Add the code for article "Kotlin Annotations" (BAEL-2579) (#6540)

core-kotlin-2/src/main/kotlin/com/baeldung/annotations/Annotations.kt

@@ -0,0 +1,7 @@
+package com.baeldung.annotations
+
+@Target(AnnotationTarget.FIELD)
+annotation class Positive
+
+@Target(AnnotationTarget.FIELD)
+annotation class AllowedNames(val names: Array<String>)

core-kotlin-2/src/main/kotlin/com/baeldung/annotations/Item.kt

@@ -0,0 +1,3 @@
+package com.baeldung.annotations
+
+class Item(@Positive val amount: Float, @AllowedNames(["Alice", "Bob"]) val name: String)

core-kotlin-2/src/main/kotlin/com/baeldung/annotations/Main.kt

@@ -0,0 +1,7 @@
+package com.baeldung.annotations
+
+fun main(args: Array<String>) {
+    val item = Item(amount = 1.0f, name = "Bob")
+    val validator = Validator()
+    println("Is instance valid? ${validator.isValid(item)}")
+}

core-kotlin-2/src/main/kotlin/com/baeldung/annotations/Validator.kt

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+package com.baeldung.annotations
+
+/**
+ * Naive annotation-based validator.
+ * @author A.Shcherbakov
+ */
+class Validator() {
+
+    /**
+     * Return true if every item's property annotated with @Positive is positive and if
+     * every item's property annotated with @AllowedNames has a value specified in that annotation.
+     */
+    fun isValid(item: Item): Boolean {
+        val fields = item::class.java.declaredFields
+        for (field in fields) {
+            field.isAccessible = true
+            for (annotation in field.annotations) {
+                val value = field.get(item)
+                if (field.isAnnotationPresent(Positive::class.java)) {
+                    val amount = value as Float
+                    if (amount < 0) {
+                        return false
+                    }
+                }
+                if (field.isAnnotationPresent(AllowedNames::class.java)) {
+                    val allowedNames = field.getAnnotation(AllowedNames::class.java)?.names
+                    val name = value as String
+                    allowedNames?.let {
+                        if (!it.contains(name)) {
+                            return false
+                        }
+                    }
+                }
+            }
+        }
+        return true
+    }
+}

core-kotlin-2/src/test/kotlin/com/baeldung/annotations/ValidationUnitTest.kt

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+package com.baeldung.annotations
+
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Assertions.assertFalse
+import org.junit.jupiter.api.Test
+
+
+class ValidationUnitTest {
+
+    @Test
+    fun whenAmountIsOneAndNameIsAlice_thenTrue() {
+        assertTrue(Validator().isValid(Item(1f, "Alice")))
+    }
+
+    @Test
+    fun whenAmountIsOneAndNameIsBob_thenTrue() {
+        assertTrue(Validator().isValid(Item(1f, "Bob")))
+    }
+
+
+    @Test
+    fun whenAmountIsMinusOneAndNameIsAlice_thenFalse() {
+        assertFalse(Validator().isValid(Item(-1f, "Alice")))
+    }
+
+    @Test
+    fun whenAmountIsMinusOneAndNameIsBob_thenFalse() {
+        assertFalse(Validator().isValid(Item(-1f, "Bob")))
+    }
+
+    @Test
+    fun whenAmountIsOneAndNameIsTom_thenFalse() {
+        assertFalse(Validator().isValid(Item(1f, "Tom")))
+    }
+
+    @Test
+    fun whenAmountIsMinusOneAndNameIsTom_thenFalse() {
+        assertFalse(Validator().isValid(Item(-1f, "Tom")))
+    }
+
+
+}