MEPX
Chapter 3 of 10All chapters
  1. 1Objects and classes
  2. 2Encapsulation
  3. 3Inheritance
  4. 4Polymorphism
  5. 5Composition over inheritance
  6. 6Design principles
  7. 7Design patterns
  8. 8State and mutation
  9. 9Testing object code
  10. 10When not to use it

Chapter 3 of 10

Inheritance

Reusing a definition, with a catch.

How it works

A subclass gets everything the parent has and may add or replace parts. It expresses an is-a relationship: a Manager is an Employee.

  • A subclass must be usable anywhere the parent is, or callers break.
  • A square subclassing a rectangle is the classic example of that rule being violated.

The cost

Inheritance is the tightest coupling available: a change to the parent reaches every descendant. Deep hierarchies become hard to reason about and harder to change.