Common Mechanisms of UML

Interaction Diagrams

An interaction diagram reveals an interaction, which consists of a set of objects and their respective relationships. It also includes the messages that may be transmitted among them.

Interaction Diagrams can be classified in to two categories:

1) Sequence Diagrams

2) Collaboration Diagrams

Sequence Diagram

A sequence diagram emphasizes the time ordering of messages between objects. 

Objects 

• Object naming 

syntax: [instanceName][:className]

• Name classes consistently with your class diagram (same classes) 
• Include instance names when objects are referred to in messages or when several objects of the same type exist in the diagram 
• The life-line represents the object's life during the interaction

Messages:

• A message is represented by an arrow between the life lines of two objects 
• The time required by the receiver object to process the message is denoted by an activation-box 
• A message is labeled at a minimum with the message name 
• Arguments and control information (conditions, iterations) may be included.

Return Values:  

Optionally indicated using a dashed arrow with a label indicating the return value

• Model a return value only when you need to refer to it elsewhere 

Example: As a parameter passed in another message 

• Prefer modeling return values as part of a method invocation 

Example: ok = isValid()

Condition:

syntax:  ‘[‘expression ']' message-label 

The message is sent only if the condition is true. 

Example:

Iteration

syntax: * [‘[‘ expression ‘]’'] message-label 

The message is sent many times to possibly multiple receiver objects.

Operator	Meaning
alt	Alternative multiple fragments: Only the one whose condition is true will execute
opt	Optional: The fragment executes only if the supplied condition is true. Equivalent to an alt only with one trace
par	Parallel: Each fragment is run in parallel
loop	Loop: The fragment may execute multiple times, and the guard indicates the basis of iteration
region	Critical region: The fragments can have only one thread executing it at once
neg	Negative: The fragments shows an invalid interaction
ref	Reference: Refers to an interaction defined on another diagram. The frame is drawn to cover the lifelines involved in the interaction. You can define parameters and a return value
sd	Sequence diagram: Used to surround an entire sequence diagram.

Collaboration Diagram 

• A collaboration diagram emphasizes the organization of the objects that participate in an interaction 
• They are isomorphic to sequence diagrams, i.e., any sequence diagram can be written as collaboration diagram and vice versa 
• Numbering of the messages can be flat 

Example: 1, 2, 3,... or nested Dewey notation
Example: 1,1, 1, 1.2, ... 2, 2.1, 2.2, ...

Uses

Interaction diagram is used: 

• To model flows of control by time ordering
• To model flows of control by organization