CODEWITHSUNDEEP
umlaggregationcompositionclass-diagram
Eve
Eve

Reputation: 31

Class Diagram: Aggregation or Composition

I'm doing an assignment on class diagram and the following shows the association between classes Order and Menu Item. Should the link be aggregation (weak) or composition (strong)?

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I'm confused because I've seen example showing aggregation. I felt it should be composition as an Order must have at least 1 Menu Item added. Am I wrong?

Upvotes: 2

Views: 5702

Answers (2)

BobRodes
BobRodes

Reputation: 6175

A lot is dependent on context. Let's use an example that should clarify the difference for you.

Suppose you are assembling a Car object in a car factory. The Car object has two Axle objects, four Wheel objects, a Motor object, and so on. All of those objects only have meaning within the context of the entire car (once you put them in, they stay there). That's composition.

Now, suppose you have a Car object in a junkyard. The Car object has the same stuff, but you can pull wheels or axles or the motor out of it and sell them separately. In that case, each part has a lifetime separate from that of the car. That's aggregation.

Upvotes: 0

sergej
sergej

Reputation: 18009

Should the link be aggregation (weak) or composition (strong)?

It can be both. The main difference is:

In case it's a composition:

  • MenuItem objects are instantiated at the instantiation time of the Order object.

  • The MenuItem objects are destructed as soon as the Order object is destructed.

  • C++ example:

    class Order {
    MenuItem menus[NUMBER_OF_MENUS];
};

In case it's a aggregation:

  • The lifetime of the MenuItem objects is independent of the Order object lifetime.

  • C++ example:

    class Order {
    MenuItem* menus[NUMBER_OF_MENUS];
};

So it's a design decision. And might also depend on the implementation language. In Java, for example, there is no distinction between both, all object variables are references.

Upvotes: 2

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