Add class composition with mixins examples

Author: SergeiIonin

core-scala/src/main/scala/com/baeldung/scala/classcompositionwithtraits/CarTraits.scala

@@ -0,0 +1,67 @@
+package com.baeldung.scala.classcompositionwithtraits
+
+import com.baeldung.scala.classcompositionwithtraits.CarTraits.{Car, Printable}
+
+object CarTraits {
+
+  /**
+   * There are a few examples of traits to describe an entity of a Car below
+   * @author Sergey Ionin
+   */
+
+  /**
+   * Trait with abstract members and one method
+   */
+  trait Car {
+    def model: String
+
+    def horsePower: Int
+  }
+
+
+  /**
+   * Another trait to be mixed into the class
+   */
+  trait Marshaller {
+    self: Car =>
+    def toJson = s"{${"\"model\""}:$model," +
+      s"\n${"\"horsePower\""}:$horsePower}"
+  }
+
+  /**
+   * The traits below will show that the matter of mixing matters (see the tests)
+   */
+  trait Printable {
+    self: Car =>
+    def print(appendix: String = ""): String =
+      s"the model $model has $horsePower HP under the hood. $appendix"
+  }
+
+  trait PrettyPrintable extends Printable {
+    self: Car =>
+    override def print(appendix: String = "You'll definitelly enjoy driving!") = super.print(appendix)
+  }
+
+  trait ShortPrintable extends Printable {
+    self: Car =>
+    override def print(appendix: String) = if (appendix.length() > 10) super.print() else
+      super.print(appendix)
+  }
+
+  // Implementations
+
+  class BMW0(val model: String, val horsePower: Int) extends Car
+
+  class BMWPrintable(val model: String, val horsePower: Int) extends Car with Printable {
+    def printInfo() = print()
+  }
+
+  object BMW {
+
+    final case class F15(model: String = "F15", horsePower: Int = 309) extends Car with Printable
+
+    final case class X7(model: String = "X7", horsePower: Int = 335) extends Car with Printable
+
+  }
+
+}

core-scala/src/test/scala/com/baeldung/scala/classcompositionwithtraits/TraitsTest.scala

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+package com.baeldung.scala.classcompositionwithtraits
+
+import org.scalatest.{Matchers, WordSpec}
+
+/**
+ * @author Sergey Ionin
+ */
+
+class TraitsTest extends WordSpec with Matchers {
+
+  import com.baeldung.scala.classcompositionwithtraits.CarTraits._
+
+  "Class that extends Car" should {
+    "inherit abstract class fields" in {
+      val bmw = new BMW0("F15", 309) with Printable
+      bmw.horsePower shouldBe 309
+      bmw.model shouldBe "F15"
+      bmw.print() shouldBe s"the model ${bmw.model} has ${bmw.horsePower} HP under the hood. "
+    }
+  }
+
+  "Objects that extends Car" should {
+    "inherit abstract class fields and methods" in {
+      val bmwX7 = BMW.X7()
+      bmwX7.horsePower shouldBe 335
+      bmwX7.model shouldBe "X7"
+      bmwX7.print() shouldBe s"the model ${bmwX7.model} has ${bmwX7.horsePower} HP under the hood. "
+    }
+  }
+
+  "Classes that extends Car with Marshaller" should {
+    "inherit abstract class fields and methods" +
+      "and be marshallable" in {
+      val bmw0 = new BMW0("F15", 309) with Marshaller
+      bmw0.toJson shouldBe "{\"model\":F15,\n\"horsePower\":309}"
+    }
+  }
+
+  // in this case the print method pf the rightmost trait calls the print method of the previously mixed trait (
+  // see super.print... in the implementation)
+  "Classes that extends Car with PrettyPrintable with ShortPrintable" should {
+    "behave differently depending from the mixing order" in {
+      val bmwPrintable1 = new BMWPrintable("F15", 309) with PrettyPrintable with ShortPrintable
+      val bmwPrintable2 = new BMWPrintable("F15", 309) with ShortPrintable with PrettyPrintable
+      bmwPrintable1.printInfo() shouldBe s"the model ${bmwPrintable1.model} has ${bmwPrintable1.horsePower}" +
+        s" HP under the hood. You'll definitelly enjoy driving!"
+      bmwPrintable2.printInfo() shouldBe s"the model ${bmwPrintable1.model} has ${bmwPrintable1.horsePower} " +
+        s"HP under the hood. "
+    }
+  }
+
+}