Listing 6-2 Abstract Point public interface Point { double getX(); double getY(); void setCartesian(double x, double y); double getR(); double getTheta(); void setPolar(double r, double theta); } The beautiful thing about Listing 6-2 is that there is no way you can tell whether the implementation is in rectangular or polar coordinates. It might be neither! And yet the interface still unmistakably represents a data structure. But it represents more than just a data structure. The methods enforce an access policy. You can read the individual coordinates
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