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The next example of relationship we're going to look at is
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the association relationship. This is a relationship in which objects of
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one class are connected to objects of another. And it's
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called an has a relationship. So it means that objects of
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one class have objects of another class. So let's see
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what that means. Let's do it by considering two classes in
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our example system. The student class and the course offering class.
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In this case, there is an association between the student and
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the course offering, because the student is registering for the
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course offering. So, in a sense, the course offering has students.
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Contains students, to indicate this fact we add a solid
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line between the student class and the course offering. And the
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fact that having a solid line doesn't really tell us
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much about the nature of the relationship, so to clarify such
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nature we can use what we call adornments that we
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can apply to associations we can add to associations to clarify
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their meaning. In particular we can add a label to
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an association and the label describes the nature of the relationship.
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In this case, for example, it clarifies that the student
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registers for CourseOffering. We can also add a triangle to
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further clarify the direction of the relationship. So in this
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case, the triangle will indicate that it's the student That registers
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for the course offering, and not the other way around. Another
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important adornment or limitation that we can put on an association,
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is multiplicity. Multiplicity defines the number of instances of one
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class that are related to one instance of the other
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class. We can define multiplicity at either end of the
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relationship. In this case, for instance, we can say that
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if we look at the student, the student can register
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for two or more course offerings. Whereas, if we look
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at the course offering, we can say that each course
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offering can have or can enroll between 1 and 50 students.
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So as you can see by adding a label, a direction,
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and multiplicity, we make it much clearer what the relationship is
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and what it means and what are its characteristics. As we
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saw when we introduced relationships, there is a different kind of
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association, kind of a specialized one, which we call aggregation. So
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here we're going to look at an example of an aggregation. So
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first of all what is an aggregation? An aggregation is a
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relationship between 2 classes in which 1 represents a larger class
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like a whole which consists of smaller classes which are
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the parts of this whole. So lets look at an example
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in the context of our system. Let's consider a Course
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and the CourseOffering. And in this case, we can see
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that the Course consists of multiple CourseOfferings. So in
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a sense, a course is a whole and the course offerings
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are the parts of this whole. So this a perfect case
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in which we will use an aggregation to express this relationship.
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So we will add a solid line with a diamond on the side of the whole class
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to indicate that the course consists of multiple course offerings.
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And as we did for associations even though we
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are not going to do it for this specific
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example, we could also in this case add multiplicity
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information on the aggregation to indicate how many classes
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of the two types are involved in the relationships.
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