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    <p>The answer to your question about the room and the wall is the answer to this question: "Can the wall exist without the room?"</p> <p>I believe your scenario is using aggregation. Can the wall exist without the room? Sure it can. One could destroy the room by destroying three walls, but that remaining wall stands on its own. I think we're down to semantics now. This answer can change depending on how <em>you</em> view the walls in <em>your</em> scenario.</p> <p>This <a href="https://softwareengineering.stackexchange.com/questions/61376/aggregation-vs-composition">link</a> shows a concise way to think about it:</p> <ol> <li>A "owns" B = <strong>Composition</strong> : B has no meaning or purpose in the system without A</li> <li>A "uses" B = <strong>Aggregation</strong> : B exists independently (conceptually) from A</li> </ol> <p>Same <a href="https://stackoverflow.com/questions/1644273/what-is-the-difference-between-aggregation-composition-and-dependency">here</a>:</p> <p><strong>Aggregation</strong> implies a relationship where the child can exist independently of the parent. Example: Class (parent) and Student (child). Delete the Class and the Students still exist.</p> <p><strong>Composition</strong> implies a relationship where the child cannot exist independent of the parent. Example: House (parent) and Room (child). Rooms don't exist separate to a House</p> <p>From <a href="http://en.wikipedia.org/wiki/Object_composition" rel="nofollow noreferrer">wikipedia</a>:</p> <p><strong>Aggregation</strong> differs from ordinary <strong>composition</strong> in that it does not imply ownership. In <strong>composition</strong>, when the owning object is destroyed, so are the contained objects. In <strong>aggregation</strong>, this is not necessarily true. For example, a university owns various departments (e.g., chemistry), and each department has a number of professors. If the university closes, the departments will no longer exist, but the professors in those departments will continue to exist. Therefore, a University can be seen as a composition of departments, whereas departments have an aggregation of professors. In addition, a Professor could work in more than one department, but a department could not be part of more than one university.</p>
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    1. POwhat kind of relationship is there between a common wall and the rooms that located next to it?
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    1. COGood answer, +1.
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    2. COI think I know the concepts that u mentioned , but based on This Concepts its hard to make a good decision for modeling such a problem,I think there is so many paradoxes here , if you destroy or delete the whole object (I mean the Room )the wall will no longer exist ,in fact it dosnt make sense so the relationship could be Composition from this point of view , on the other hand,in this example we can see that there is one Common instance at part section that is shared between two whole objects ,and as far as i know this is not permitted in the composition,so which option is correct to choose?
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    3. CO@siamak I believe your scenario is using aggregation. Can the wall exist without the room? Sure it can. One could destroy the room by destroying three walls, but that remaining wall stands on its own. I think we're down to semantics now. This answer can change depending on how *you* view the walls in *your* scenario.
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