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Object-Oriented Software Engineering

Practical Software Development using UML and Java

Chapter 8: Modelling Interactions and Behaviour

8.1 Interaction Diagrams

Interaction diagrams are used to model the dynamic aspects of a software system

• They help you to visualize how the system runs.
• An interaction diagram is often built from a use case and a class diagram. —The objective is to show how a set of objects accomplish the required interactions with an actor.

Elements found in interaction diagrams

• Instances of classes —Shown as boxes with the class and object identifier underlined
• Actors —Use the stick-person symbol as in use case diagrams
• Messages —Shown as arrows from actor to object, or from object to object

Creating interaction diagrams

You should develop a class diagram and a use case model before starting to create an interaction diagram.

• There are two kinds of interaction diagrams: — Sequence diagrams — Communication diagrams

Sequence diagrams

A sequence diagram shows the sequence of messages exchanged by the set of objects performing a certain task

• The objects are arranged horizontally across the diagram.
• An actor that initiates the interaction is often shown on the left.
• The vertical dimension represents time.
• A vertical line, called a lifeline , is attached to each object or actor.
• The lifeline becomes a broad box, called an activation box during the live activation period.
• A message is represented as an arrow between activation boxes of the sender and receiver. —A message is labelled and can have an argument list and a return value.

Sequence diagrams –

same example, more details

Sequence diagrams –

an example with object deletion

• If an object’s life ends, this is shown with an X at the end of the lifeline

Communication diagrams – an example

Communication diagrams –

same example, more details

Communication links

• A communication link can exist between two objects whenever it is possible for one object to send a message to the other one.
• Several situations can make this message exchange possible: 1. The classes of the two objects have an association between them. - This is the most common case. - If all messages are sent in the same direction, then probably the association can be made unidirectional.

Other communication links

4. The receiving object is global.
   • This is the case when a reference to an object can be obtained using a static method.
   • The stereotype «global», or a [G] symbol is used in this case.
5. The objects communicate over a network.
   • We suggest to write «network».

How to choose between using a sequence

or communication diagram

Sequence diagrams

• Make explicit the time ordering of the interaction. —Use cases make time ordering explicit too —So sequence diagrams are a natural choice when you build an interaction model from a use case.
• Make it easy to add details to messages. —Communication diagrams have less space for this

Communication diagrams and patterns

A communication diagram can be used to represent aspects of a design pattern

8.2 State Diagrams

A state diagram describes the behaviour of a system , some part of a system, or an individual object****.

• At any given point in time, the system or object is in a certain state. —Being in a state means that it is will behave in a specific way in response to any events that occur.
• Some events will cause the system to change state. —In the new state, the system will behave in a different way to events.
• A state diagram is a directed graph where the nodes are states and the arcs are transitions.