Delegated Properties in Kotlin (#2357)

This produces a fake Database representation - working in terms of
hardcoded data, a Delegate that loads values from this fake Database,
and then a class with fields delegated to the fake database.

kotlin/src/main/kotlin/com/baeldung/kotlin/delegates/Database.kt

@@ -0,0 +1,35 @@
+package com.baeldung.kotlin.delegates
+
+val data = arrayOf<MutableMap<String, Any?>>(
+        mutableMapOf(
+                "id" to 1,
+                "name" to "George",
+                "age" to 4
+        ),
+        mutableMapOf(
+                "id" to 2,
+                "name" to "Charlotte",
+                "age" to 2
+        )
+)
+
+class NoRecordFoundException(id: Int) : Exception("No record found for id $id") {
+    init {
+        println("No record found for ID $id")
+    }
+}
+
+fun queryForValue(field: String, id: Int): Any {
+    println("Loading record $id from the fake database")
+    val value = data.firstOrNull { it["id"] == id }
+            ?.get(field) ?: throw NoRecordFoundException(id)
+    println("Loaded value $value for field $field of record $id")
+    return value
+}
+
+fun update(field: String, id: Int, value: Any?) {
+    println("Updating field $field of record $id to value $value in the fake database")
+    data.firstOrNull { it["id"] == id }
+            ?.put(field, value)
+            ?: throw NoRecordFoundException(id)
+}

kotlin/src/main/kotlin/com/baeldung/kotlin/delegates/DatabaseDelegate.kt

@@ -0,0 +1,13 @@
+package com.baeldung.kotlin.delegates
+
+import kotlin.properties.ReadWriteProperty
+import kotlin.reflect.KProperty
+
+class DatabaseDelegate<in R, T>(private val field: String, private val id: Int) : ReadWriteProperty<R, T> {
+    override fun getValue(thisRef: R, property: KProperty<*>): T =
+            queryForValue(field, id) as T
+
+    override fun setValue(thisRef: R, property: KProperty<*>, value: T) {
+        update(field, id, value)
+    }
+}

kotlin/src/main/kotlin/com/baeldung/kotlin/delegates/User.kt

@@ -0,0 +1,6 @@
+package com.baeldung.kotlin.delegates
+
+class User(val id: Int) {
+    var name: String by DatabaseDelegate("name", id)
+    var age: Int by DatabaseDelegate("age", id)
+}

kotlin/src/test/kotlin/com/baeldung/kotlin/delegates/DatabaseDelegatesTest.kt

@@ -0,0 +1,26 @@
+package com.baeldung.kotlin.delegates
+
+import org.junit.Test
+import kotlin.test.assertEquals
+
+class DatabaseDelegatesTest {
+    @Test
+    fun testGetKnownFields() {
+        val user = User(1)
+        assertEquals("George", user.name)
+        assertEquals(4, user.age)
+    }
+
+    @Test
+    fun testSetKnownFields() {
+        val user = User(2)
+        user.age = 3
+        assertEquals(3, user.age)
+    }
+
+    @Test(expected = NoRecordFoundException::class)
+    fun testGetKnownField() {
+        val user = User(3)
+        user.name
+    }
+}