Chapter 5. Reducing Costs with Duck Typing

The purpose of object-oriented design is to reduce the cost of change. Now that you know messages are at the design center of your application, and now that you are committed to the construction of rigorously defined public interfaces, you can combine these two ideas into a powerful design technique that further reduces your costs.

This technique is known as duck typing. Duck types are public interfaces that are not tied to any specific class. These across-class interfaces add enormous flexibility to your application by replacing costly dependencies on class with more forgiving dependencies on messages.