[impr-flatten-vs-flatmap] add flatten()

Author: sk1418

core-kotlin-modules/core-kotlin-collections-2/src/test/kotlin/com/baeldung/mapvsflatmap/MapVsFlatMapUnitTest.kt

@@ -0,0 +1,80 @@
+package com.baeldung.mapvsflatmapvsflatten
+
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class MapVsFlatMapVsFlattenUnitTest {
+
+    @Test
+    fun `map should convert each element to another element`() {
+        val order = Order(
+            listOf(OrderLine("Tomato", 2), OrderLine("Garlic", 3), OrderLine("Chives", 2))
+        )
+
+        val names = order.lines.map { it.name }
+        val totalPrice = order.lines.map { it.price }.sum()
+
+        assertThat(names).containsExactly("Tomato", "Garlic", "Chives")
+        assertEquals(7, totalPrice)
+    }
+
+    @Test
+    fun `flatMap should flatten the one-to-many relation as expected`() {
+        val orders = listOf(
+            Order(listOf(OrderLine("Garlic", 1), OrderLine("Chives", 2))),
+            Order(listOf(OrderLine("Tomato", 3), OrderLine("Garlic", 4))),
+            Order(listOf(OrderLine("Potato", 5), OrderLine("Chives", 6))),
+        )
+
+        val lines: List<OrderLine> = orders.flatMap { it.lines }
+        val names = lines.map { it.name }.distinct()
+        assertThat(names).containsExactlyInAnyOrder("Garlic", "Chives", "Tomato", "Potato")
+    }
+
+    @Test
+    fun `flatten should flatten the nested collections without transformation`() {
+        val orderLines = listOf(
+            listOf(OrderLine("Garlic", 1), OrderLine("Chives", 2)),
+            listOf(OrderLine("Tomato", 3), OrderLine("Garlic", 4)),
+            listOf(OrderLine("Potato", 5), OrderLine("Chives", 6)),
+        )
+
+        val lines: List<OrderLine> = orderLines.flatten()
+        val expected = listOf(
+            OrderLine("Garlic", 1),
+            OrderLine("Chives", 2),
+            OrderLine("Tomato", 3),
+            OrderLine("Garlic", 4),
+            OrderLine("Potato", 5),
+            OrderLine("Chives", 6),
+        )
+        assertThat(lines).hasSize(6).isEqualTo(expected)
+    }
+
+    @Test
+    fun `flatMap should get same result of map and then flatten`() {
+        val orders = listOf(
+            Order(listOf(OrderLine("Garlic", 1), OrderLine("Chives", 2))),
+            Order(listOf(OrderLine("Tomato", 3), OrderLine("Garlic", 4))),
+            Order(listOf(OrderLine("Potato", 5), OrderLine("Chives", 6))),
+        )
+
+        val expected = listOf(
+            OrderLine("Garlic", 1),
+            OrderLine("Chives", 2),
+            OrderLine("Tomato", 3),
+            OrderLine("Garlic", 4),
+            OrderLine("Potato", 5),
+            OrderLine("Chives", 6),
+        )
+
+        val resultMapAndFlatten: List<OrderLine> = orders.map { it.lines }.flatten()
+        val resultFlatMap:List<OrderLine> = orders.flatMap { it.lines }
+
+        assertThat(resultFlatMap).isEqualTo(resultMapAndFlatten).hasSize(6).isEqualTo(expected)
+    }
+}
+
+data class Order(val lines: List<OrderLine>)
+data class OrderLine(val name: String, val price: Int)