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And that's exactly what we're going to look at next,

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relationships in the class diagram, how they're represented and

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what they mean. First of all relationships as the

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name says, describe interactions between classes or between objects in

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my system. And we will describe three main types

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of relationships. The first one is called a Dependency

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relationship. And we can express that as X uses

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Y and we represent it with a dashed directed line.

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So when we have such a line between two classes

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that means that the first class uses the second one. And

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we're going to provide an example of a dependency in a

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minute. The second type of relationship is an association that can

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also be an aggregation. We'll see what the distinction is.

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But basically, what this means is that we can express that

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as a X has a y. So x contains a

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y. And if it is in association, we indicate it with

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a solid undirected line. If it's an aggregation,

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we indicate it in the same way, but with

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a diamond at one of the ends. Finally, the

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third type of relationship is what is called Generalization.

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And this can be expressed as x is a

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y. So this is the relationship that expresses inheritance.

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Specialization between two classes. It's represented with a solid

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directed line with a large open arrow head at

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the end. Going from the more specialized class to

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the less specialized class. So going from the subclass to

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the super class. So now let's look at each relationship

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in more detail using our example, our course management system.