Common Mechanisms of UML

Interactions 

• Objects and Roles 
• Links 
• Messages 
• Sequencing 
• Creation, Modification and Destruction 
• Common Modeling Techniques

Objects and Roles 

• Objects are either concrete things or prototypical things 
• Concrete thing represents something in real world while prototypical thing stands for any object of a given class 
• In interactions, the interactors are usually prototypical things that play particular roles 

Example: 

• When we say "Dhawan is the man of the match", we are referring to a particular instance of the player class. This is an example for concrete things 
• When we say "a player is selected on merit", we are referring to any instance of player class. This is an example for prototypical things

Link

Link is 

• Semantic connection among objects 
• Instance of an association, i.e., the association between instances of two classes represents a link 

Link specifies a path to send and receive messages between two objects 

• Association: Object is visible by association 
• Self: Object is visible because it is the dispatcher of the operation 
• Global: Object is visible because it is in enclosing scope 
• Local: Object is visible because it is in a local scope 
• Parameter: Object is visible because it is a parameter

Messages 

Indicates an action to be done (requested by the sender to the receiver) 

Different types of messages are 

• Call: Invokes an operation on an object; an object may send a message to itself, resulting in the local invocation of an operation 
• Return: Returns a value to the caller 
• Send: Sends a signal to an object 
• Create: Creates an object 
• Destroy: Destroys an object. An object may destroy itself

Sequencing

• A sequence is a stream of messages exchanged between objects 
• Sequence must have a beginning Messages are ordered in sequence by time and are visualized by prefixing the message with a sequence number with a colon separator 
• Flow of control are two types: 

A procedural or nested flow of control is rendered by using a filled solid arrowhead 

A flat flow of control is rendered by using a stick arrowhead

Creation, Modification and Destruction 

Enabled by adding the following constraints to the element

• New - This instance or link is created while the closing interaction is executed 
• Destroyed - This situation or link is destroyed before the closing interaction is  completed
• Transient- This situation or link is created while the closing interaction is executed, but destroyed before the execution is completed

Common Modeling Techniques

Modeling a Flow of Control 

To model a flow of control: 

• Determine what interaction must be modeled 
• Analyze which objects take part in the interaction, set their initial properties, which includes their attribute values, state, and role 
• If the model highlights the structural organization of these objects, identify the links that associate them which are relevant to the paths of communication 
• Indicate the nature of the links using the UMLs standard stereotyped and constraints wherever necessary 
• Specify the messages from object to object in time order 
• Distinguish the different kind of messages wherever necessary. Also, include parameters and return values to pass on the interaction details 
• Enhance each object with its state and role at every stage in time

Interaction at a Glance