SCALA-282: Merge two maps in Scala (#503)

Author: thomasduxbury

scala-core-collections/src/main/scala/com/baeldung/scala/mergemaps/CombineIterables.scala

@@ -0,0 +1,12 @@
+package com.baeldung.scala.mergemaps
+
+object CombineIterables {
+
+  def combineIterables[K, V](
+    a: Map[K, Iterable[V]],
+    b: Map[K, Iterable[V]]
+  ): Map[K, Iterable[V]] = {
+    a ++ b.map { case (k, v) => k -> (v ++ a.getOrElse(k, Iterable.empty)) }
+  }
+
+}

scala-core-collections/src/test/scala/com/baedung/scala/mergemaps/MergeMapsUnitTest.scala

@@ -0,0 +1,25 @@
+package com.baedung.scala.mergemaps
+
+import com.baeldung.scala.mergemaps.CombineIterables
+import org.scalatest.{FlatSpec, Matchers}
+
+class MergeMapsUnitTest extends FlatSpec with Matchers {
+
+  "++ operator" should "merge maps preferring duplicates from second map" in {
+    val firstMap = Map(1 -> "Apple", 5 -> "Banana", 3 -> "Orange");
+    val secondMap = Map(5 -> "Pear", 4 -> "Cherry");
+
+    firstMap ++ secondMap should equal(
+      Map(1 -> "Apple", 5 -> "Pear", 3 -> "Orange", 4 -> "Cherry")
+    )
+  }
+
+  "combineIterables" should "combine values for duplicate keys" in {
+    val firstMap = Map(1 -> List(1, 2, 3))
+    val secondMap = Map(1 -> List(4, 5))
+
+    CombineIterables.combineIterables(firstMap, secondMap) should equal(
+      Map(1 -> List(4, 5, 1, 2, 3))
+    )
+  }
+}