diff --git a/Kotlin/src/main/kotlin/com/gildedrose/GildedRose.kt b/Kotlin/src/main/kotlin/com/gildedrose/GildedRose.kt
index 077867b6..35cf583d 100644
--- a/Kotlin/src/main/kotlin/com/gildedrose/GildedRose.kt
+++ b/Kotlin/src/main/kotlin/com/gildedrose/GildedRose.kt
@@ -1,6 +1,6 @@
 package com.gildedrose
 
-class GildedRose(var items: Array<Item>) {
+class GildedRose(val items: Array<Item>) {
 
     fun updateQuality() {
         for (i in items.indices) {
diff --git a/Kotlin/src/test/kotlin/com/gildedrose/GildedRoseTest.kt b/Kotlin/src/test/kotlin/com/gildedrose/GildedRoseTest.kt
index 24ae793e..334632cd 100644
--- a/Kotlin/src/test/kotlin/com/gildedrose/GildedRoseTest.kt
+++ b/Kotlin/src/test/kotlin/com/gildedrose/GildedRoseTest.kt
@@ -1,10 +1,23 @@
 package com.gildedrose
 
-import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.*
 import org.junit.jupiter.api.Test
 
 internal class GildedRoseTest {
 
+    val items = arrayOf(
+        Item("+5 Dexterity Vest", 10, 20), //
+        Item("Aged Brie", 2, 0), //
+        Item("Elixir of the Mongoose", 5, 7), //
+        Item("Sulfuras, Hand of Ragnaros", 0, 80), //
+        Item("Sulfuras, Hand of Ragnaros", -1, 80),
+        Item("Backstage passes to a TAFKAL80ETC concert", 15, 20),
+        Item("Backstage passes to a TAFKAL80ETC concert", 10, 49),
+        Item("Backstage passes to a TAFKAL80ETC concert", 5, 49),
+        // this conjured item does not work properly yet
+        Item("Conjured Mana Cake", 3, 6)
+    )
+
     @Test
     fun foo() {
         val items = arrayOf<Item>(Item("foo", 0, 0))
@@ -14,6 +27,37 @@ internal class GildedRoseTest {
 
     }
 
+    @Test
+    fun allItemsHaveASellInValue() {
+        items.forEach {
+            assertNotNull(it.sellIn)
+        }
+
+    }
+
+    private val gildedRose = GildedRose(items)
+
+    @Test
+    fun qualityIsNeverNegative() {
+        (1..100).forEach {
+            gildedRose.updateQuality()
+            items.forEach {
+                assertTrue(it.quality >= 0, "The quality of an item is never negative")
+            }
+        }
+    }
+
+    @Test
+    fun agedBrieIncreasesInQuality() {
+        items.first { it.name == "Aged Brie" }.also { agedBrie ->
+            while (agedBrie.quality < 50) {
+                val quality = agedBrie.quality
+                gildedRose.updateQuality()
+                assertTrue(agedBrie.quality > quality)
+            }
+        }
+    }
+
 }