Encapsulate Collection

Encapsulate access to a collection field to prevent direct modification.

“A method returns a collection. Make it return a read-only view and provide add/remove methods.”

Protect your lists.

graph LR
    A[Public List] -->|Refactoring| B[Private List + Add/Remove Methods]

When to apply (Smells)

• Leaking internal state: Clients modify the collection directly without the owner knowing.

Motivation

• Control: The owning class should control membership of the collection.

Mechanics (Steps)

1. Add add() and remove() methods.
2. Return a Read-Only copy (or iterator) in the getter.

Explanation

Problem

A class returns a reference to a collection field. getSkills(). The client then adds to it. person.getSkills().add('Java').

Solution

Return an unmodifiable list. person.getSkills() (read-only). Provide person.addSkill('Java').

Pros and Cons

Pros	Cons
Encapsulation	Slightly more code

Code example

Typescript

Before

After

PHP

Before

After