add checks in order to make sure negative coins don't occur

add configurable price
use persistent data containers to keep track of items

src/main/java/dev/plex/shop/item/AbstractItem.kt

@@ -1,18 +1,67 @@
 package dev.plex.shop.item
 
 import dev.plex.cache.DataUtils
+import dev.plex.listener.PlexListener
 import dev.plex.player.PlexPlayer
+import dev.plex.util.PlexLog
+import dev.plex.util.minimessage.SafeMiniMessage
+import org.bukkit.Material
+import org.bukkit.NamespacedKey
+import org.bukkit.event.EventHandler
+import org.bukkit.event.player.PlayerInteractEvent
+import org.bukkit.inventory.EquipmentSlot
 import org.bukkit.inventory.ItemStack
+import org.bukkit.inventory.meta.ItemMeta
+import org.bukkit.persistence.PersistentDataContainer
 import java.util.concurrent.CompletableFuture
 
-abstract class AbstractItem(val item: ItemStack, private val cost: Double)
+abstract class AbstractItem(val item: ItemStack, val cost: Double, val itemTag: NamespacedKey) : PlexListener()
 {
     fun purchase(player: PlexPlayer)
     {
+        if (player.coins < this.cost) {
+            player.player?.sendMessage(SafeMiniMessage.mmDeserialize("<red>You need ${this.cost - player.coins} more coins to purchase this!"))
+            return
+        }
+        if (player.player?.inventory!!.filter { it?.type != Material.AIR }.size == 53)
+        {
+            player.player?.sendMessage(SafeMiniMessage.mmDeserialize("<red>Your inventory is currently full!"))
+            return
+        }
         player.coins.minus(this.cost);
         player.player?.inventory?.addItem(this.item);
         CompletableFuture.runAsync {
             DataUtils.update(player);
         }
     }
+
+    abstract fun interact(event: PlayerInteractEvent)
+
+    @EventHandler
+    fun onInteract(event: PlayerInteractEvent)
+    {
+        if (event.hand != EquipmentSlot.HAND)
+        {
+            return
+        }
+
+        if (event.item == null)
+        {
+            return
+        }
+
+        if (!event.item!!.hasItemMeta())
+        {
+            return
+        }
+
+        val meta: ItemMeta = event.item!!.itemMeta
+        val dataContainer: PersistentDataContainer = meta.persistentDataContainer
+        if (!dataContainer.has(itemTag))
+        {
+            return
+        }
+
+        interact(event)
+    }
 }

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

@@ -1,8 +1,20 @@
 package dev.plex.shop.item.impl
 
+import dev.plex.Plex
 import dev.plex.shop.item.AbstractItem
+import org.bukkit.NamespacedKey
+import org.bukkit.entity.Fireball
+import org.bukkit.event.player.PlayerInteractEvent
 import org.bukkit.inventory.ItemStack
 
-class FireballItem(item: ItemStack, cost: Double) : AbstractItem(item, cost)
+class FireballItem(item: ItemStack, cost: Double) : AbstractItem(item, cost, ITEM_TAG)
 {
+    companion object {
+        val ITEM_TAG: NamespacedKey = NamespacedKey(Plex.get(), "fireball_item")
+    }
+
+    override fun interact(event: PlayerInteractEvent)
+    {
+        event.player.launchProjectile(Fireball::class.java)
+    }
 }