Almost Never Use Inheritance

Inheritance is safe only within a package or when a class is explicitly designed for extension.

Think twice before going for inheritance. And if you do decide to go for it. Firstly, inheritance must strictly follow a “is-a” relationship.

Inheritance breaks encapsulation in the sense that a subclass depends on the implementation details of its superclass. (So it is relatively safe within a package because it is considered private within a package.) So when a class is designed for extension, you must document throughly not only what a method does but also how a method does it so that a subclass can be properly implemented.