A qualifier is an attribute or list of attributes of the association end. When you add a qualifier to an association end, it narrows down its multiplicity. For example, the relationship between classes Bank and Cashier has a multiplicity of one-to-many, in other words, one Bank employs many Cashiers. The addition of a qualifier called Employee Code would reduce the multiplicity to one-to-one, in other words, a Bank plus an Employee Code corresponds to a particular Cashier.
One-to-many and many-to-many associations are qualified at the "many" end of the association. It is permissible (although somewhat rare), to have a qualifier on each end of a single association.
Notation
A qualifier is shown as a small box on the association end that it qualifies. The qualifier box is part of the association, not part of the class. The qualifier attributes are drawn within the qualifier box. There can be one or more attributes, shown one to a line.
Setting qualifiers for an association end
1 Open the definition of the association.
The Qualifier property is not presented in the Class Roles (Association End) grid. You can specify qualifiers only by opening the full Association End definition, as described forthwith.
2 Place your cursor in the name box of the specific association end that you want to specify qualifier(s) for, and click the Define button at the bottom of the grid. You can actually place your cursor in any of the Association End's cells except the Package cell and the Class cell. (If you place your cursor in either of these two cells, clicking define will open the definition of the package or class of the association end, respectively, not the definition of the Association End.)
3 Specify the attributes that form the qualifier in the Attributes listof box within the Qualifier tab of the Association End definition. You can click the Choices button to get a list of attributes of the target class, and select and drag appropriate attributes into the listof box.
