apply the item hierarchy

Java-Knight/src/main/java/org/project/item/SellableItem.java

@@ -0,0 +1,16 @@
+package org.project.item;
+
+import org.project.entity.Entity;
+
+public class SellableItem extends Item{
+    private final int Price;
+
+    public SellableItem (String name, Entity owner, int price) {
+        super(name, owner);
+        this.Price = price;
+    }
+
+    public int GetPrice() {
+        return Price;
+    }
+}