Mastering Scala requires understanding the fundamental differences between abstract classes and traits. Both concepts share commonalities, but their unique characteristics set them apart. Traits are similar to Java interfaces, offering more flexibility with implementation capabilities. Abstract classes, on the other hand, can contain both abstract and concrete methods. Neither traits nor abstract classes can be instantiated in the main method, and classes that extend either must implement declared methods.

Traits can include implemented methods and variables, while abstract classes can feature both abstract and concrete methods along with variables. However, using a constructor parameter in a trait results in a compilation error, whereas it functions correctly in an abstract class. Scala's multiple inheritance is resolved through trait linearization, allowing for the amalgamation of multiple traits that jointly alter a method.

Scala provides enhanced versatility compared to abstract classes, making traits a suitable choice when possible. In situations where a base class with a constructor parameter is required, abstract classes can be utilized instead.

Source: https://dev.to/jackwilltech/unlock-scala-in-5-minutes-traits-vs-abstract-classes-4lp8