such a kotlin moment

setup shop command
setup shop listener
setup item registration

src/main/java/dev/plex/shop/ShopMenu.kt

@@ -1,9 +1,12 @@
 package dev.plex.shop
 
+import com.google.common.collect.Maps
 import dev.plex.player.PlexPlayer
 import dev.plex.shop.item.AbstractItem
+import dev.plex.util.item.ItemBuilder
 import dev.plex.util.minimessage.SafeMiniMessage
 import org.bukkit.Bukkit
+import org.bukkit.Material
 import org.bukkit.entity.Player
 import org.bukkit.inventory.Inventory
 
@@ -13,22 +16,35 @@ import org.bukkit.inventory.Inventory
  * @since 11:21 PM [10-06-2022]
  *
  */
-class ShopMenu {
-    companion object {
-        val ITEMS = mapOf<Int, AbstractItem>()
-    }
-
-    fun open(plexPlayer: PlexPlayer)
+class ShopMenu
+{
+    companion object
     {
-        val player: Player? = plexPlayer.player
-        val inventory: Inventory = constructInventory()
-        player?.openInventory(inventory)
-    }
+        val ITEMS: HashMap<Int, AbstractItem> = Maps.newHashMap()
+        fun registerItem(index: Int, item: AbstractItem)
+        {
+            ITEMS.put(index, item);
+        }
 
-    private fun constructInventory(): Inventory
-    {
-        val inventory: Inventory = Bukkit.createInventory(null, 54, SafeMiniMessage.mmDeserialize("<gold>Shop"))
-        ITEMS.forEach { (t, u) ->  inventory.setItem(t, u.item) }
-        return inventory
+        fun open(plexPlayer: PlexPlayer)
+        {
+            val player: Player? = plexPlayer.player
+            val inventory: Inventory = constructInventory()
+            player?.openInventory(inventory)
+        }
+
+        private fun constructInventory(): Inventory
+        {
+            val inventory: Inventory = Bukkit.createInventory(null, 54, SafeMiniMessage.mmDeserialize("<gold>Shop"))
+            ITEMS.forEach { (t, u) -> inventory.setItem(t, u.item) }
+            for (i in 0 until inventory.size)
+            {
+                if (inventory.getItem(i) == null || inventory.getItem(i)?.type == Material.AIR)
+                {
+                    inventory.setItem(i, ItemBuilder(Material.GRAY_STAINED_GLASS_PANE).displayName(SafeMiniMessage.mmDeserialize("<!italic> ")).build())
+                }
+            }
+            return inventory
+        }
     }
 }

src/main/java/dev/plex/shop/item/impl/FireballItem.kt

@@ -0,0 +1,8 @@
+package dev.plex.shop.item.impl
+
+import dev.plex.shop.item.AbstractItem
+import org.bukkit.inventory.ItemStack
+
+class FireballItem(item: ItemStack, cost: Double) : AbstractItem(item, cost)
+{
+}