Forms of Interitance (Chapter 8)

All objects eventually inherit from Object, which provides useful methods such as equals and toString.

In general we want to satisfy substitutability: if B is a subclass of A, anywhere we expect an instance of A we can use an instance of B.

Inheritance gets used for a number of purposes in typical object-oriented programming:

specialization

specification -- the superclass just specifies which methods should be available but doesn't give code. This is supported in java by interfaces and abstract methods.

construction -- the superclass is just used to provide behavior, but instances of the subclass don't really act like the superclass. Violates substitutability. Exmample: defining Stack as a subclass of Vector. This is not clean -- better to define Stack as having a field that holds a vector.

extension -- subclass adds new methods, and perhaps redefines inherited ones as well.

limitation -- the subclass restricts the inherited behavior. Violates substitutability. Example: defining Queue as a subclass of Dequeue.

combination -- multiple inheritance. Provided in part by implementing multiple interfaces.

See the book's description of benefits and costs of inheritance.