Java Practices->Designing for subclassing

Designing for subclassing

To create an extendable class:

all constructors, readObject, and clone must not invoke an overridable method (one that is not static, private, or final)
if a method depends on an implementation of a overridable method, this dependence must be explicitly stated in its javadoc
don't implement Serializable unless you absolutely need to
don't implement Cloneable unless you absolutely need to

Consider using an "interface + default implementation" pair. This will allow users to choose the desired style of inheritance:

subclass the default implementation directly
use the default implementation as a field, and forward calls to it
ignore the default implementation, and implement the interface entirely from scratch