Add test for DefaultSellInBehavior

Java/src/test/java/com/gildedrose/behavior/sellin/DefaultSellInBehaviorTest.java

@@ -0,0 +1,45 @@
+package com.gildedrose.behavior.sellin;
+
+import com.gildedrose.Item;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class DefaultSellInBehaviorTest {
+
+    private SellInBehavior sellInBehavior;
+
+    @BeforeEach
+    public void setUp() throws Exception {
+        sellInBehavior = new DefaultSellInBehavior();
+    }
+
+    @Test
+    void decreaseSellIn() {
+        Item item = getItem(10);
+        sellInBehavior.processSellInUpdate(item);
+
+        assertEquals(9, item.sellIn);
+    }
+
+    @Test
+    void decreaseSellInZero() {
+        Item item = getItem(0);
+        sellInBehavior.processSellInUpdate(item);
+
+        assertEquals(-1, item.sellIn);
+    }
+
+    @Test
+    void decreaseNegativeSellIn() {
+        Item item = getItem(-1);
+        sellInBehavior.processSellInUpdate(item);
+
+        assertEquals(-2, item.sellIn);
+    }
+
+    private Item getItem(int sellIn) {
+        return new Item("SomeItem", sellIn, 0);
+    }
+}