AmanWhy Immutable Classes?
Immutable classes are beneficial because they are inherently thread-safe, easy to reason about, and prevent accidental changes to the object's state. An immutable object’s state cannot be modified after it is created, making it a valuable design pattern, especially in multi-threaded environments.
Traditional Approach to Creating an Immutable Class
Consider the following Employee class:
final class Employee {
private final long id;
private final String name;
private final double salary;
public Employee(long id, String name, double salary) {
this.id = id;
this.name = name;
this.salary = salary;
}
public long getId() {
return id;
}
public String getName() {
return name;
}
public double getSalary() {
return salary;
}
}
In this traditional approach:
- The class is marked
finalto prevent subclassing. - All fields are
privateandfinal, ensuring they cannot be changed after initialization. - Only getter methods are provided to access the field values.
While this approach works well, it involves writing boilerplate code for constructors, getters, and sometimes equals, hashCode, and toString methods.
Using Lombok to Eliminate Boilerplate
Lombok can drastically reduce the amount of code you need to write. Here's how you can achieve the same functionality with Lombok:
import lombok.AllArgsConstructor;
import lombok.Getter;
@AllArgsConstructor
@Getter
final class Employee {
private final long id;
private final String name;
private final double salary;
}
This version uses Lombok annotations to generate the constructor and getters automatically:
-
@AllArgsConstructorgenerates a constructor with all fields as parameters. -
@Gettergenerates getter methods for each field.
Simplifying Even Further with @Value
Lombok’s @Value annotation is a more powerful alternative that combines multiple features to create an immutable class:
import lombok.Value;
@Value
class Employee {
long id;
String name;
double salary;
}
With @Value, Lombok automatically:
- Makes the class
final. - Makes all fields
privateandfinalby default. - Generates an all-arguments constructor.
- Generates getters for all fields.
- Implements
equals,hashCode, andtoStringmethods.
This reduces your class definition to just the fields, with all the necessary code automatically generated.
Handling Field Updates with @With
Immutable objects do not allow modification of their state. However, in certain cases, you may need to create a modified copy of the object, such as updating an employee's salary. Without Lombok, this could look like:
@Value
class Employee {
long id;
String name;
double salary;
}
class Main {
public static void main(String... args) {
var emp = new Employee(1L, "Aman", 10_000.0);
emp = updateSalary(emp, 12_0000.0);
}
public Employee updateSalary(Employee emp, long newSalary) {
return new Employee(emp.getId(), emp.getName(), newSalary);
}
}
This is straightforward but tedious, especially when dealing with classes that have many fields.
Lombok's @With annotation simplifies this:
import lombok.Value;
import lombok.With;
@Value
class Employee {
long id;
String name;
@With double salary;
}
class Main {
public static void main(String... args) {
var emp = new Employee(1L, "Aman", 10_000.0);
emp = updateSalary(emp, 12_0000.0);
}
public Employee updateSalary(Employee emp, double newSalary) {
return emp.withSalary(newSalary);
}
}
The @With annotation generates a method that returns a new instance of the class with the specified field updated, leaving the rest unchanged.
The Final De-lomboked Version
The de-lomboked version of our Employee class (i.e., what Lombok generates under the hood) would look like this:
final
class Employee {
private final long id;
private final String name;
private final double salary;
public Employee(long id, String name, double salary) {
this.id = id;
this.name = name;
this.salary = salary;
}
public Employee withSalary(double salary) {
return this.salary == salary ? this : new Employee(this.id, this.name, salary);
}
public long getId() {
return this.id;
}
public String getName() {
return this.name;
}
public double getSalary() {
return this.salary;
}
@Override
public boolean equals(final Object o) {
if (o == this) return true;
if (!(o instanceof Employee)) return false;
final Employee other = (Employee) o;
if (this.getId() != other.getId()) return false;
final Object this$name = this.getName();
final Object other$name = other.getName();
if (this$name == null ? other$name != null : !this$name.equals(other$name)) return false;
return Double.compare(this.getSalary(), other.getSalary()) == 0;
}
@Override
public int hashCode() {
final int PRIME = 59;
int result = 1;
final long $id = this.getId();
result = result * PRIME + (int) ($id >>> 32 ^ $id);
final Object $name = this.getName();
result = result * PRIME + ($name == null ? 43 : $name.hashCode());
final long $salary = Double.doubleToLongBits(this.getSalary());
result = result * PRIME + (int) ($salary >>> 32 ^ $salary);
return result;
}
@Override
public String toString() {
return "Employee(id=" + this.getId() + ", name=" + this.getName() + ", salary=" + this.getSalary() + ")";
}
}
Common Pitfalls When Using Lombok for Immutability
While Lombok simplifies creating immutable classes, it's important to note some potential pitfalls:
-
Mutable Collections: If your class contains mutable collections (like
ListorMap), Lombok’s@Valueannotation won’t protect you from modifications to these collections. Consider using immutable versions likeCollections.unmodifiableListorList.of()for initialization. - Complex Objects: If your class fields are complex objects that themselves are mutable, the immutability of the class is compromised. Ensure that all fields are either primitive, immutable objects, or properly encapsulated to prevent mutation.
Performance Considerations
While immutability offers significant benefits, it’s important to consider the performance impact, particularly in scenarios involving frequent updates:
- Memory Usage: Creating new instances every time a change is needed can lead to higher memory usage. However, this trade-off is often worth it for the benefits of thread safety and reduced complexity.
- Garbage Collection: The creation of multiple short-lived objects can put additional pressure on the garbage collector. Ensure that your application’s memory management is optimized for such scenarios.
Conclusion
Lombok's @Value and @With annotations offer a powerful and concise way to create immutable classes in Java, eliminating the need for boilerplate code and making your code more readable and maintainable. By leveraging these annotations, you can focus on the logic of your application rather than the mechanics of class design.
