MEPX
Chapter 2 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 2 of 10

Encapsulation

Hiding how, exposing what.

Why hide anything

If callers only use the methods, the internals can change freely. If they reach into the fields, every internal detail becomes a promise you cannot break.

  • Expose behaviour rather than data: account.withdraw(50) not account.balance -= 50.
  • A getter and setter for every field is encapsulation in name only.

Invariants

An invariant is something that must always be true, such as a balance never going negative. Encapsulation exists so one place can enforce it rather than every caller remembering.