Multiplicity of a Role
The multiplicity specifies the range of allowable cardinalities a set may assume. This is primarily indicated for each role that classes play in an association. It can assume the values *, 0..1, 1, 1..*, 2..*, 4..10, etc. The default value is *.
The cardinality is the number of elements in a set.
The multiplicity expresses the minimum and maximum number of instances of a class that can be linked by the association to each instance of the other class.
The usual multiplicities are "1", "0..1", "*" or "0..*", "1..*", and "M..N" where "M" and "N" are integers:
• The "1" multiplicity indicates that one and only one instance of the class is linked by this association to each instance of the other class.
• The "0..1" multiplicity indicates that at most one instance of the class can be linked by this association to each instance of the other class.
• The "*" or "0..*" multiplicity indicates that any number of instances of the class can be linked by the association to each instance of the other class.
• The "1..*" multiplicity indicates that at least one instance of the class is linked by the association to each instance of the other class.
• The "M..N" multiplicity indicates that at least M instances and at most N instances of the class are linked by the association to each instance of the other class.
1 | One and one only |
0 / 1 | Zero or one |
M..N | From M to N (natural integer) |
* | From zero to several |
0..* | From zero to several |
1..* | From one to several |
The following example illustrates the significance of the different multiplicities:
• 0..1 : An order corresponds to zero or at most one invoice.
• * : No restriction is placed on the number of invoices corresponding to an order.
• 1, Each order has one and only one corresponding invoice.
• 1..* : Each order has one or more corresponding invoices.
Specifying role multiplicity
To specify association end multiplicity:
Right-click the line between the association and the class and select
Multiplicity.
The multiplicity now appears on the role.
If the menu you see does not propose multiplicity, verify that you clicked on the line indicating the association end and not on the association.
