Favor immutability to retain control of your objects
TL;DR: Use immutable objects to prevent unexpected changes caused by aliasing.
Problems
- Unexpected mutations
- Difficult bug tracking
- Unpredictable code behavior
- Reduced code predictability
- Increased coupling
- Compromised thread safety
Solutions
- Use immutable objects
- Implement defensive copying
- Favor functional programming
Refactorings
Context
Aliasing happens when multiple references point to the same mutable object.
This can lead to unexpected changes in them when one part of the code modifies the object, affecting all references.
Immutable objects mitigate this risk by ensuring you cannot change their internal representation once you create an object.
Collection Aliasing is a notable example of this issue.
Sample Code
Wrong
public class Person {
private String name;
}
public void modifyPerson(Person person) {
person.setName("Cosmo Kramer");
}
public static void main(String[] args) {
Person p1 = new Person("Newman");
Person p2 = p1; // p1 and p2 refer to the same object
modifyPerson(p1);
System.out.println(p1.name()); // Output: Cosmo Kramer
System.out.println(p2.name()); // Output: Cosmo Kramer (unexpected)
}
Right
public class ImmutablePerson {
private final String name;
public ImmutablePerson(String name) {
this.name = name;
}
}
public ImmutablePerson withName(String newName) {
return new ImmutablePerson(newName);
}
public static void main(String[] args) {
ImmutablePerson p1 = new ImmutablePerson("Newman");
ImmutablePerson p2 = p1; // p1 and p2 refer to the same object
// Modifying p1 creates a new object
ImmutablePerson p3 = p1.withName("Cosmo Kramer");
// but this is a bad practice
// since only constructors should create new objects
// A better option is
ImmutablePerson p3 = new ImmutablePerson("Cosmo Kramer");
System.out.println(p1.name()); // Output: Newman
System.out.println(p2.name()); // Output: Newman
System.out.println(p3.name()); // Output: Cosmo Kramer
}
Detection
[X] Semi-Automatic
You can detect this smell by reviewing your code for mutable objects shared across different parts of your program.
Tags
- Mutability
Level
[x] Intermediate
AI Generation
AI generators might introduce this smell if they're not specifically trained to prioritize immutability and avoid aliasing issues.
AI Detection
AI detectors identify this smell by analyzing code for mutable shared objects and suggesting immutable alternatives.
They need specific instructions on the context and the importance of immutability in the codebase.
Try Them!
Remember AI Assistants make lots of mistakes
ChatGPT Claude
Perplexity
Gemini
Conclusion
Using immutable objects and avoiding aliasing can significantly improve your code's predictability, reduces bugs, and improves thread safety.
It requires a shift in thinking and the benefits of immutability far outweigh the initial learning curve.
Relations
More Info
Disclaimer
Code Smells are my opinion.
Credits
Photo by Natural Photos on Unsplash
Immutability changes everything.
Pat Helland
This article is part of the CodeSmell Series.
Top comments (5)
Do you think using a Factory pattern would work in this example? Or do you think that it could have similar side effects?
as long as it creates immutable objetcts, Factory pattern is fine
Thank you!
Is the "Immutable" prefix required part of a right code?
No. In fact is a code smell. All Objects should be immutable by default.
The example is here to show the differences