14 Things for Comparing Mutable vs Immutable Objects

In programming, we encounter objects of different types frequently. For example, some objects are mutable, and some are immutable. These concepts describe two very different types of objects in programming. Understanding the difference between them is important because they have major implications when working with these types of objects.

Mutable objects are objects whose value can be changed during runtime, while an immutable object is an object you cannot change. Therefore, you must create a new object with an immutable object if you want to change its contents.

When you create a mutable object, you can change its contents without creating a new object. For example, if you have a list of numbers, you can add new numbers to the list or remove numbers from the list. A new instance of an object is created with immutable objects, such as int or double, if you change its initial value.

This article will show the differences between mutable and immutable objects and how you can use them to make your codebase more secure, maintainable, and extensible.

What are mutable objects?


Generally, the term mutable means “changeable.” In programming, an object whose internal state can be changed after creation is called a mutable object. For example, in C#, all the types are mutable except for String and value types such as integer, double, float, and char.

The mutable objects are modifiable during runtime.

Why do you need mutable objects?

You need the mutable objects in a situation when you have to transform a class from one state to another. Mutable objects allow you to make changes in place without redefining the object or creating a new object. 

Why should you use the mutable objects?

You should use the mutable objects to make the memory usage efficient during the program execution. When creating an application, you should prefer using mutable objects as it will prevent you from creating a new variable each time you want a new value. Furthermore, when you want to let the user change the values on runtime, the mutable objects are the best to use.

What are immutable objects?

The term immutable means “unchangeable.” An object whose value is unmodifiable once initialized is said to be immutable. In C#, a String and value types such as integer, double, float, and char are immutable objects. The immutability implies that the value of an object must not change once assigned.

You need to redefine or recreate an immutable object to change its value.

Is immutability a good thing?

Immutability is good as your code is less error-prone and ensures thread safety. In addition, it allows you to parallelize your code and run it in a distributed fashion for fast processing and making the most of your multicore CPU. Immutability also prevents the objects from any accidental or intentional changes.

Why should you use immutable objects?

You should use immutable objects to preserve the state of an object you set. For example, a thread interference cannot corrupt or take the object to an inconsistent state if immutable. Furthermore, you get better encapsulation with immutable objects.

What are the advantages of immutable objects?

The first and foremost advantage of immutable objects is their unchangeable state. Other advantages include:

  • Thread safety – It is a significant benefit of using immutable objects. The immutable objects achieve thread safety as the object state does not change, so no matter how many threads use it concurrently, its state will not change. If any thread needs the object with an altered state, it will have to create a new object. So, there is no synchronization issue. This way, immutability guarantees thread safety.
  • Easy design and implementation – Immutable classes are easier to create, design, implement and use. 
  • No state invalidation – Once you have defined and validated the state of an immutable object, there is no way it will change by any thread or the background process without your knowledge. This way, the programs having immutable objects have high security. You know that your objects will remain safe before entering an invalid state.
  • Better encapsulation – Encapsulation is the fundamental part of object-oriented programming. Without it, the existence of OOP becomes pointless. With the help of immutable objects, you can achieve better encapsulation while passing the objects to different methods, and you will be sure that their state will not change.
  • Easy debugging – Whenever an error occurs while debugging the code, you know that the immutable objects will not change; hence they can not cause errors. So, while debugging, you skip the code having an immutable object and make debugging more straightforward and faster.
  • Easy testing – A code base with immutable objects is simpler to test because your code has fewer side effects that automatically lead to less confusing code paths that are easier to track.
  • Readable and maintainable code – Immutability makes your code more readable and maintainable as none of the parts of your code will be affected by the immutable objects. In addition, you can lessen the intermingled and moving parts in your code using immutability which makes your code clean and easy to understand.

What are the disadvantages of immutable objects?

With all the benefits mentioned in the above section, immutable objects have only one disadvantage: When you need to change the state of an object, you have to create a new object which is an overhead and causes frequent garbage collection. Moreover, if these objects are large, they can be costly.

What are the advantages of mutable objects?

Although immutable objects have many benefits, and it is always better to use them, mutable objects also serve some purposes and have some advantages, which are as follows:

  • Easy to change – When you use mutable objects, they allow you to change their state repeatedly without redeclaring them. 
  • Memory efficient – Using mutable objects is an efficient way to use memory as you do not need to create a new object whenever you need an object with an altered state.

What are the disadvantages of mutable objects?

While reading this article, you might observe that using mutable objects is not recommended. There are specific reasons behind it, some of which are as follows:

  • No thread safety – The internal state of the mutable objects can be changed by any thread whenever it accesses it. This causes the synchronization issue as no two threads can access it simultaneously. 
  • Reduced maintainability – When you use mutable objects, your code becomes difficult to maintain as the state of your objects keeps changing throughout the code, reducing the throughput. 
  • Difficult to debug and test – Mutable objects make it difficult to test and debug your code as more code paths lead to confusion. It is also difficult to debug as the object’s state changes frequently.
  • Reduced code readability – Mutability also reduces code readability because of the changing object’s state.

Mutable vs. immutable objects – detailed comparison

The mutable and immutable objects are both usable in different situations and serve different purposes. You use one when an object’s state can change and the other to preserve an object’s state. However, here is a detailed comparison of mutable and immutable objects to help you understand the concepts better:

  1. In mutability, you can change the fields after object initialization, whereas immutability does not allow you to do so.
  2. A mutable object refers to the variable, whereas you can think of an immutable object as the value.
  3. Mutable classes generally have a method to modify the object’s state. Immutable classes do not have such a method.
  4. Mutable have getter and setter accessors, whereas immutables are read-only and have only getter accessors.
  5. The process of creating a mutable class is simple and comprises two steps:
    – Creating a method that modifies the object.
    – Defining the Getter and Setter methods.
  6. The process of creating an immutable class is a bit complex and comprises various steps:
    – Declaring the class as sealed so no other class could inherit it.
    – Make all the fields private so no other class can access them directly.
    – Defining the Getter method while skipping the Setter.
    – Make all the mutable fields read-only so they can be assigned only once.
  7. The immutable objects do not require synchronization and guarantee thread safety, whereas the mutable objects need some synchronization as their state is not maintained.
  8. To change the state of an immutable object, you need to redeclare and redefine the object that adds an overhead over the compiler and causes inefficient use of memory. You can change the mutable objects without recreating them, which is a more efficient way to utilize the memory.
  9. Immutability becomes costly if the objects are large.
  10. Immutable code is easier to create, implement, debug and test than mutable code.
  11. Mutability makes the code harder to understand and trace.
  12. Immutables are quicker to access but expensive to change, whereas mutable are difficult to access and easier to change.
  13. Mutability is useful in cases where parallel programming is needed. Immutability supports functional programming and is highly implementable in situations where multiple threads access the object concurrently.
  14. Immutability provides better encapsulation. Mutability does not contribute to encapsulation.

If immutable objects are good, why do people keep creating mutable objects?

Although, immutable objects are better than mutable ones in many ways. They can come in handy in concurrent applications and make your code more readable, useable, simple, and easy to debug.

But, when you have large objects, creating a single copy becomes expensive. So, creating multiple copies of an object becomes less efficient and slows down your computer’s operation. In such a situation, a mutable object is better than an immutable object.

Moreover, in some scenarios, the speed of the application determines its usability. So, using the mutable objects will help increase the speed of your software hence fulfilling one of the primary system requirements. 

Furthermore, while representing real-world entities, mutable objects help you to implement the domain model more easily and straightforwardly.

However, besides all the legitimate reasons mentioned above, programmers keep using mutable objects because they do not want to change or stay updated about new tools and practices.


Being a developer, you must have the know-how of the fundamentals of functional programming and keep updating your programming skills while learning.

This article intends to explain two of the most fundamental concepts of functional programming, i.e., mutable and immutable objects.

Although both mutable and immutable objects fit best in certain circumstances, it is recommended that you use immutable objects to improve the implementation of core OOP and write better, more manageable, maintainable, and testable code with fewer errors and bugs.

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