Program to an Interface, not an Implementation

• You should program to an interface, not an implementation of a class. This will lead to the code depending on abstractions and not concrete classes.

• You can tell that the design is flexible enough if you can easily extend it without breaking any existing code.

• There’s a flexible way to establish collaboration between objects:

  1. Determine what exactly one object needs from the other: which methods does it execute?
  2. Describe these methods in a new interface or abstract class
  3. Make the class that is a dependency implement this interface
  4. Make the second class dependent on this interface rather than on the concrete class