What is the difference between aggregation and composition

In a composite aggregation, an object may be a part of only one composite at a time. For example, your car consists of wheels, engine, gearbox, steering, and the main body, etc. It is an assembly, and the other parts are its constituents. Here, car to the wheel is one Aggregation, car to the engine is another aggregation, car to gearbox another, and so on. This type of pairing helps you to define the multiplicity of the constituent part within the assembly as its outcome.

The number of objects can also be depicted. A car needs a wheel to function correctly. However, we cannot say the same with a car. The same logic can be applied to bike, bicycle, or any other vehicle but not a particular car. Here, the wheel object is meaningful even without the car object. It is known as an aggregation relationship. Represented by a solid diamond next to assembly class. Function The deletion of assembly doesn't affect its parts. If the owning class object is deleted, it could significantly affect the containing class object.

Aggregation can be defined as a relationship with an assembly class to constituent class. Aggregation is also used to specify the multiplicity of objects such as one-to-one, one-to-many and many-to-many. When an object of class X is associated with a single object of class Y, in that case there exist one-to-one relationship. Similarly, if a single object of a class is associated with multiple objects of a class, then this relationship is referred to as one-to-many. For example, a car consists of wheels, engine, gearbox, steering, main body, etc.

A car is an assembly, and the other parts are its constituents. Car to wheels is one aggregation, car to engines is another aggregation, and so on. We use cookies to offer you a better experience. By visiting our website, you agree to the use of cookies as described in our Cookie Policy. Free Download. Learn UML. Draw UML. Turn every software project into a successful one. Try Visual Paradigm Free. All rights reserved. Legal Privacy statement. The quote is correct but the comment is not. UML does not define garbage collection system.

This also falls apart when an object is the component of two other objects, such as the ball in a ball-joint joining two artificial limbs.

The Component relationship is about functional dependence. I agree that my answer is severely lacking - but so is the WikiPedia article The difference between composition and aggregation is clear. The problem with aggregation is that it's unclear how it is different from ordinary association.

Kilian Foth k 43 43 gold badges silver badges bronze badges. Pawan Pawan 3 3 silver badges 3 3 bronze badges. What is the difference between aggregation and association that is neither composition nor aggregation? This is the correct answer. Compositions are a critical building block of many basic data structures Aggregation collection differs from ordinary composition in that it does not imply ownership.

UML notations for different kind of dependency between two classes Composition : Since Engine is part-of Car, relationship between them is Composition. Premraj Premraj 8 8 silver badges 17 17 bronze badges.

Pretty sure books can exist without libraries. Bad example! Here the List of Employees is the part of Organization Object. How can this be aggregation? How is association different from aggregation? I love this answer. It finally explained to me properly what the difference between aggregation and composition is. Steven A. Lowe Steven A. Lowe Lazarus Lazarus 2 2 silver badges 6 6 bronze badges.

How about this simple example: An array of objects is a composition. An array of pointers to objects is an aggregation. Respectfully disagree, gnat. This is a helpful example of how the two could be implemented. People learn better with examples.