20.4. Reuse Through Association

Inheritance is not the only way to reuse code. Association occurs when an object stores a reference to one or more objects from a different class in one of its instance variables (attributes). The object is thus able to execute code on the objects it embeds within itself.

For example, our LabeledPoint example could have been implemented as follows:

The first thing to notice is that LabeledPoint does not inherit from Point in the code above. Instead, its constructor creates a Point and stores a reference to it in its point instance variable so that it can be used by the other methods.

Next, notice how the distanceFromOrigin() method uses the code in Point` by invoking it. We say that ``LabeledPoint’s distanceFromOrigin() delegates its implementation to Point’s implementation.

Finally, notice how the __str__() method also uses the code in Point by calling str(self.point).

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