A relationship between two objects indicates that one can send messages to the other. Generally, an object both sends and receives messages. This concept is graphically noted through the lack of an arrowhead on the link.
Link denotes the qualities of one object that are available in some way to another object. A link is an instance of an association, analogous to an object being an instance of a class. A link exists between two objects if, and only if, there is an association between their corresponding classes.
Adding a message to the link between two objects
1 Open the link’s Associative dialog box: Right-click the link and choosing Associative, or selecting the link and then click Edit, Associative.
This is not the standard definition dialog that you may be familiar with opening by double-clicking on the line (or choosing Edit after right-mouse clicking on the line).
2 Type the name of the message in the Message/Stimulus field.
3 Enter a sequence number in the Sequence field. The number you enter is the global number of the message on the diagram.
The numbers can include decimal points (for example, 2.1.3.4.5). Letters can not be used (for example, a.b.c and 1.a.2 are invalid).
4 Select a type of synchronization in the Synchronize box.
5 Determine which way the message is to be sent (from which object to which object).
If the message is sent from the object on the left to the one on the right, clear Reverse.
If the message is going the other way, select Reverse.
6 Click Add. The message is added to the link.
7 You can edit the message’s definition, including specifying a method that the message invokes, select it, and then click Define,