• Bad effects of public fields
• Non-final classes have two interfaces

Designing a class to be extended

• Choose the access for each part of a class design carefully: public, protected, private.
• If your design will have subclasses, design your protected parts carefully.

Bad effects of public fields

• Fields can be modified at any time by a programmer.
• No way to add functionality.

Non-final classes have two interfaces

• Public interface for programmers using the class.
• Protected interface for programmers extending the class.