Implement behavior with abstract state pattern

Defining the behaviour of a class without defining any of the data structure leaves your class very flexible. By delaying the specification of implementation details but providing a class interface that is already functional you manage to stick to the Once and only once principle and also create a class that allows subclasses to adapt to changing space and efficiency requirments.

What to do

When you are defining your base class implement all the functionality that the public interface requires. Every time you find that you require some data from the class define an abstract accessor method instead of an instance variable. Later a class that extends your base class can fill in all the details of the data structure and the accessor methods. This also leaves your class very flexible to accommodate new special cases and variations.