§Using immutable objects

An immutable object is an object that cannot be modified after it was created.

Immutable objects have two great advantages:

• Code based on immutable objects is clearer and likelier to be correct. Bugs involving unexpected changes simply can’t occur.
• Multiple threads can safely access immutable objects concurrently.

In Lagom, immutable objects are required in several places, such as:

• Service request and response types
• Persistent entity commands, events, and states
• Publish and subscribe messages

§Mutable vs. immutable

Here is an example of a mutable object:

class MutableUser {
  private var name: String  = null
  private var email: String = null

  def getName: String = name

  def setName(name: String): Unit =
    this.name = name

  def getEmail: String = email

  def setEmail(email: String): Unit =
    this.email = email
}

The setter methods can be used to modify the object after construction.

Here, by contrast, is an immutable object:

final case class ImmutableUser(name: String, email: String)

All fields are final and are assigned at construction time. There are no setter methods.

As you can see, Scala’s case class is very convenient for writing immutable classes.

Note that contained members of an immutable class must also be immutable. Scala has nice immutable collections that you should use in your immutable classes. Be aware of that scala.collection.Seq is not guaranteed to be immutable. Instead you should use scala.collection.immutable.Seq or concrete immutable implementations such as List or Vector.