15.3 Common UML Interaction Diagram Notation
Illustrating Participants with Lifeline Boxes
In the UML, the boxes you've seen in the prior sample interaction diagrams are called lifeline boxes. Their precise UML definition is subtle, but informally they represent the participants in the interactionrelated parts defined in the context of some structure diagram, such as a class diagram. It is not precisely accurate to say that a lifeline box equals an instance of a class, but informally and practically, the participants will often be interpreted as such. Therefore, in this text I'll often write something like "the lifeline representing a Sale instance," as a convenient shorthand. See Figure 15.5 for common cases of notation.
Figure 15.5 Lifeline boxes to show participants in interactions.
Basic Message Expression Syntax
Interaction diagrams show messages between objects; the UML has a standard syntax for these message expressions: 
return = message(parameter : parameterType) : returnType
Parentheses are usually excluded if there are no parameters, though still legal.
Type information may be excluded if obvious or unimportant.
initialize(code) initialize d = getProductDescription(id) d = getProductDescription(id:ItemID) d = getProductDescription(id:ItemID) : ProductDescription
In the world of OO design patterns, there is one that is especially common, called the Singleton pattern. It is explained later, but an implication of the pattern is that there is only one instance of a class instantiatednever two. In other words, it is a "singleton" instance. In a UML interaction diagram (sequence or communication), such an object is marked with a '1' in the upper right corner of the lifeline box. It implies that the Singleton pattern is used to gain visibility to the objectthe meaning of that won't be clear at this time, but will be upon reading its description on p. 442. See Figure 15.6.
Figure 15.6 Singletons in interaction diagrams.
Singleton p. 442