Begin work for proper API

src/main/java/de/epiceric/shopchest/api/ShopChest.java

@@ -0,0 +1,29 @@
+package de.epiceric.shopchest.api;
+
+import org.bukkit.OfflinePlayer;
+
+/**
+ * The plugin's main entry point
+ * 
+ * @since 1.13
+ */
+public interface ShopChest {
+
+    /**
+     * Gets an instance of the shop manager
+     * 
+     * @return the shop manager
+     */
+    ShopManager getShopManager();
+
+    /**
+     * Gets the amount of shops the given player is allowed to have
+     * <p>
+     * If the player has no shop limit {@link Integer#MAX_VALUE} is returned.
+     * 
+     * @param player the player
+     * @return the shop limit
+     */
+    int getShopLimit(OfflinePlayer player);
+    
+}

src/main/java/de/epiceric/shopchest/api/ShopManager.java

@@ -0,0 +1,134 @@
+package de.epiceric.shopchest.api;
+
+import java.util.Collection;
+import java.util.Optional;
+
+import org.bukkit.Location;
+import org.bukkit.OfflinePlayer;
+import org.bukkit.World;
+
+import de.epiceric.shopchest.api.event.ShopReloadEvent;
+import de.epiceric.shopchest.api.exceptions.ChestNotFoundException;
+import de.epiceric.shopchest.api.exceptions.NoSpaceAboveChestException;
+import de.epiceric.shopchest.api.shop.Shop;
+import de.epiceric.shopchest.api.shop.ShopProduct;
+
+/**
+ * Collection of methods to get, add or remove shops
+ * 
+ * @since 1.13
+ */
+public interface ShopManager {
+
+    /**
+     * Gets all shops
+     * 
+     * @return a collection of shops
+     */
+    Collection<Shop> getShops();
+
+    /**
+     * Gets the shop by its ID
+     * 
+     * @param id the shop's ID
+     * @return the shop or an empty optional if there is no shop
+     * @since 1.13
+     */
+    Optional<Shop> getShop(int id);
+
+    /**
+     * Gets the shop at the given location
+     * 
+     * @param location the shop's chest location
+     * @return the shop or an empty optional if there is no shop
+     * @since 1.13
+     */
+    Optional<Shop> getShop(Location location);
+
+    /**
+     * Gets all shops by the given player
+     * 
+     * @param vendor the player
+     * @return a collection of shops
+     * @since 1.13
+     */
+    Collection<Shop> getShops(OfflinePlayer vendor);
+
+    /**
+     * Gets all shops in the given world
+     * 
+     * @param world the world
+     * @return a collection of shops
+     * @since 1.13
+     */
+    Collection<Shop> getShops(World world);
+
+    /**
+     * Creates a shop and adds it to the database
+     * <p>
+     * When {@code buyPrice} is zero, a player cannot buy from the shop.
+     * When {@code sellPrice} is zero, a player cannot sell to the shop.
+     * You cannot have {@code buyPrice} and {@code sellPrice} be zero.
+     * 
+     * @param vendor    the shop's vendor
+     * @param product   the shop's product
+     * @param location  the shop's chest location.
+     *                  Can be either chest if it's on a double chest
+     * @param buyPrice  the price a player can buy the product for.
+     * @param sellPrice the price a player can sell the product for.
+     * @throws ChestNotFoundException when there is no chest at the given location
+     * @throws NoSpaceAboveChestException when there is no empty space above the chest
+     * @return the shop if created successfully or an empty optional if not
+     * @since 1.13
+     * @see ShopManager#addAdminShop(ShopProduct, Location, double, double)
+     */
+    Shop addShop(OfflinePlayer vendor, ShopProduct product, Location location, double buyPrice, double sellPrice)
+            throws ChestNotFoundException, NoSpaceAboveChestException;
+
+    /**
+     * Creates an admin shop and adds it to the database
+     * <p>
+     * When {@code buyPrice} is zero, a player cannot buy from the shop.
+     * When {@code sellPrice} is zero, a player cannot sell to the shop.
+     * You cannot have {@code buyPrice} and {@code sellPrice} be zero.
+     * 
+     * @param product   the shop's product
+     * @param location  the shop's chest location.
+     *                  Can be either chest if it's on a double chest
+     * @param buyPrice  the price a player can buy the product for.
+     * @param sellPrice the price a player can sell the product for.
+     * @throws ChestNotFoundException when there is no chest at the given location
+     * @throws NoSpaceAboveChestException when there is no empty space above the chest
+     * @return the created admin shop
+     * @since 1.13
+     * @see ShopManager#addShop(OfflinePlayer, ShopProduct, Location, double, double)
+     */
+    Shop addAdminShop(ShopProduct product, Location location, double buyPrice, double sellPrice)
+            throws ChestNotFoundException, NoSpaceAboveChestException;
+
+    /**
+     * Removes a shop from the database
+     * 
+     * @param shop the shop to remove
+     * @since 1.13
+     */
+    void removeShop(Shop shop);
+    
+    /**
+     * Removes a shop from the database by its ID
+     * 
+     * @param shopId the id of the shop to remove
+     * @since 1.13
+     */
+    void removeShop(int shopId);
+
+    /**
+     * Asynchronously reloads all shops from the database
+     * <p>
+     * This does not trigger the {@link ShopReloadEvent}.
+     * 
+     * @since 1.13
+     */
+    void reloadShops();
+
+}

src/main/java/de/epiceric/shopchest/api/event/ShopBuySellEvent.java

@@ -0,0 +1,59 @@
+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.api.shop.Shop;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+
+/**
+ * Called when a player buys or sells something from or to a shop
+ */
+public class ShopBuySellEvent extends ShopEvent implements Cancellable {
+    private Type type;
+    private int amount;
+    private double price;
+    private boolean cancelled;
+
+    public ShopBuySellEvent(Player player, Shop shop, Type type, int amount, double price) {
+        super(player, shop);
+        this.type = type;
+        this.amount = amount;
+        this.price = price;
+    }
+
+    /**
+     * @return Whether the player buys or sells something
+     */
+    public Type getType() {
+        return type;
+    }
+
+    /**
+     * @return The amount which might be modified because of automatic item amount calculation
+     */
+    public int getAmount() {
+        return amount;
+    }
+
+    /**
+     * @return The price which might be modified because of automatic item amount calculation
+     */
+    public double getPrice() {
+        return price;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+
+    public enum Type {
+        BUY,
+        SELL;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopCreateEvent.java

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+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.api.shop.Shop;
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+
+/**
+ * Called when a player creates a shop (clicks on a chest)
+ */
+public class ShopCreateEvent extends ShopEvent implements Cancellable {
+    private double creationPrice;
+    private boolean cancelled;
+
+    public ShopCreateEvent(Player player, Shop shop, double creationPrice) {
+        super(player, shop);
+        this.creationPrice = creationPrice;
+    }
+    /**
+     * @return The price the player has to pay in order to create the shop (only if the event is not cancelled)
+     */
+    public double getCreationPrice() {
+        return creationPrice;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+
+}

src/main/java/de/epiceric/shopchest/api/event/ShopEvent.java

@@ -0,0 +1,42 @@
+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.api.shop.Shop;
+import org.bukkit.entity.Player;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+
+public abstract class ShopEvent extends Event {
+    private static final HandlerList handlers = new HandlerList();
+    
+    private Shop shop;
+    private Player player;
+
+    public ShopEvent(Player player, Shop shop) {
+        this.player = player;
+        this.shop = shop;
+    }
+
+    /**
+     * @return Shop which is involved in this event
+     */
+    public Shop getShop() {
+        return shop;
+    }
+
+    /**
+     * @return Player who is involved in this event
+     */
+    public Player getPlayer() {
+        return player;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+}

src/main/java/de/epiceric/shopchest/api/event/ShopExtendEvent.java

@@ -0,0 +1,37 @@
+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.api.shop.Shop;
+
+import org.bukkit.Location;
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+
+/**
+ * Called when a player extends a shop (making a chest a double chest)
+ */
+public class ShopExtendEvent extends ShopEvent implements Cancellable {
+    private boolean cancelled;
+    private Location newChestLocation;
+
+    public ShopExtendEvent(Player player, Shop shop, Location newChest) {
+        super(player, shop);
+        this.newChestLocation = newChest;
+    }
+
+    /**
+     * @return Location of the placed chest
+     */
+    public Location getNewChestLocation() {
+        return newChestLocation;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopInfoEvent.java

@@ -0,0 +1,26 @@
+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.api.shop.Shop;
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+
+/**
+ * Called when a player retrieves information about a shop (clicks on a chest)
+ */
+public class ShopInfoEvent extends ShopEvent implements Cancellable {
+    private boolean cancelled;
+
+    public ShopInfoEvent(Player player, Shop shop) {
+        super(player, shop);
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopInitializedEvent.java

@@ -0,0 +1,27 @@
+package de.epiceric.shopchest.api.event;
+
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+
+public class ShopInitializedEvent extends Event {
+    private static final HandlerList handlers = new HandlerList();
+    
+    private int amount;
+
+    public ShopInitializedEvent(int amount) {
+        this.amount = amount;
+    }
+
+    public int getAmount() {
+        return amount;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopOpenEvent.java

@@ -0,0 +1,26 @@
+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.api.shop.Shop;
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+
+/**
+ * Called when a player opens a shop (clicks on a chest)
+ */
+public class ShopOpenEvent extends ShopEvent implements Cancellable {
+    private boolean cancelled;
+
+    public ShopOpenEvent(Player player, Shop shop) {
+        super(player, shop);
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopPreCreateEvent.java

@@ -0,0 +1,84 @@
+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.api.shop.ShopProduct;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+
+/**
+ * Called when a player wants to create a shop (enters the command)
+ */
+public class ShopPreCreateEvent extends Event implements Cancellable {
+    private static final HandlerList handlers = new HandlerList();
+
+    private Player player;
+    private ShopProduct product;
+    private double buyPrice;
+    private double sellPrice;
+    private boolean admin;
+    private boolean cancelled;
+
+    public ShopPreCreateEvent(Player player, ShopProduct product, double buyPrice, double sellPrice, boolean admin) {
+        this.player = player;
+        this.product = product;
+        this.buyPrice = buyPrice;
+        this.sellPrice = sellPrice;
+        this.admin = admin;
+    }
+
+    /**
+     * @return the player
+     */
+    public Player getPlayer() {
+        return player;
+    }
+
+    /**
+     * @return the product
+     */
+    public ShopProduct getProduct() {
+        return product;
+    }
+
+    /**
+     * @return the buyPrice
+     */
+    public double getBuyPrice() {
+        return buyPrice;
+    }
+
+    /**
+     * @return the sellPrice
+     */
+    public double getSellPrice() {
+        return sellPrice;
+    }
+
+    /**
+     * @return the admin
+     */
+    public boolean isAdminShop() {
+        return admin;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopPreInfoEvent.java

@@ -0,0 +1,46 @@
+package de.epiceric.shopchest.api.event;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+
+/**
+ * Called when a player wants to retrieve information about a shop (enters the command)
+ */
+public class ShopPreInfoEvent extends Event implements Cancellable {
+    private static final HandlerList handlers = new HandlerList();
+
+    private Player player;
+    private boolean cancelled;
+
+    public ShopPreInfoEvent(Player player) {
+        this.player = player;
+    }
+
+    /**
+     * @return Player who is involved in this event
+     */
+    public Player getPlayer() {
+        return player;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        this.cancelled = cancel;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopPreOpenEvent.java

@@ -0,0 +1,46 @@
+package de.epiceric.shopchest.api.event;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+
+/**
+ * Called when a player wants to open a shop (enters the command)
+ */
+public class ShopPreOpenEvent extends Event implements Cancellable {
+    private static final HandlerList handlers = new HandlerList();
+
+    private Player player;
+    private boolean cancelled;
+
+    public ShopPreOpenEvent(Player player) {
+        this.player = player;
+    }
+
+    /**
+     * @return Player who is involved in this event
+     */
+    public Player getPlayer() {
+        return player;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        this.cancelled = cancel;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopPreRemoveEvent.java

@@ -0,0 +1,46 @@
+package de.epiceric.shopchest.api.event;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+
+/**
+ * Called when a player wants to remove a shop (enters the command)
+ */
+public class ShopPreRemoveEvent extends Event implements Cancellable {
+    private static final HandlerList handlers = new HandlerList();
+
+    private Player player;
+    private boolean cancelled;
+
+    public ShopPreRemoveEvent(Player player) {
+        this.player = player;
+    }
+
+    /**
+     * @return Player who is involved in this event
+     */
+    public Player getPlayer() {
+        return player;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopReloadEvent.java

@@ -0,0 +1,46 @@
+package de.epiceric.shopchest.api.event;
+
+import org.bukkit.command.CommandSender;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+
+/**
+ * Called when a player reloads the shops
+ */
+public class ShopReloadEvent extends Event implements Cancellable {
+    private static final HandlerList handlers = new HandlerList();
+
+    private CommandSender sender;
+    private boolean cancelled;
+
+    public ShopReloadEvent(CommandSender sender) {
+        this.sender = sender;
+    }
+
+    /**
+     * @return Sender who triggered the reload
+     */
+    public CommandSender getSender() {
+        return sender;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopRemoveAllEvent.java

@@ -0,0 +1,56 @@
+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.shop.Shop;
+import org.bukkit.OfflinePlayer;
+import org.bukkit.command.CommandSender;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.Event;
+import org.bukkit.event.HandlerList;
+
+import java.util.List;
+
+public class ShopRemoveAllEvent extends Event implements Cancellable {
+    private static final HandlerList handlers = new HandlerList();
+
+    private CommandSender sender;
+    private OfflinePlayer vendor;
+    private List<Shop> shops;
+    private boolean cancelled;
+
+    public ShopRemoveAllEvent(CommandSender sender, OfflinePlayer vendor, List<Shop> shops) {
+        this.sender = sender;
+        this.vendor = vendor;
+        this.shops = shops;
+    }
+
+    public CommandSender getSender() {
+        return sender;
+    }
+
+    public OfflinePlayer getVendor() {
+        return vendor;
+    }
+
+    public List<Shop> getShops() {
+        return shops;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+
+    @Override
+    public void setCancelled(boolean cancelled) {
+        this.cancelled = cancelled;
+    }
+}

src/main/java/de/epiceric/shopchest/api/event/ShopRemoveEvent.java

@@ -0,0 +1,26 @@
+package de.epiceric.shopchest.api.event;
+
+import de.epiceric.shopchest.api.shop.Shop;
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+
+/**
+ * Called when a player removes a shop (clicks on a chest)
+ */
+public class ShopRemoveEvent extends ShopEvent implements Cancellable {
+    private boolean cancelled;
+
+    public ShopRemoveEvent(Player player, Shop shop) {
+        super(player, shop);
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancel) {
+        cancelled = cancel;
+    }
+}

src/main/java/de/epiceric/shopchest/api/exceptions/ChestNotFoundException.java

@@ -0,0 +1,12 @@
+package de.epiceric.shopchest.api.exceptions;
+
+import org.bukkit.Location;
+
+public class ChestNotFoundException extends Exception {
+    private static final long serialVersionUID = -6446875473671870708L;
+
+    public ChestNotFoundException(Location location) {
+        super(String.format("No chest found in world '%s' at x=%d y=%d z=%d", location.getWorld().getName(),
+                location.getBlockX(), location.getBlockY(), location.getBlockZ()));
+    }
+}

src/main/java/de/epiceric/shopchest/api/exceptions/NoSpaceAboveChestException.java

@@ -0,0 +1,12 @@
+package de.epiceric.shopchest.api.exceptions;
+
+import org.bukkit.Location;
+
+public class NoSpaceAboveChestException extends Exception {
+    private static final long serialVersionUID = 3718475607700458355L;
+
+    public NoSpaceAboveChestException(Location location) {
+        super(String.format("No space above chest in world '%s' at x=%d y=%d z=%d", location.getWorld().getName(),
+                location.getBlockX(), location.getBlockY(), location.getBlockZ()));
+    }
+}

src/main/java/de/epiceric/shopchest/api/shop/Shop.java

@@ -0,0 +1,153 @@
+package de.epiceric.shopchest.api.shop;
+
+import org.bukkit.Location;
+import org.bukkit.OfflinePlayer;
+import org.bukkit.World;
+import org.bukkit.inventory.Inventory;
+
+import de.epiceric.shopchest.api.ShopManager;
+
+/**
+ * Represents a shop
+ * 
+ * @since 1.13
+ * @see ShopManager#addShop(OfflinePlayer, ShopProduct, Location, double, double)
+ * @see ShopManager#addAdminShop(ShopProduct, Location, double, double)
+ */
+public interface Shop {
+
+    /**
+     * Gets this shop's ID
+     * 
+     * @return the ID
+     * @since 1.13
+     */
+    int getId();
+
+    /**
+     * Gets the player who created this shop
+     * 
+     * @return the vendor
+     * @since 1.13
+     */
+    OfflinePlayer getVendor();
+
+    /**
+     * Gets the product this shop is buying or selling
+     * 
+     * @return the product
+     * @since 1.13
+     */
+    ShopProduct getProduct();
+
+    /**
+     * Gets the location of this shop's chest
+     * <p>
+     * If the shop is on a double chest, it returns one of the chest's location.
+     * 
+     * @return the location
+     * @since 1.13
+     */
+    Location getLocation();
+
+    /**
+     * Gets the world this shop is located in
+     * 
+     * @return the world
+     * @since 1.13
+     */
+    default World getWorld() {
+        return getLocation().getWorld();
+    }
+
+    /**
+     * Gets whether this shop is on a double chest
+     * 
+     * @return whether the shop is on a double chest
+     * @since 1.13
+     */
+    boolean isDoubleChest();
+
+    /**
+     * Gets the inventory of this shop's chest
+     * 
+     * @return the inventory
+     * @since 1.13
+     */
+    Inventory getInventory();
+
+    /**
+     * Gets whether this shop is an admin shop
+     * 
+     * @return whether this shop is an admin shop
+     * @since 1.13
+     */
+    boolean isAdminShop();
+
+    /**
+     * Sets whether this shop is an admin shop
+     * 
+     * @param adminShop whether this shop should be an admin shop
+     * @since 1.13
+     */
+    void setAdminShop(boolean adminShop);
+
+    /**
+     * Gets the price for which a player can buy the product from this shop
+     * 
+     * @return the buy price
+     * @since 1.13
+     */
+    double getBuyPrice();
+
+    /**
+     * Sets the price for which a player can sell the product to this shop
+     * <p>
+     * If set to zero, a player cannot buy from this shop.
+     * 
+     * @param buyPrice the buy price
+     * @throws IllegalStateException when a player can neither buy nor sell from this shop
+     * @since 1.13
+     */
+    void setBuyPrice(double buyPrice);
+
+    /**
+     * Gets whether a player can buy from this shop
+     * 
+     * @return whether buying is enabled
+     * @since 1.13
+     */
+    default boolean canPlayerBuy() {
+        return getBuyPrice() > 0;
+    }
+
+    /**
+     * Gets the price for which a player can sell the product to this shop
+     * 
+     * @return the sell price
+     * @since 1.13
+     */
+    double getSellPrice();
+
+    /**
+     * Sets the price for which a player can sell the product to this shop
+     * <p>
+     * If set to zero, a player cannot sell to this shop.
+     * 
+     * @param sellPrice the sell price
+     * @throws IllegalStateException when a player can neither buy nor sell from this shop
+     * @since 1.13
+     */
+    void setSellPrice(double sellPrice);
+
+    /**
+     * Gets whether a player can sell to this shop
+     * 
+     * @return whether selling is enabled
+     * @since 1.13
+     */
+    default boolean canPlayerSell() {
+        return getSellPrice() > 0;
+    }
+
+}

src/main/java/de/epiceric/shopchest/api/shop/ShopProduct.java

@@ -0,0 +1,54 @@
+package de.epiceric.shopchest.api.shop;
+
+import org.bukkit.inventory.ItemStack;
+
+/**
+ * Represents the item that can be bought or sold in one transaction
+ * 
+ * @since 1.13
+ */
+public interface ShopProduct {
+
+    /**
+     * Gets the {@link ItemStack} with an amount of one
+     * 
+     * @return the item
+     * @since 1.13
+     */
+    ItemStack getItemStack();
+
+    /**
+     * Sets the {@link ItemStack}
+     * <p>
+     * The passed item stack will be cloned and its amount set to one.
+     * 
+     * @param itemStack the item
+     * @since 1.13
+     */
+    void setItemStack(ItemStack itemStack);
+
+    /**
+     * Gets the amount of items bought or sold in one transaction
+     * 
+     * @return the amount
+     * @since 1.13
+     */
+    int getAmount();
+
+    /**
+     * Sets the amount of items bought or sold in one transaction
+     * 
+     * @param amount the amount
+     * @since 1.13
+     */
+    void setAmount(int amount);
+
+    /**
+     * Gets the localized name of this product's item in the configured langauge file
+     * 
+     * @return the localized name
+     * @since 1.13
+     */
+    String getLocalizedName();
+    
+}