Has-A
It is often useful to draw several class diagrams, and show how they are associated.
We do this using arrows from one class diagram to another.
Composition/Aggregation Arrows¶
One of the most useful arrows is the Composition/Aggregation or "HAS-A" arrow.
HAS-A Aggregation Arrow¶
When a class has a field whose type is another class, we can use an arrow with a diamond on one end. * The relationship is called aggregation because the class is made of other elements. * The relationship is also known as has-a because the class "has a" field for the other class.
In the partial class diagram below, Dog has-a Tail.
The diamond belongs on the class that owns the other object.
Practice Exercise¶
The
>on the owned side of the has-a relationship is optional. Either way makes sense.
Aggregation and Composition¶
Composition is a special type of aggregation which denotes a strong dependency between the owning class and the composite (owned) class. * Without the composite class, the owning class does not function.
In the example below, the Dog has both a Tail and a Heart, and will not function without the Heart.
Multiplicity¶
Sometimes we show the multiplicity of each class in a relationship by adding indicators to the has-a arrow.
multiplicity¶
The number of instances of one class linked to one instance of the other class.
For example, a Dog can have between 0 and 4 legs.
Options for multiplicity are listed below.
| Indicator | Meaning |
|---|---|
| 0..1 | Zero or one |
| 1 | Exactly one |
| 0..* | Zero or more |
| 1..* | One or more |
| * | One or more |
| n | Only n (where n > 1) |
| 0..n | Zero to n (where n > 1) |
| 1..n | One to n (where n > 1) |
| Exactly one (default) |