[enum-extension-fun] create a function for all enum classes (#769)

Author: sk1418

core-kotlin-modules/core-kotlin-lang-oop-3/src/test/kotlin/com/baeldung/extensionOfEnum/EnumExtensionFunctionsUnitTest.kt

@@ -0,0 +1,59 @@
+package com.baeldung.extensionOfEnum
+
+import org.junit.jupiter.api.Test
+import kotlin.enums.EnumEntries
+import kotlin.test.assertEquals
+
+enum class Level {
+    A, B, C, D, E
+}
+
+enum class WorkingDay(val n: Int) {
+    MON(1), TUE(2), WED(3), THU(4), FRI(5);
+}
+
+
+class EnumExtensionFunctionsUnitTest {
+
+    @Test
+    fun `when extending enum Array, then get expected result`() {
+        fun <E : Enum<E>> Array<E>.joinTheirNames(): String {
+            return joinToString { it.name }
+        }
+
+        assertEquals("A, B, C, D, E", Level.values().joinTheirNames())
+        assertEquals("MON, TUE, WED, THU, FRI", WorkingDay.values().joinTheirNames())
+    }
+
+    @Test
+    fun `when extending EnumEntries, then get expected result`() {
+        fun EnumEntries<*>.joinTheirNames(): String {
+            return joinToString { it.name }
+        }
+
+        assertEquals("A, B, C, D, E", Level.entries.joinTheirNames())
+        assertEquals("MON, TUE, WED, THU, FRI", WorkingDay.entries.joinTheirNames())
+    }
+
+
+    inline fun <reified E : Enum<E>> Enum.Companion.joinTheirNames(): String {
+        return enumValues<E>().joinToString { it.name }
+    }
+
+    @Test
+    fun `when extending Enum's Companion object, then get expected result`() {
+        assertEquals("A, B, C, D, E", Enum.joinTheirNames<Level>())
+        assertEquals("MON, TUE, WED, THU, FRI", Enum.joinTheirNames<WorkingDay>())
+    }
+
+    inline fun <reified E : Enum<E>> joinEnumNames(): String {
+        return enumValues<E>().joinToString { it.name }
+    }
+
+    @Test
+    fun `when using a generic function, then get expected result`() {
+        assertEquals("A, B, C, D, E", joinEnumNames<Level>())
+        assertEquals("MON, TUE, WED, THU, FRI", joinEnumNames<WorkingDay>())
+    }
+
+}