What is the difference between Composition and Association relationship?

Question

In OOP, what is the difference between composition (denoted by filled diamond in UML) and association (denoted by empty diamond in UML) relationship between classes. I'm a bit confused. What is aggregation? Can I have a convincing real world example?

Answer 1

Usually, composition means that the lifetime of the contained object is bounded by that of the container, whereas association is a reference to an object which may exist independently.

However, this is just the practice I've observed. I hate to admit it, but ploughing through the UML2 spec isn't high on my list of fun stuff to do!

Answer 2

Independent existence.

An Invoice is composed of line items.

What's a line item that's not on an invoice? It's -- well -- it's nothing. It can't exist independently.

On the other hand, an Invoice is associated with a Customer.

Customer has an independent existence, with or without an invoice.

If the two things have independent existence, they may be associated.

If one thing cannot exist independently, then it is part of a composition.

Answer 3

Owning and using.

Composition: the object with the reference owns the object referred to, and is responsible for its "lifetime", its destruction (and often creation, though it may be passed in). Also known as a has-a relationship.

Association: the object with the reference uses the object referred to, may not be an exclusive user, and isn't responsible for he referred-to object's lifetime. Also known as a uses-a relationship.

The OP comments:

Can you provide a real world example. Also, what is aggregation? – Marc

Aggregation: an Association that is from whole to part, and that can't be cyclic.

Examples:

Composition: a Car has-an Engine, a Person has-an Address. Basically, must have, controls lifetime.

Association: A Car has-a Driver, some class instance has-an ErrorLogger. Lifetime not controlled, may be shared.

Aggregation: A DOM (Document Object Model, that is the objects that make up a tree of HTML elements) Node has-a (an array of) child Nodes. The Node is top (well, higher) level; it "contains" its children, they don't contain it.

Answer 4

Composition means a part of the entity state is encapsulated by another type but it is conceptualy part of the entity state. For example you may have a address type and a employee entity type that includes a address.

Association means that a entity type is assocciated with another entity type but the assocciated entity is conceptualy not part of the entity state. For example a employee may be assocciated with a company.

Answer 5

Here go a few examples:

• I am an employee of a company, hence I am associated to that company. I am not part of it, nor do I compose it, but am related to it, however.

• I am composed of organs, which unless are transplanted, will die with me. This is composition, which is a very strong bind between objects. Basically objects are composed by other objects. The verb says everything.

• There is also another less bound kind of composition, called aggregation. An aggregation is when objects are composed by other objects, but their life cycles are not necessarily tied. Using an extreme example, a Lego toy is an aggregation of parts. Even though the toy can be dismantled, its parts can be recombined to make a different toy.

Answer 6

Composition is a stricter relationship than aggregation. Composition means that something is so strongly related to something else that they cannot basically exist independently, or if they can, they live in different contexts.

Real world example: you define a GUI window, and then a text field where to write something. Between the class defining the GUI and the class defining the text field there's composition. Together, they compose a widget which can be seen as an entity on its own. Suppose you delete the window, and you delete the text field as well.

Aggregation is different, in the sense that the link between the two entities is temporary, unstable, and occasional. A real world example. Suppose you have a database of objects containing multiple data instances. Now you run some filter to collect the data instances obeying a given criterium, and the resulting instances are pushed into a graphical list so that the user can see them. When the graphical widget receives the objects, it can form an aggregation of these entities, and present them. If the user closes the window with the graphical list, and the latter get deleted, the data objects should not be deleted. Maybe they are displayed somewhere else, or you still need them.

Also, in general, composition is defined at creation time. Aggregation is instead defined later in the object lifetime.

Answer 7

COMPOSITION

Imagine a software firm that is composed of different Business Units (or departments) like Storage BU, Networking BU. Automobile BU. The life time of these Business Units is governed by the lifetime of the organization. In other words, these Business Units cannot exist independently without the firm. This is COMPOSITION. (ie the firm is COMPOSED OF business units)

ASSOCIATION

The software firm may have external caterers serving food to the employees. These caterers are NOT PART OF the firm. However, they are ASSOCIATED with the firm. The caterers can exist even if our software firm is closed down. They may serve another firm! Thus the lifetime of caterers is not governed by the lifetime of the software firm. This is typical ASSOCIATION

AGGREGATION

Consider a Car manufacturing unit. We can think of Car as a whole entity and Car Wheel as part of the Car. (at this point, it may look like composition..hold on) The wheel can be created weeks ahead of time, and it can sit in a warehouse before being placed on a car during assembly. In this example, the Wheel class's instance clearly lives independently of the Car class's instance. Thus, unlike composition, in aggregation, life cycles of the objects involved are not tightly coupled.