An event that is triggered when an operation is invoked. A call event is usually synchronous and targeted. Catcher. An object that chooses to receive an asynchronous event. Also called listener or subscriber. 

Change Event. 

An event that changes the state of an object. Any call, signal or time event can qualify as a change event.

Collaboration Diagram.

An interaction diagram that focuses on the organization of objects that must cooperate, by exchanging messages, to satisfy a certain behavior.

Dynamic Modeling. 

Represents the interaction of the building blocks of the information system with each other and with the outside world to satisfy the behavioral requirements of the system. It models the structure of the system as it moves through space and time.

Error-Handler. 

See Event-Handler.

 Event. 

An action by one object that interrupts the existing condition of one or more other objects. Event types are call, signal, and time. Any event that changes the state of an object is called a change event.

Event-Handler. 

A method that implements the response of an object to an event raised by another object. If the event is an exception (an error) then the method is called an error-handler.

Fork

When a single flow of action splits into parallel timelines (sequence diagram) or parallel set of activities (activity diagram). Each fork must be balanced by a merge, where parallel timelines or activities are joined back into a single flow. 

Interface

 The services that an object offers to the outside world, as well as how these services are structured and arranged. 

Lifetime.

 In a sequence diagram, an empty rectangle that specifies when an object is instantiated, how long it lives, and when it is destroyed.

Listener. 

An object that chooses to receive an asynchronous event. Also called catcher or subscriber.

Message.

Instruction and information sent to an object in the expectation that the recipient will carry out a certain action. A message is the invocation, by the sender, of an operation for which the recipient is responsible. The data included in the message are defined—both in type and in order—by the parameters specified in the invoked operation. After the operation is complete, the recipient of the message may return a result to the sender. 

Method

How an operation is implemented or actually carried out by the object when it receives a message. In call events, both the operation and the method exist within one object. In signal and time events, the message sender specifies the event and the recipient implements it.

Operation.

What the object does, identified by a name and one or more parameters. An operation may also send back a return value to the object from which it receives the message. Parameters. Specify the data that must be supplied to an object to carry out a specific operation. (Parameters are also called arguments.) The type and the ordinal position of each value that must be received for each parameter is decided by the message.

Parameters.

Specify the data that must be supplied to an object to carry out a specific operation. (Parameters are also called arguments.) The type and the ordinal position of each value that must be received for each parameter is decided by the operation defined in the interface of the class to which the object belongs.

Responsibility. 

Any action that an object must take in response to a correct message. Responsibilities of an object are defined in the interface of the class to which it belongs. 

Return Value. 

The response that a message may invoke from the receiving object after an operation is complete. Each operation may have only one return value. The return value, however, may be complex or might even be an object or a set of objects. 

Sequence Diagram.

An interaction diagram that represents the interaction between objects, or between actors and objects ordered in time. A sequence diagram is composed of a timeline, objects that interact across this timeline, and the messages that they exchange. 

Signal Event.

An event that disrupts the normal flow of actions. A signal event is synchronous and anonymous.

State.

The condition of an object at a certain stage from a specific viewpoint. A state is a snapshot of the object at a (usually important) point in time. An object may arrive at a certain state repeatedly, or it might have multiple states simultaneously (depending on the viewpoint).

Statechart Diagram.

Adiagram that is composed of the states of an object and the flow of events that change its state. A statechart diagram is the only dynamic model that can illustrate the milestones in the lifetime of one class of objects in its entirety. 

Subscriber. 

An object that chooses to receive an asynchronous event. Also called listener or catcher

Swimlane.

A vertical partition on the activity diagram that organizes responsibilities for actions depicted by the diagram.

Synchronization Bar.

 A horizontal line on the activity diagram where parallel actions fork or merge. 

Synchronous.

An interaction in which both the action and the response to it occur in the same timeframe (or are in-phase). A synchronous event is one whose triggering action occurs immediately before the event is “fired” and neither side of the interaction can gain control until the operation is 

complete—that is to say, “the event runs its course.”

Targeted Message.

A message that is targeted towards a specific object. (See also Anonymous Message.)

Time Event.

An event that is triggered by the passage time. Like signal events, time events are asynchronous and anonymous; unlike signal events, they do not result from a prior action in the ordinary sense of the word 

Timeline.

An axis in the sequence diagram that signifies the chronology of events

Creation & Instantiation.

 Even though these two terms are often used interchangeably, sometimes we should (at least) try to keep them separate. When a patient does not exist on the hospital’s file—in other words, when a patient is new to the hospital—it is created. When the information system needs to exchange messages with the Patient object, it is instantiated, regardless of whether the hospital considers the patient “new” or “existing.” In a play, an actor “instantiates” a role when he or she appears on the stage, and “destroys” the role when he or she leaves the stage. But neither the actor nor the role are created onstage or die offstage. Nevertheless, it is hopeless to expect that anybody can follow the distinction strictly and faithfully. We may create both patients (for the hospital) and Patient objects (for exchanging messages within the information system). Our only real option is to interpret the word based on its context.

Events.

Even in books on information systems (such as this one), usage of the term “event” runs from loose and general to very specific. Any action, any occurrence, can legitimately be called an event. Any action that changes the state of an object is a change event. Sending a message to an object is an event because, at the very least, it turns the state of the object from Idle (doing nothing before receiving the message) into Active (after receiving it). The most specialized type of events are signal and time events that need event-handlers. As usual with confusing terms, you often have to rely on the context to grasp the implication.

Events & Messages.

 Events are always related to messages. (As we emphasized, messages are the only mechanism by which objects in a virtual world interact.) The possible confusion lies in the direction of the message: call events result from receiving messages, while signal and time events send them. You send a message to a waiter to provide you with a menu, while the clerk at the train station sends you (and most likely others like you) a message when your train is delayed or canceled. As a result, sometimes we say that a message raises an event while, at other times, we may refer to an “event message.”

Lifetime.

The term may be used in two different ways. If you have “instantiated” the application (you are running the program, in other words), “lifetime” refers to the lifespan of an object while the system is running. (Sequence diagrams represent this definition.) But the term might go beyond this definition: A bank account starts life when it is opened and ceases to exist when it is closed—regardless of whether the bank’s information system is running or not. In other words, the persistent—or, more or less, business—meaning of “lifetime” is not the same as its transitory (role-playing) sense when the application is running.

State. 

Any change to the “condition” of an object changes its state. If you change the address of a Patient, its state (in programmer’s vocabulary) changes from Clean to Dirty and must be saved to the database. But usually it has a narrower meaning and refers to a “significant” change or a milestone from a specific viewpoint. Therefore, depending on the viewpoint, an object may have multiple states at the same time: a car may be simultaneously Empty or Full, Stationary or Moving, New or Old or Refurbished. For each viewpoint, you need a different statechart diagram. (Moreover, some states, such as New, cannot be regained, while others, Empty for example, can be repeated for the same object.)