280: Mainline

Data abstraction and the notion of a public interface fit together as follows:

• Channeling instance-variable accesses and assignments through access methods isolates you from the details of class implementation.
• When you define private instance variables, you force all instance-variable accesses and assignments to go through the constructors, getters, setters, and other instance methods in the public interface, thus providing a means to ensure that you practice data abstraction.