Merge pull request #818 from sk1418/private-constructor

[private-constructor] private constructor

Author: theangrydev

core-kotlin-modules/core-kotlin-lang-4/src/test/kotlin/com/baeldung/privateConstructor/PrivateConstructorUnitTest.kt

@@ -0,0 +1,49 @@
+package com.baeldung.privateConstructor
+
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class Student private constructor(val name: String, val age: Int) {
+
+    companion object {
+        fun createInstance(pair: Pair<String, Int>): Student {
+            return Student(pair.first.uppercase(), pair.second)
+        }
+    }
+}
+
+data class StudentData private constructor(val name: String, val age: Int) {
+
+    companion object {
+        fun createInstance(pair: Pair<String, Int>): StudentData {
+            return StudentData(pair.first.uppercase(), pair.second)
+        }
+    }
+}
+
+class PrivateConstructorUnitTest {
+    @Test
+    fun `when using private constructor then instance can only be created via companion function`() {
+        // val kai = Student("Kai", 18)
+        // Kotlin: Cannot access '<init>': it is private in 'Student'
+
+        val kai = Student.createInstance("Kai" to 18)
+
+        assertEquals("KAI", kai.name)
+        assertEquals(18, kai.age)
+    }
+
+    @Test
+    fun `when using private constructor in data class then copy() exposes the private constructor`() {
+        // val kaiData = StudentData("Kai", 18)
+        // Kotlin: Cannot access '<init>': it is private in 'StudentData'
+
+        val kaiData = StudentData.createInstance("Kai" to 18)
+        assertEquals("KAI", kaiData.name)
+        assertEquals(18, kaiData.age)
+
+        val liam = kaiData.copy(name = "Liam", age = 20)
+        assertEquals("Liam", liam.name)
+        assertEquals(20, liam.age)
+    }
+}