Implement the check part of a transaction

plugin/src/main/java/de/epiceric/shopchest/transaction/Actor.java

@@ -1,4 +1,39 @@
 package de.epiceric.shopchest.transaction;
 
+import org.bukkit.inventory.ItemStack;
+
 public interface Actor {
+
+    /**
+     * Check if this {@link Actor} have a specified amount of money
+     *
+     * @param moneyAmount The money amount required
+     * @return Whether the {@link Actor} have the money
+     */
+    boolean hasMoney(double moneyAmount);
+
+    /**
+     * Check if this {@link Actor} have a specified amount of an item
+     *
+     * @param itemStack The {@link ItemStack} that represent the product
+     * @param amount    The amount to check
+     * @return Whether the {@link Actor} have the amount of this item stack or more
+     */
+    boolean hasProductAmount(ItemStack itemStack, int amount);
+
+    /**
+     * Check if this {@link Actor} have enough inventory space to accept this item stack with this amount
+     *
+     * @param itemStack The {@link ItemStack} that represent the product
+     * @param amount    The amount to check
+     * @return Whether the {@link Actor} have the space to accept this amount of this item stack in its inventory
+     */
+    boolean hasEnoughInventorySpace(ItemStack itemStack, int amount);
+
+    /**
+     * Send a message to this {@link Actor}
+     *
+     * @param message The message to send
+     */
+    void sendMessage(String message);
 }

plugin/src/main/java/de/epiceric/shopchest/transaction/Transaction.java

@@ -1,19 +1,26 @@
 package de.epiceric.shopchest.transaction;
 
+import de.epiceric.shopchest.language.LanguageUtils;
+import de.epiceric.shopchest.language.Message;
 import org.bukkit.inventory.ItemStack;
 
 public class Transaction {
 
+    // A direct transaction is initiated by the buyer
+    // A indirect transaction is initiated by the seller
+    private final boolean directTransaction;
     private final Actor buyer, seller;
     private final ItemStack itemStack;
     private int amount;
-    private final double buyPrice, sellPrice;
+    private final double moneyAmountRequired, moneyAmountGiven;
 
     public void apply() {
-        if(!check()) {
+        if (!check()) {
             return;
         }
 
+        // Call buy or sell event
+
         transferItems();
 
         transferMoney();
@@ -23,12 +30,42 @@ public class Transaction {
         // log
     }
 
+    /**
+     * Check if the transaction can be performed and inform actors
+     *
+     * @return {@code true} if the transaction can be performed
+     */
     private boolean check() {
         // Check buyer money
+        if (!buyer.hasMoney(moneyAmountRequired)) {
+            if (directTransaction) {
+                buyer.sendMessage(LanguageUtils.getMessage(Message.NOT_ENOUGH_MONEY));
+            } else {
+                seller.sendMessage(LanguageUtils.getMessage(Message.VENDOR_NOT_ENOUGH_MONEY));
+            }
+            return false;
+        }
 
         // Check seller item quantity
+        if (!seller.hasProductAmount(itemStack, amount)) {
+            if (directTransaction) {
+                buyer.sendMessage(LanguageUtils.getMessage(Message.OUT_OF_STOCK));
+            } else {
+                seller.sendMessage(LanguageUtils.getMessage(Message.NOT_ENOUGH_ITEMS));
+            }
+            return false;
+        }
 
         // Check buyer inventory space
+        if (!buyer.hasEnoughInventorySpace(itemStack, amount)) {
+            if (directTransaction) {
+                buyer.sendMessage(LanguageUtils.getMessage(Message.NOT_ENOUGH_INVENTORY_SPACE));
+            } else {
+                seller.sendMessage(LanguageUtils.getMessage(Message.CHEST_NOT_ENOUGH_INVENTORY_SPACE));
+            }
+            return false;
+        }
+        return true;
     }
 
     private void transferItems() {