Encapsulate Collection

Removes smells

Referenced by patterns

Symptom

A class exposes a collection field directly (getter returning the internal array, public field, etc.) — callers can mutate the collection from outside.

Goal

A class's internal collection is never returned directly; callers add or remove via methods on the class, and reads return a snapshot or iterator.

Before the refactoring

class Person {
  courses;
  getCourses() { return this.courses; }
}

After the refactoring

class Person {
  #courses = [];
  courses()       { return [...this.#courses]; }
  enroll(course)  { this.#courses.push(course); }
  drop(course)    { this.#courses = this.#courses.filter(c => c !== course); }
}

Example source: Illustrative example written for this site, not a quotation from any source.

Pressure

Invariants on the collection (size, uniqueness, ordering) can't be enforced; bugs hide where one consumer mutates the collection in a way another assumed wouldn't happen.

Tradeoff

Returning a shallow copy on every read can hide bugs where callers expected mutation to be reflected — be explicit about the contract.

Relief

The owner can enforce invariants (uniqueness, ordering, max size); refactoring the collection's internal shape is local.

Trap

Returning a shallow copy on every read without telling callers — they expect mutations to be reflected, get silent no-ops, and bugs appear in unexpected places.