Similar to interfaces in Java, traits are used to define object types by specifying the signature of the supported methods. Like in Java 8, Scala allows traits to be partially implemented; i.e. it is possible to define default implementations for some methods. In contrast to classes, traits may not have constructor parameters. Here is an example:

trait Similarity {
  def isSimilar(x: Any): Boolean
  def isNotSimilar(x: Any): Boolean = !isSimilar(x)

This trait consists of two methods isSimilar and isNotSimilar. While isSimilar does not provide a concrete method implementation (it is abstract in the terminology of Java), method isNotSimilar defines a concrete implementation. Consequently, classes that integrate this trait only have to provide a concrete implementation for isSimilar. The behavior for isNotSimilar gets inherited directly from the trait. Traits are typically integrated into a class (or other traits) with a mixin class composition:

class Point(xc: Int, yc: Int) extends Similarity {
  var x: Int = xc
  var y: Int = yc
  def isSimilar(obj: Any) =
    obj.isInstanceOf[Point] &&
    obj.asInstanceOf[Point].x == x
object TraitsTest extends App {
  val p1 = new Point(2, 3)
  val p2 = new Point(2, 4)
  val p3 = new Point(3, 3)
  val p4 = new Point(2, 3)
  // Point's isNotSimilar is defined in Similarity

Here is the output of the program:

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