add tests to ensure current behavior

Java/src/test/java/com/gildedrose/GildedRoseTest.java

@@ -6,12 +6,119 @@ import static org.junit.jupiter.api.Assertions.assertEquals;
 
 class GildedRoseTest {
 
+    // Once the sell by date has passed, Quality degrades twice as fast
     @Test
-    void foo() {
-        Item[] items = new Item[] { new Item("foo", 0, 0) };
-        GildedRose app = new GildedRose(items);
-        app.updateQuality();
-        assertEquals("fixme", app.items[0].name);
+    void testUpdateQualityDegradesTwiceAsFast() {
+        var normalItem = new Item("Other Item", 0, 10);
+        GildedRose gildedRose = new GildedRose(new Item[]{normalItem});
+
+        gildedRose.updateQuality();
+
+        assertEquals(8, normalItem.quality);
+        assertEquals(-1, normalItem.sellIn);
+    }
+
+    // The Quality of an item is never negative
+    @Test
+    void testUpdateQualityDoesNotGetNegative() {
+        var normalItem = new Item("Other Item", 10, 0);
+        GildedRose gildedRose = new GildedRose(new Item[]{normalItem});
+
+        gildedRose.updateQuality();
+
+        assertEquals(0, normalItem.quality);
+        assertEquals(9, normalItem.sellIn);
+    }
+
+    // "Aged Brie" actually increases in Quality the older it gets
+    @Test
+    void testUpdateQualityIncreasesForAgedBrie() {
+        var agedBrie = new Item("Aged Brie", 10, 20);
+        GildedRose gildedRose = new GildedRose(new Item[]{agedBrie});
+
+        gildedRose.updateQuality();
+
+        assertEquals(21, agedBrie.quality);
+        assertEquals(9, agedBrie.sellIn);
+    }
+
+    // The Quality of an item is never more than 50
+    @Test
+    void testUpdateQualityDoesNotIncreaseMoreThan50() {
+        var agedBrie = new Item("Aged Brie", 10, 50);
+        GildedRose gildedRose = new GildedRose(new Item[]{agedBrie});
+
+        gildedRose.updateQuality();
+
+        assertEquals(50, agedBrie.quality);
+        assertEquals(9, agedBrie.sellIn);
+    }
+
+    // "Sulfuras", being a legendary item, never has to be sold or decreases in Quality
+    @Test
+    void testUpdateQualityForSulfurasWithSellIn() {
+        var sulfuras = new Item("Sulfuras, Hand of Ragnaros", 5, 80);
+        GildedRose gildedRose = new GildedRose(new Item[]{sulfuras});
+
+        gildedRose.updateQuality();
+
+        assertEquals(80, sulfuras.quality); // Sulfuras quality should never change
+        assertEquals(5, sulfuras.sellIn);
     }
 
+    // Normal behaviour if more than 10 days to sell
+    @Test
+    void testUpdateQualityForBackstagePassesWithSellInMoreThan10() {
+        var backstagePasses = new Item("Backstage passes to a TAFKAL80ETC concert", 15, 20);
+        GildedRose gildedRose = new GildedRose(new Item[]{backstagePasses});
+
+        gildedRose.updateQuality();
+
+        assertEquals(21, backstagePasses.quality);
+        assertEquals(14, backstagePasses.sellIn);
+    }
+    // Quality increases by 2 when there are 10 days or less
+    @Test
+    void testUpdateQualityForBackstagePassesWithSellInLessThan10() {
+        var backstagePasses = new Item("Backstage passes to a TAFKAL80ETC concert", 10, 20);
+        GildedRose gildedRose = new GildedRose(new Item[]{backstagePasses});
+
+        gildedRose.updateQuality();
+
+        assertEquals(22, backstagePasses.quality);
+        assertEquals(9, backstagePasses.sellIn);
+    }
+    // Quality increases by 3 when there are 5 days or less
+    @Test
+    void testUpdateQualityForBackstagePassesWithSellInLessThan5() {
+        var backstagePasses = new Item("Backstage passes to a TAFKAL80ETC concert", 5, 20);
+        GildedRose gildedRose = new GildedRose(new Item[]{backstagePasses});
+
+        gildedRose.updateQuality();
+
+        assertEquals(23, backstagePasses.quality);
+        assertEquals(4, backstagePasses.sellIn);
+    }
+    // Quality drops to 0 after the concert
+    @Test
+    void testUpdateQualityForBackstagePassesWithSellInLessThan0() {
+        var backstagePasses = new Item("Backstage passes to a TAFKAL80ETC concert", 0, 20);
+        GildedRose gildedRose = new GildedRose(new Item[]{backstagePasses});
+
+        gildedRose.updateQuality();
+
+        assertEquals(0, backstagePasses.quality);
+        assertEquals(-1, backstagePasses.sellIn);
+    }
+
+    @Test
+    void testUpdateQualityForOtherItem() {
+        var normalItem = new Item("Other Item", 10, 10);
+        GildedRose gildedRose = new GildedRose(new Item[]{normalItem});
+
+        gildedRose.updateQuality();
+
+        assertEquals(9, normalItem.quality);
+        assertEquals(9, normalItem.sellIn);
+    }
 }