Traits

In simple words#

Learning objectives#

• Define traits with abstract and concrete methods
• Mix in multiple traits
• Understand stackable traits
• Choose traits vs abstract classes

Explanation#

A trait can declare abstract methods and provide concrete ones.

class A extends B with C with D mixes in several traits.

Traits can call super to stack behaviour; the rightmost trait runs first (linearization).

Scala 3 traits can take parameters; sealed traits define closed hierarchies.

Code example: Mixing notification behaviours#

1trait Notifier:
2  def send(to: String, msg: String): String
3

4trait Logging extends Notifier:
5  abstract override def send(to: String, msg: String): String =
6    val result = super.send(to, msg)
7    println(s"[log] $result")
8    result
9

10trait Uppercase extends Notifier:
11  abstract override def send(to: String, msg: String): String =
12    super.send(to, msg.toUpperCase)
13

14class SmsNotifier extends Notifier:
15  def send(to: String, msg: String): String = s"SMS to $to: $msg"
16

17val n = new SmsNotifier with Logging with Uppercase
18n.send("+91 98xxxxxx01", "order shipped")

[log] SMS to +91 98xxxxxx01: ORDER SHIPPED

This example is illustrative. Copy or download it and run it in your own environment; the expected output is shown above.

Step-by-step explanation#

1. Notifier declares the contract.
2. Logging and Uppercase wrap send with super calls.
3. Uppercase (rightmost) runs first, then Logging.
4. Behaviour is composed without changing SmsNotifier.

Real-world use case#

Common mistakes#

Best practices#

• Small focused traits
• sealed traits for ADTs
• Prefer composition
• Document stacking order

Remember this#

Practice exercise#

1trait Greeter:
2  def greet(name: String) = s"Namaste, $name"
3

4class Shop extends Greeter
5Shop().greet("Asha")

Mini quiz#

Want more? Practise all Scala MCQs · Scala interview questions

Related topics#

My notes#

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