Finding the Stinky Parts of Your Code: Code Smell 256 - Mutable Getters

30 Jun 2024

Using getters is a significant issue. Exposing internals is a major problem

TL;DR: Don't expose your internals and lose control


  • Mutability
  • Unexpected Changes
  • Ripple Effects
  • Thread unsafety
  • Encapsulation Principle violation


  1. Return shallow copies of your collections


Immutable objects are essential in functional and object-oriented programming.

Once created, their state cannot be altered.

This is key to keeping object integrity and ensuring thread safety in multithreaded applications.

Mutable getters allow callers to access and modify the internal state of an object, leading to potential corruption and unexpected behavior.

When you break encapsulation, you take responsibility away from an object. Integrity is lost.

Returning a page in a book is like an immutable copy. It cannot be edited, like a human memory.

You can edit some memories by bringing them from long-term memory.

Sample Code


public class Person {
    private List<String> hobbies;

    public Person(List<String> hobbies) {
        this.hobbies = hobbies;

    public List<String> getHobbies() {
        return hobbies;


public class Person {
    private List<String> hobbies;

    public Person(List<String> hobbies) {
        this.hobbies = new ArrayList<>(hobbies);

    public List<String> hobbies() {
        // This returns a shallow copy
        // This is usually not a big performance issue
        return new ArrayList<>(hobbies);


  • [x]Semi-Automatic

You can detect mutable getters by examining the return types of your getters.

If they return mutable collections or objects, you need to refactor them to return immutable copies or use immutable data structures.


  • Mutability


  • [x]Intermediate

AI Generation

AI generators might create this smell if they prioritize simplicity and brevity over best practices.

They not always consider the implications of returning mutable objects.

AI Detection

AI tools can detect this smell if you instruct them to look for getters returning mutable objects or collections.

They can suggest returning copies or using immutable types to fix the issue.


Getters are a code smell, but something you need to return objects you hold.

You can do it at your own risk, but retain the tracking of those collections.

Avoid mutable getters to protect your object integrity and encapsulation.

By returning immutable copies or using immutable types, you can prevent unintended modifications and ensure your objects remain reliable and predictable.


Code Smell 68 - Getters

Code Smell 109 - Automatic Properties

Disclaimer: Code Smells are my opinion.

Credits: Photo by Suzanne D. Williams on Unsplash

The best programmers write only easy programs.

Michael A. Jackson

Software Engineering Great Quotes

This article is part of the CodeSmell Series.

How to Find the Stinky Parts of your Code