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1. Introduction

When working with objects in Java, understanding the difference between mutable and immutable objects is crucial. These concepts impact the behavior and design of your Java code.

In this tutorial, let’s explore the definitions, examples, advantages, and considerations of both mutable and immutable objects.

2. Immutable Objects

Immutable objects are objects whose state cannot be changed once they are created. Once an immutable object is instantiated, its values and properties remain constant throughout its lifetime.

Let’s explore some examples of built-in immutable classes in Java.

2.1. String Class

The immutability of Strings in Java ensures thread safety, enhances security, and helps with the efficient use of memory through the String Pool mechanism.

public void givenImmutableString_whenConcatString_thenNotSameAndCorrectValues() {
    String originalString = "Hello";
    String modifiedString = originalString.concat(" World");

    assertNotSame(originalString, modifiedString);

    assertEquals("Hello", originalString);
    assertEquals("Hello World", modifiedString);

In this example, the concat() method creates a new String, and the original String remains unchanged.

2.2. Integer Class

In Java, the Integer class is immutable, meaning its values cannot be changed once they are set. However, when you perform operations on an Integer, a new instance is created to hold the result.

public void givenImmutableInteger_whenAddInteger_thenNotSameAndCorrectValue() {
    Integer immutableInt = 42;
    Integer modifiedInt = immutableInt + 8;

    assertNotSame(immutableInt, modifiedInt);

    assertEquals(42, (int) immutableInt);
    assertEquals(50, (int) modifiedInt);

Here, the + operation creates a new Integer object, and the original object remains immutable.

2.3. Advantages of Immutable Objects

Immutable objects in Java offer several advantages that contribute to code reliability, simplicity, and performance. Let’s understand some of the benefits of using immutable objects:

  • Thread Safety: Immutability inherently ensures thread safety. Since the state of an immutable object cannot be modified after creation, it can be safely shared among multiple threads without the need for explicit synchronization. This simplifies concurrent programming and reduces the risk of race conditions.
  • Predictability and Debugging: The constant state of immutable objects makes code more predictable. Once created, an immutable object’s values remain unchanged, simplifying reasoning about code behavior.
  • Facilitates Caching and Optimization: Immutable objects can be easily cached and reused. Once created, an immutable object’s state does not change, allowing for efficient caching strategies.

Therefore, developers can design more robust, predictable, and efficient systems using immutable objects in their Java applications.

3. Creating Immutable Objects

To create an immutable object, let’s consider an example of a class named ImmutablePerson. The class is declared as final to prevent extension, and it contains private final fields with no setter methods, adhering to the principles of immutability.

public final class ImmutablePerson {
    private final String name;
    private final int age;

    public ImmutablePerson(String name, int age) { = name;
        this.age = age;

    public String getName() {
        return name;

    public int getAge() {
        return age;

Now, let’s consider what happens when we attempt to modify the name of an instance of ImmutablePerson:

ImmutablePerson person = new ImmutablePerson("John", 30);

The attempt to modify the name of an ImmutablePerson instance will result in a compilation error. This is because the class is designed to be immutable, with no setter methods allowing changes to its state after instantiation.

The absence of setters and the declaration of the class as final ensure the immutability of the object, providing a clear and robust way to handle a constant state throughout its lifecycle.

4. Mutable Objects

Mutable objects in Java are entities whose state can be modified after their creation. This mutability introduces the concept of changeable internal data, allowing values and properties to be altered during the object’s lifecycle.

Let’s explore a couple of examples to understand their characteristics.

4.1. StringBuilder Class

The StringBuilder class in Java represents a mutable sequence of characters. Unlike its immutable counterpart, String, a StringBuilder allows the dynamic modification of its content.

public void givenMutableString_whenAppendElement_thenCorrectValue() {
    StringBuilder mutableString = new StringBuilder("Hello");
    mutableString.append(" World");

    assertEquals("Hello World", mutableString.toString());

Here, the append method directly alters the internal state of the StringBuilder object, showcasing its mutability.

4.2. ArrayList Class

The ArrayList class is another example of a mutable object. It represents a dynamic array that can grow or shrink in size, allowing the addition and removal of elements.

public void givenMutableList_whenAddElement_thenCorrectSize() {
    List<String> mutableList = new ArrayList<>();

    assertEquals(1, mutableList.size());

The add method modifies the state of the ArrayList by adding an element, exemplifying its mutable nature.

4.3. Considerations

While mutable objects offer flexibility, they come with certain considerations that developers need to be mindful of:

  • Thread Safety: Mutable objects may require additional synchronization mechanisms to ensure thread safety in a multi-threaded environment. Without proper synchronization, concurrent modifications can lead to unexpected behavior.
  • Complexity in Code Understanding: The ability to modify the internal state of mutable objects introduces complexity in code understanding. Developers need to be cautious about the potential changes to an object’s state, especially in large codebases.
  • State Management Challenges: Managing the internal state of mutable objects requires careful consideration. Developers should track and control changes to ensure the object’s integrity and prevent unintended modifications.

Despite these considerations, mutable objects provide a dynamic and flexible approach, allowing developers to adapt the state of an object based on changing requirements.

5. Mutable vs. Immutable Objects

When contrasting mutable and immutable objects, several factors come into play. Let’s explore the fundamental differences between these two types of objects:

Criteria Mutable Objects Immutable Objects
Modifiability Can be changed after creation Remain constant once created
Thread Safety May require synchronization for thread safety Inherently thread-safe
Predictability May introduce complexity in understanding Simplifies reasoning and debugging
Performance Impact Can impact performance due to synchronization Generally has a positive impact on performance

5.1. Choosing Between Mutability and Immutability

The choice between mutability and immutability relies on the application’s requirements. If adaptability and frequent changes are necessary, opt for mutable objects. However, if consistency, safety, and a stable state are priorities, immutability is the way to go.

Consider the concurrency aspect in multitasking scenarios. Immutability simplifies data sharing among tasks without the complexities of synchronization.

Additionally, assess your application’s performance needs. While immutable objects generally enhance performance, weigh whether this boost is more significant than the flexibility offered by mutable objects, especially in situations with infrequent data changes.

Maintaining the right balance ensures your code aligns effectively with your application’s demands.

6. Conclusion

In conclusion, the choice between mutable and immutable objects in Java plays a crucial role in shaping the reliability, efficiency, and maintainability of your code. While immutability provides thread safety, predictability, and other advantages, mutability offers flexibility and dynamic state changes.

Assessing your application’s requirements and considering factors such as concurrency, performance, and code complexity will help in making the appropriate choice for designing resilient and efficient Java applications.

You can find the examples used in this article over on GitHub.

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