Scaling shapes

exercise No. 166

Q:

We want shapes to become scalable:

In the above example the rectangle is being shrunk to 50% of its original size whereas the circle's radius is being doubled. Add a corresponding scale(...) method to the inheritance hierarchy which allows for operation chaining as well.

Provide appropriate unit tests.

A:

This task is pretty much straightforward. Since scaling requires specific details (like radius or width and height) a scale() method cannot be implemented on top level of our inheritance hierarchy. We thus start by defining an abstract method in our class Shape: 

/**
  *
  * @param factor Scale the current shape by this value.
  * @return The current object.
  */
public abstract Shape scale(double factor);

This method has to be implemented in our two concrete classes Circle and Rectangle.

Sensible unit tests may be based on the observation that:

  • A shape's perimeter grows linear with the scaling factor.

  • A shape's area grows linear with the scaling factor's square.

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