Fixes a lot of nullability issues

src/main/java/net/knarcraft/bookswithoutborders/BooksWithoutBorders.java

@@ -74,6 +74,7 @@ public class BooksWithoutBorders extends JavaPlugin {
      *
      * @return <p>The console's console sender</p>
      */
+    @NotNull
     public static ConsoleCommandSender getConsoleSender() {
         return consoleSender;
     }
@@ -85,7 +86,8 @@ public class BooksWithoutBorders extends JavaPlugin {
      * @param getPublic <p>Whether to get available public books</p>
      * @return <p>A list of available books</p>
      */
-    public static List<String> getAvailableBooks(CommandSender sender, boolean getPublic) {
+    @NotNull
+    public static List<String> getAvailableBooks(@NotNull CommandSender sender, boolean getPublic) {
         if (getPublic) {
             return new ArrayList<>(publicBooksList);
         } else if (sender instanceof Player player) {
@@ -191,6 +193,7 @@ public class BooksWithoutBorders extends JavaPlugin {
      *
      * @return <p>An instance of this plugin</p>
      */
+    @NotNull
     public static BooksWithoutBorders getInstance() {
         return booksWithoutBorders;
     }
@@ -229,7 +232,7 @@ public class BooksWithoutBorders extends JavaPlugin {
      * @param commandName <p>The name of the command to register</p>
      * @param executor    <p>The executor to register for the command</p>
      */
-    private void registerCommand(String commandName, CommandExecutor executor) {
+    private void registerCommand(@NotNull String commandName, @NotNull CommandExecutor executor) {
         PluginCommand pluginCommand = this.getCommand(commandName);
         if (pluginCommand != null) {
             pluginCommand.setExecutor(executor);
@@ -270,6 +273,7 @@ public class BooksWithoutBorders extends JavaPlugin {
      *
      * @return <p>The server's item factory</p>
      */
+    @NotNull
     public static ItemFactory getItemFactory() {
         return itemFactory;
     }
@@ -313,7 +317,7 @@ public class BooksWithoutBorders extends JavaPlugin {
      * @param sender  <p>The sender to send the message to</p>
      * @param message <p>The message to send</p>
      */
-    public static void sendSuccessMessage(CommandSender sender, String message) {
+    public static void sendSuccessMessage(@NotNull CommandSender sender, @NotNull String message) {
         sender.sendMessage(getSuccessColor() + message);
     }
 
@@ -323,7 +327,7 @@ public class BooksWithoutBorders extends JavaPlugin {
      * @param sender  <p>The sender to send the message to</p>
      * @param message <p>The message to send</p>
      */
-    public static void sendErrorMessage(CommandSender sender, String message) {
+    public static void sendErrorMessage(@NotNull CommandSender sender, @NotNull String message) {
         sender.sendMessage(getErrorColor() + message);
     }
 

src/main/java/net/knarcraft/bookswithoutborders/command/CommandBooksWithoutBorders.java

@@ -24,7 +24,7 @@ import static net.knarcraft.bookswithoutborders.config.BooksWithoutBordersConfig
 public class CommandBooksWithoutBorders implements TabExecutor {
 
     @Override
-    public boolean onCommand(CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
         sender.sendMessage(getCommandColor() + "[] denote optional parameters");
         sender.sendMessage(getCommandColor() + "<> denote required parameters");
         sender.sendMessage(getCommandColor() + "{} denote required permission");
@@ -42,7 +42,7 @@ public class CommandBooksWithoutBorders implements TabExecutor {
      *
      * @param sender <p>The console which sent the command</p>
      */
-    private void showConsoleCommands(CommandSender sender) {
+    private void showConsoleCommands(@NotNull CommandSender sender) {
         sender.sendMessage(getCommandColor() + "Commands:");
         showCommandInfo("deletePublicBook", sender);
         showCommandInfo("givePublicBook", sender);
@@ -54,7 +54,7 @@ public class CommandBooksWithoutBorders implements TabExecutor {
      *
      * @param sender <p>The player which sent the command</p>
      */
-    private void showPlayerCommands(CommandSender sender) {
+    private void showPlayerCommands(@NotNull CommandSender sender) {
         //Lists all commands
         Material bookPriceType = BooksWithoutBordersConfig.getBookPriceType();
         double bookPriceQuantity = BooksWithoutBordersConfig.getBookPriceQuantity();
@@ -97,7 +97,7 @@ public class CommandBooksWithoutBorders implements TabExecutor {
      * @param command <p>The command to get information about</p>
      * @param sender  <p>The sender asking to see command info</p>
      */
-    private void showCommandInfo(String command, CommandSender sender) {
+    private void showCommandInfo(@NotNull String command, @NotNull CommandSender sender) {
         PluginCommand pluginCommand = BooksWithoutBorders.getInstance().getCommand(command);
         if (pluginCommand != null) {
             String permission = pluginCommand.getPermission();
@@ -116,7 +116,8 @@ public class CommandBooksWithoutBorders implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, @NotNull String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
+                                      @NotNull String[] arguments) {
         return new ArrayList<>();
     }
 

src/main/java/net/knarcraft/bookswithoutborders/command/CommandClear.java

@@ -51,7 +51,7 @@ public class CommandClear implements TabExecutor {
     @Nullable
     @Override
     public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
-                                      @NotNull String[] args) {
+                                      @NotNull String[] arguments) {
         return new ArrayList<>();
     }
 

src/main/java/net/knarcraft/bookswithoutborders/command/CommandCopy.java

@@ -26,7 +26,8 @@ import java.util.Objects;
 public class CommandCopy implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
@@ -37,14 +38,14 @@ public class CommandCopy implements TabExecutor {
             return false;
         }
 
-        if (args.length < 1) {
+        if (arguments.length < 1) {
             BooksWithoutBorders.sendErrorMessage(player, "You must specify the number of copies to be made!");
             return false;
         }
 
         try {
             ItemStack heldBook = InventoryHelper.getHeldBook(player, true);
-            int copies = Integer.parseInt(args[0]);
+            int copies = Integer.parseInt(arguments[0]);
             if (copies <= 0) {
                 throw new NumberFormatException("Number of copies must be larger than 0");
             }
@@ -64,7 +65,7 @@ public class CommandCopy implements TabExecutor {
      * @param heldBook <p>The book to be copied</p>
      * @return <p>True if the copying was successful</p>
      */
-    private boolean performCopy(int copies, Player player, ItemStack heldBook) {
+    private boolean performCopy(int copies, @NotNull Player player, @NotNull ItemStack heldBook) {
         //Make sure the player owns the book if authorOnlyCopy is enabled
         if (BooksWithoutBordersConfig.getAuthorOnlyCopy() &&
                 !player.hasPermission("bookswithoutborders.bypassAuthorOnlyCopy")) {
@@ -95,7 +96,7 @@ public class CommandCopy implements TabExecutor {
      * @param copies <p>The number of copies to create for the player</p>
      * @return <p>True if the payment failed</p>
      */
-    private boolean paymentUnSuccessful(Player player, int copies) {
+    private boolean paymentUnSuccessful(@NotNull Player player, int copies) {
         return BooksWithoutBordersConfig.booksHavePrice() &&
                 !player.hasPermission("bookswithoutborders.bypassBookPrice") &&
                 EconomyHelper.cannotPayForBookPrinting(player, copies);
@@ -109,7 +110,7 @@ public class CommandCopy implements TabExecutor {
      * @param copies   <p>The number of copies requested</p>
      * @return <p>True if the book was successfully copied</p>
      */
-    private boolean copyNextGenerationBook(BookMeta bookMeta, Player player, int copies) {
+    private boolean copyNextGenerationBook(@NotNull BookMeta bookMeta, @NotNull Player player, int copies) {
         //Copy the vanilla behavior of refusing copying any further
         if (bookMeta.getGeneration() == BookMeta.Generation.COPY_OF_COPY ||
                 bookMeta.getGeneration() == BookMeta.Generation.TATTERED) {
@@ -140,10 +141,10 @@ public class CommandCopy implements TabExecutor {
 
     @Override
     public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-                                      @NotNull String[] args) {
+                                      @NotNull String[] arguments) {
-        int argumentCount = args.length;
+        int argumentCount = arguments.length;
         if (argumentCount == 1) {
-            return TabCompletionHelper.filterMatchingStartsWith(TabCompletionTypeHelper.getNumbers(1, 20), args[0]);
+            return TabCompletionHelper.filterMatchingStartsWith(TabCompletionTypeHelper.getNumbers(1, 20), arguments[0]);
         }
         return new ArrayList<>();
     }

src/main/java/net/knarcraft/bookswithoutborders/command/CommandDecrypt.java

@@ -99,8 +99,9 @@ public class CommandDecrypt implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, @NotNull String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-        int argumentCount = args.length;
+                                      @NotNull String[] arguments) {
+        int argumentCount = arguments.length;
         if (argumentCount == 1) {
             List<String> info = new ArrayList<>();
             info.add("<password>");

src/main/java/net/knarcraft/bookswithoutborders/command/CommandDelete.java

@@ -22,7 +22,8 @@ import java.util.List;
 public class CommandDelete implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] arguments) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         if (!(sender instanceof Player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
@@ -39,7 +40,7 @@ public class CommandDelete implements TabExecutor {
      * @param deletePublic <p>Whether to delete a public book</p>
      * @return <p>True if the book was deleted successfully</p>
      */
-    boolean deleteBook(@NotNull CommandSender sender, @NotNull String[] arguments, boolean deletePublic) {
+    protected boolean deleteBook(@NotNull CommandSender sender, @NotNull String[] arguments, boolean deletePublic) {
         String command = deletePublic ? "deletepublicbook" : "deletebook";
         if (PagedBookIndex.displayPage(arguments, sender, deletePublic, command)) {
             return true;
@@ -69,7 +70,7 @@ public class CommandDelete implements TabExecutor {
      * @param fileName <p>The file name of the book</p>
      * @param isPublic <p>Whether the book to delete is public or not</p>
      */
-    public void performBookDeletion(CommandSender sender, String fileName, Boolean isPublic) {
+    public void performBookDeletion(@NotNull CommandSender sender, @NotNull String fileName, @NotNull Boolean isPublic) {
         //If the file name is an index of the load list, load the book
         try {
             int loadListIndex = Integer.parseInt(fileName);
@@ -104,8 +105,9 @@ public class CommandDelete implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, @NotNull String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-        return doTabCompletion(sender, args, false);
+                                      @NotNull String[] arguments) {
+        return doTabCompletion(sender, arguments, false);
     }
 
     /**
@@ -116,7 +118,8 @@ public class CommandDelete implements TabExecutor {
      * @param deletePublic <p>Whether to delete a public book</p>
      * @return <p>A list of available arguments</p>
      */
-    protected List<String> doTabCompletion(CommandSender sender, String[] args, boolean deletePublic) {
+    @NotNull
+    protected List<String> doTabCompletion(@NotNull CommandSender sender, @NotNull String[] args, boolean deletePublic) {
         int argumentCount = args.length;
         if (argumentCount == 1) {
             return TabCompletionHelper.filterMatchingContains(BooksWithoutBorders.getAvailableBooks(sender, deletePublic),

src/main/java/net/knarcraft/bookswithoutborders/command/CommandDeletePublic.java

@@ -18,8 +18,8 @@ public class CommandDeletePublic extends CommandDelete implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] arguments) {
-        return doTabCompletion(sender, args, true);
+        return doTabCompletion(sender, arguments, true);
     }
 
 }

src/main/java/net/knarcraft/bookswithoutborders/command/CommandEncrypt.java

@@ -13,6 +13,7 @@ import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.meta.BookMeta;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -23,25 +24,28 @@ import java.util.List;
 public class CommandEncrypt implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
-        if (performPreChecks(sender, args, 1, "You must specify a key to encrypt a book!") == null) {
+                             @NotNull String[] arguments) {
+        if (performPreChecks(sender, arguments, 1, "You must specify a key to encrypt a book!") == null) {
             return false;
         }
 
-        EncryptionStyle encryptionStyle = args.length == 2 ? EncryptionStyle.getFromString(args[1]) : EncryptionStyle.SUBSTITUTION;
+        EncryptionStyle encryptionStyle = arguments.length == 2 ? EncryptionStyle.getFromString(arguments[1]) : EncryptionStyle.SUBSTITUTION;
-        return encryptBook(encryptionStyle, (Player) sender, args[0], "");
+        return encryptBook(encryptionStyle, (Player) sender, arguments[0], "");
     }
 
     /**
      * Performs necessary pre-checks before going through with the encryption
      *
      * @param sender                <p>The sender trying to encrypt a book</p>
-     * @param args                  <p>The arguments given</p>
+     * @param arguments             <p>The arguments given</p>
      * @param necessaryArguments    <p>How many arguments is the minimum requirement</p>
      * @param missingArgumentsError <p>The error to show if a required argument is missing</p>
      * @return <p>The metadata of the book to encrypt, or null if any checks fail</p>
      */
-    BookMeta performPreChecks(CommandSender sender, String[] args, int necessaryArguments, String missingArgumentsError) {
+    @Nullable
+    protected BookMeta performPreChecks(@NotNull CommandSender sender, @NotNull String[] arguments,
+                                        int necessaryArguments, @NotNull String missingArgumentsError) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return null;
@@ -53,7 +57,7 @@ public class CommandEncrypt implements TabExecutor {
             return null;
         }
 
-        int argumentCount = args.length;
+        int argumentCount = arguments.length;
         if (argumentCount < necessaryArguments) {
             BooksWithoutBorders.sendErrorMessage(player, missingArgumentsError);
             return null;
@@ -85,7 +89,8 @@ public class CommandEncrypt implements TabExecutor {
      * @param group           <p>The group to encrypt for</p>
      * @return <p>True if the book was encrypted successfully</p>
      */
-    boolean encryptBook(EncryptionStyle encryptionStyle, Player player, String key, String group) {
+    protected boolean encryptBook(@NotNull EncryptionStyle encryptionStyle, @NotNull Player player, @NotNull String key,
+                                  @NotNull String group) {
         ItemSlot heldSlot = InventoryHelper.getHeldSlotBook(player, false, false, true, true);
         ItemStack encryptedBook = EncryptionHelper.encryptBook(player, heldSlot == ItemSlot.MAIN_HAND, key, encryptionStyle, group);
 
@@ -98,8 +103,10 @@ public class CommandEncrypt implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] args) {
+    @NotNull
-        return doTabCompletion(args, false);
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
+                                      @NotNull String[] arguments) {
+        return doTabCompletion(arguments, false);
     }
 
     /**
@@ -109,7 +116,8 @@ public class CommandEncrypt implements TabExecutor {
      * @param groupEncrypt <p>Whether to auto-complete for group encryption</p>
      * @return <p>The strings to auto-complete</p>
      */
-    protected List<String> doTabCompletion(String[] args, boolean groupEncrypt) {
+    @NotNull
+    protected List<String> doTabCompletion(@NotNull String[] args, boolean groupEncrypt) {
         int argumentsCount = args.length;
 
         List<String> encryptionStyles = new ArrayList<>();

src/main/java/net/knarcraft/bookswithoutborders/command/CommandFormat.java

@@ -20,7 +20,8 @@ import java.util.List;
 public class CommandFormat implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
@@ -32,7 +33,14 @@ public class CommandFormat implements TabExecutor {
         }
 
         ItemStack heldBook = InventoryHelper.getHeldBook(player, true);
-        heldBook.setItemMeta(BookFormatter.formatPages((BookMeta) heldBook.getItemMeta()));
+        BookMeta meta = (BookMeta) heldBook.getItemMeta();
+
+        if (meta == null) {
+            BooksWithoutBorders.sendErrorMessage(sender, "Unable to get metadata from the held book!");
+            return false;
+        }
+
+        heldBook.setItemMeta(BookFormatter.formatPages(meta));
 
         BooksWithoutBorders.sendSuccessMessage(sender, "Book formatted!");
 
@@ -40,7 +48,8 @@ public class CommandFormat implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, @NotNull String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
+                                      @NotNull String[] arguments) {
         return new ArrayList<>();
     }
 

src/main/java/net/knarcraft/bookswithoutborders/command/CommandGive.java

@@ -13,6 +13,7 @@ import org.bukkit.command.TabExecutor;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -25,7 +26,8 @@ public class CommandGive implements TabExecutor {
     private final BooksWithoutBorders booksWithoutBorders = BooksWithoutBorders.getInstance();
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] arguments) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         if (!(sender instanceof Player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
@@ -43,7 +45,8 @@ public class CommandGive implements TabExecutor {
      * @param folder     <p>The folder containing the book to load</p>
      * @return <p>True if the book was given successfully</p>
      */
-    boolean giveBook(@NotNull CommandSender sender, String[] arguments, boolean givePublic, String folder) {
+    boolean giveBook(@NotNull CommandSender sender, @NotNull String[] arguments, boolean givePublic,
+                     @NotNull String folder) {
         String command = givePublic ? "givepublicbook" : "givebook";
         if (PagedBookIndex.displayPage(arguments, sender, givePublic, command)) {
             return true;
@@ -100,8 +103,9 @@ public class CommandGive implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, @NotNull String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-        return doTabCompletion(sender, args, false);
+                                      @NotNull String[] arguments) {
+        return doTabCompletion(sender, arguments, false);
     }
 
     /**
@@ -112,7 +116,8 @@ public class CommandGive implements TabExecutor {
      * @param listPublic <p>Whether to list public files or player files</p>
      * @return <p>A list of available choices</p>
      */
-    protected List<String> doTabCompletion(CommandSender sender, String[] args, boolean listPublic) {
+    @Nullable
+    protected List<String> doTabCompletion(@NotNull CommandSender sender, @NotNull String[] args, boolean listPublic) {
         Server server = booksWithoutBorders.getServer();
 
         int argumentCount = args.length;
@@ -157,8 +162,9 @@ public class CommandGive implements TabExecutor {
      * @param copies          <p>The number of copies the player wants to give</p>
      * @return <p>True if the book was successfully given</p>
      */
-    private boolean loadAndGiveBook(String bookIdentifier, CommandSender sender, Player receivingPlayer,
+    private boolean loadAndGiveBook(@NotNull String bookIdentifier, @NotNull CommandSender sender,
-                                    String isSigned, String folder, String copies) throws NumberFormatException {
+                                    @NotNull Player receivingPlayer, @NotNull String isSigned, @NotNull String folder,
+                                    @NotNull String copies) throws NumberFormatException {
         String bookToLoad = InputCleaningHelper.cleanString(bookIdentifier);
         ItemStack newBook = BookLoader.loadBook(sender, bookToLoad, isSigned, folder, Integer.parseInt(copies));
         if (newBook != null) {

src/main/java/net/knarcraft/bookswithoutborders/command/CommandGivePublic.java

@@ -4,6 +4,7 @@ import org.bukkit.command.Command;
 import org.bukkit.command.CommandSender;
 import org.bukkit.command.TabExecutor;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.List;
 
@@ -13,13 +14,16 @@ import java.util.List;
 public class CommandGivePublic extends CommandGive implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, String[] arguments) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         return giveBook(sender, arguments, true, "public");
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] args) {
+    @Nullable
-        return doTabCompletion(sender, args, true);
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
+                                      @NotNull String[] arguments) {
+        return doTabCompletion(sender, arguments, true);
     }
 
 }

src/main/java/net/knarcraft/bookswithoutborders/command/CommandGroupEncrypt.java

@@ -17,8 +17,9 @@ import java.util.List;
 public class CommandGroupEncrypt extends CommandEncrypt implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
-        BookMeta bookMetadata = performPreChecks(sender, args, 2,
+                             @NotNull String[] arguments) {
+        BookMeta bookMetadata = performPreChecks(sender, arguments, 2,
                 "You must specify a group name and key to encrypt a book!");
 
         if (bookMetadata == null) {
@@ -32,13 +33,14 @@ public class CommandGroupEncrypt extends CommandEncrypt implements TabExecutor {
             return false;
         }
 
-        EncryptionStyle encryptionStyle = args.length == 3 ? EncryptionStyle.getFromString(args[2]) : EncryptionStyle.SUBSTITUTION;
+        EncryptionStyle encryptionStyle = arguments.length == 3 ? EncryptionStyle.getFromString(arguments[2]) : EncryptionStyle.SUBSTITUTION;
-        return encryptBook(encryptionStyle, (Player) sender, args[1], args[0]);
+        return encryptBook(encryptionStyle, (Player) sender, arguments[1], arguments[0]);
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] args) {
+    public @NotNull List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command,
-        return doTabCompletion(args, true);
+                                               @NotNull String alias, @NotNull String[] arguments) {
+        return doTabCompletion(arguments, true);
     }
 
 }

src/main/java/net/knarcraft/bookswithoutborders/command/CommandLoad.java

@@ -22,7 +22,8 @@ import java.util.List;
 public class CommandLoad implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] arguments) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         return loadBook(sender, arguments, "player", false);
     }
 
@@ -95,32 +96,34 @@ public class CommandLoad implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, @NotNull String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-        return doTabCompletion(sender, args, false);
+                                      @NotNull String[] arguments) {
+        return doTabCompletion(sender, arguments, false);
     }
 
     /**
      * Performs the actual tab completion
      *
      * @param sender     <p>The sender of the command</p>
-     * @param args       <p>The arguments given</p>
+     * @param arguments  <p>The arguments given</p>
      * @param loadPublic <p>Whether to list public files or player files</p>
      * @return <p>A list of available choices</p>
      */
-    protected List<String> doTabCompletion(CommandSender sender, String[] args, boolean loadPublic) {
+    @NotNull
-        int argumentCount = args.length;
+    protected List<String> doTabCompletion(@NotNull CommandSender sender, @NotNull String[] arguments, boolean loadPublic) {
+        int argumentCount = arguments.length;
         if (argumentCount == 1) {
             //Return list of books
             return TabCompletionHelper.filterMatchingContains(BooksWithoutBorders.getAvailableBooks(sender, loadPublic),
-                    args[0]);
+                    arguments[0]);
         } else if (argumentCount == 2) {
             //Number of copies
-            return TabCompletionHelper.filterMatchingStartsWith(TabCompletionTypeHelper.getBooleansAndNumbers(1, 3), args[1]);
+            return TabCompletionHelper.filterMatchingStartsWith(TabCompletionTypeHelper.getBooleansAndNumbers(1, 3), arguments[1]);
         } else if (argumentCount == 3) {
             //Signed
             try {
-                Integer.parseInt(args[1]);
+                Integer.parseInt(arguments[1]);
-                return TabCompletionHelper.filterMatchingStartsWith(TabCompletionTypeHelper.getBooleans(), args[2]);
+                return TabCompletionHelper.filterMatchingStartsWith(TabCompletionTypeHelper.getBooleans(), arguments[2]);
             } catch (NumberFormatException e) {
                 return new ArrayList<>();
             }

src/main/java/net/knarcraft/bookswithoutborders/command/CommandLoadPublic.java

@@ -13,13 +13,15 @@ import java.util.List;
 public class CommandLoadPublic extends CommandLoad implements CommandExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, String[] arguments) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         return loadBook(sender, arguments, "public", true);
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-        return doTabCompletion(sender, args, true);
+                                      @NotNull String[] arguments) {
+        return doTabCompletion(sender, arguments, true);
     }
 
 }

src/main/java/net/knarcraft/bookswithoutborders/command/CommandReload.java

@@ -16,7 +16,8 @@ import java.util.List;
 public class CommandReload implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         if (BooksWithoutBordersConfig.loadConfig()) {
             BooksWithoutBorders.sendSuccessMessage(sender, "BooksWithoutBorders configuration reloaded!");
         } else {
@@ -31,7 +32,8 @@ public class CommandReload implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, @NotNull String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
+                                      @NotNull String[] arguments) {
         return new ArrayList<>();
     }
 

src/main/java/net/knarcraft/bookswithoutborders/command/CommandSave.java

@@ -33,19 +33,19 @@ import static net.knarcraft.bookswithoutborders.config.BooksWithoutBordersConfig
 public class CommandSave implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, String[] arguments) {
-        return saveHeldBook(sender, args, false);
+        return saveHeldBook(sender, arguments, false);
     }
 
     /**
      * Saves the player's held book if it exists
      *
      * @param sender     <p>The sender of the command</p>
-     * @param args       <p>The arguments given</p>
+     * @param arguments  <p>The arguments given</p>
      * @param savePublic <p>Whether to save the book in the public directory or the player directory</p>
      * @return <p>True if a book was saved successfully</p>
      */
-    boolean saveHeldBook(CommandSender sender, String[] args, boolean savePublic) {
+    boolean saveHeldBook(@NotNull CommandSender sender, @NotNull String[] arguments, boolean savePublic) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
@@ -54,7 +54,7 @@ public class CommandSave implements TabExecutor {
         ItemSlot holdingSlot = InventoryHelper.getHeldSlotBook(player, false, false, false, false);
         if (holdingSlot != ItemSlot.NONE) {
             ItemStack holdingItem = InventoryHelper.getHeldItem(player, holdingSlot == ItemSlot.MAIN_HAND);
-            boolean duplicate = args.length == 1 && Boolean.parseBoolean(args[0]);
+            boolean duplicate = arguments.length == 1 && Boolean.parseBoolean(arguments[0]);
             saveBook(player, holdingItem, duplicate, savePublic);
             return true;
         } else {
@@ -71,7 +71,7 @@ public class CommandSave implements TabExecutor {
      * @param overwrite          <p>Whether to overwrite any existing books</p>
      * @param saveToPublicFolder <p>Whether to save the book to the public folder instead of the player folder</p>
      */
-    public void saveBook(Player player, ItemStack heldBook, boolean overwrite, boolean saveToPublicFolder) {
+    public void saveBook(@NotNull Player player, @NotNull ItemStack heldBook, boolean overwrite, boolean saveToPublicFolder) {
         BookMeta book = (BookMeta) heldBook.getItemMeta();
         if (book == null) {
             BooksWithoutBorders.sendErrorMessage(player, "Unable to get metadata for your held book!");
@@ -156,8 +156,9 @@ public class CommandSave implements TabExecutor {
     }
 
     @Override
+    @NotNull
     public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-                                      @NotNull String[] args) {
+                                      @NotNull String[] arguments) {
         return new ArrayList<>();
     }
 

src/main/java/net/knarcraft/bookswithoutborders/command/CommandSavePublic.java

@@ -11,8 +11,9 @@ import org.jetbrains.annotations.NotNull;
 public class CommandSavePublic extends CommandSave implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
-        return saveHeldBook(sender, args, true);
+                             @NotNull String[] arguments) {
+        return saveHeldBook(sender, arguments, true);
     }
 
 }

src/main/java/net/knarcraft/bookswithoutborders/command/CommandSetAuthor.java

@@ -12,6 +12,7 @@ import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.meta.BookMeta;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -22,13 +23,14 @@ import java.util.List;
 public class CommandSetAuthor implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
         }
 
-        if (args.length < 1) {
+        if (arguments.length < 1) {
             BooksWithoutBorders.sendErrorMessage(player, "Too few command arguments!");
             return false;
         }
@@ -48,7 +50,7 @@ public class CommandSetAuthor implements TabExecutor {
             return false;
         }
 
-        String author = ColorHelper.translateColorCodes(String.join(" ", args), ColorConversion.RGB);
+        String author = ColorHelper.translateColorCodes(String.join(" ", arguments), ColorConversion.RGB);
         bookMetaData.setAuthor(author);
         heldBook.setItemMeta(bookMetaData);
         BooksWithoutBorders.sendSuccessMessage(player, "Book author set to " + author + "!");
@@ -56,8 +58,10 @@ public class CommandSetAuthor implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] args) {
+    @Nullable
-        if (args.length == 1) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
+                                      @NotNull String[] arguments) {
+        if (arguments.length == 1) {
             return null;
         } else {
             return new ArrayList<>();

src/main/java/net/knarcraft/bookswithoutborders/command/CommandSetBookPrice.java

@@ -26,35 +26,36 @@ public class CommandSetBookPrice implements TabExecutor {
     private List<String> paymentTypes;
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         //Clear the current price
-        if (args.length == 0) {
+        if (arguments.length == 0) {
             clearItemPrice(sender);
             return true;
         }
 
         //Warn about missing arguments
-        if (args.length < 2) {
+        if (arguments.length < 2) {
             BooksWithoutBorders.sendErrorMessage(sender, "[Item/Eco] and [quantity] must be specified!");
             return false;
         }
 
         //Warn about invalid argument
-        if (!args[0].equalsIgnoreCase("Item") && !args[0].equalsIgnoreCase("Eco")) {
+        if (!arguments[0].equalsIgnoreCase("Item") && !arguments[0].equalsIgnoreCase("Eco")) {
             BooksWithoutBorders.sendErrorMessage(sender, "Price type must be \"Item\" or \"Eco\"!");
             return false;
         }
 
         try {
-            double price = Double.parseDouble(args[1]);
+            double price = Double.parseDouble(arguments[1]);
             if (price <= 0) {
                 BooksWithoutBorders.sendErrorMessage(sender, "[quantity] must be greater than 0!");
                 return false;
             }
 
-            if (args[0].equalsIgnoreCase("Item")) {
+            if (arguments[0].equalsIgnoreCase("Item")) {
                 return setItemPrice(sender, price);
-            } else if (args[0].equalsIgnoreCase("Eco")) {
+            } else if (arguments[0].equalsIgnoreCase("Eco")) {
                 return setEconomyPrice(sender, price);
             }
         } catch (NumberFormatException e) {
@@ -68,7 +69,7 @@ public class CommandSetBookPrice implements TabExecutor {
      *
      * @param sender <p>The sender of the command</p>
      */
-    private void clearItemPrice(CommandSender sender) {
+    private void clearItemPrice(@NotNull CommandSender sender) {
         BooksWithoutBordersConfig.setBookPriceType(null);
         BooksWithoutBordersConfig.setBookPriceQuantity(0);
         booksWithoutBorders.getConfig().set("Options.Price_to_create_book.Item_type", "Item type name");
@@ -85,7 +86,7 @@ public class CommandSetBookPrice implements TabExecutor {
      * @param price  <p>The new price</p>
      * @return <p>True if the price was changed successfully</p>
      */
-    private boolean setItemPrice(CommandSender sender, double price) {
+    private boolean setItemPrice(@NotNull CommandSender sender, double price) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "[Item] price can only be used by a player!");
             return false;
@@ -117,7 +118,7 @@ public class CommandSetBookPrice implements TabExecutor {
      * @param price  <p>The new price</p>
      * @return <p>True if the price was changed successfully</p>
      */
-    private boolean setEconomyPrice(CommandSender sender, double price) {
+    private boolean setEconomyPrice(@NotNull CommandSender sender, double price) {
         if (EconomyHelper.setupEconomy()) {
             BooksWithoutBordersConfig.setBookPriceQuantity(price);
             BooksWithoutBordersConfig.setBookPriceType(Material.AIR);
@@ -136,16 +137,17 @@ public class CommandSetBookPrice implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
+                                      @NotNull String[] arguments) {
         if (paymentTypes == null) {
             initializeTabCompleteLists();
         }
 
-        int argumentCount = args.length;
+        int argumentCount = arguments.length;
         if (argumentCount == 1) {
-            return TabCompletionHelper.filterMatchingStartsWith(paymentTypes, args[0]);
+            return TabCompletionHelper.filterMatchingStartsWith(paymentTypes, arguments[0]);
         } else if (argumentCount == 2) {
-            return TabCompletionHelper.filterMatchingStartsWith(TabCompletionTypeHelper.getNumbers(1, 3), args[1]);
+            return TabCompletionHelper.filterMatchingStartsWith(TabCompletionTypeHelper.getNumbers(1, 3), arguments[1]);
         }
         return new ArrayList<>();
     }

src/main/java/net/knarcraft/bookswithoutborders/command/CommandSetGeneration.java

@@ -21,7 +21,7 @@ public class CommandSetGeneration implements TabExecutor {
 
     @Override
     public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
-                             @NotNull String[] args) {
+                             @NotNull String[] arguments) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
@@ -32,14 +32,14 @@ public class CommandSetGeneration implements TabExecutor {
             return false;
         }
 
-        if (args.length < 1) {
+        if (arguments.length < 1) {
             BooksWithoutBorders.sendErrorMessage(player, "You must specify the new generation for your book!");
             return false;
         }
 
         BookMeta.Generation generation;
         try {
-            generation = BookMeta.Generation.valueOf(args[0]);
+            generation = BookMeta.Generation.valueOf(arguments[0]);
         } catch (IllegalArgumentException exception) {
             BooksWithoutBorders.sendErrorMessage(player, "Invalid book generation specified!");
             return false;
@@ -60,8 +60,8 @@ public class CommandSetGeneration implements TabExecutor {
     @Nullable
     @Override
     public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
-                                      @NotNull String[] args) {
+                                      @NotNull String[] arguments) {
-        if (args.length == 1) {
+        if (arguments.length == 1) {
             List<String> generations = new ArrayList<>();
             for (BookMeta.Generation generation : BookMeta.Generation.values()) {
                 generations.add(generation.name());

src/main/java/net/knarcraft/bookswithoutborders/command/CommandSetLore.java

@@ -24,13 +24,14 @@ import java.util.List;
 public class CommandSetLore implements TabExecutor {
 
     @Override
-    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label, @NotNull String[] args) {
+    public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
+                             @NotNull String[] arguments) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
         }
 
-        if (args.length < 1) {
+        if (arguments.length < 1) {
             BooksWithoutBorders.sendErrorMessage(player, "Missing a command argument!");
             return false;
         }
@@ -42,7 +43,7 @@ public class CommandSetLore implements TabExecutor {
         }
 
         //Treat all arguments as lore input
-        String rawLore = String.join(" ", args);
+        String rawLore = String.join(" ", arguments);
 
         //Format lore
         rawLore = ColorHelper.translateColorCodes(rawLore, ColorConversion.RGB);
@@ -62,7 +63,8 @@ public class CommandSetLore implements TabExecutor {
     }
 
     @Override
-    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias, String[] args) {
+    public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
+                                      @NotNull String[] arguments) {
         //TODO: Figure out if there is a better way to display that an argument is required
         List<String> options = new ArrayList<>();
         options.add("<new lore>");

src/main/java/net/knarcraft/bookswithoutborders/command/CommandSetTitle.java

@@ -24,13 +24,13 @@ public class CommandSetTitle implements TabExecutor {
 
     @Override
     public boolean onCommand(@NotNull CommandSender sender, @NotNull Command command, @NotNull String label,
-                             @NotNull String[] args) {
+                             @NotNull String[] arguments) {
         if (!(sender instanceof Player player)) {
             BooksWithoutBorders.sendErrorMessage(sender, "This command can only be used by a player!");
             return false;
         }
 
-        if (args.length < 1) {
+        if (arguments.length < 1) {
             BooksWithoutBorders.sendErrorMessage(player, "Too few command arguments!");
             return false;
         }
@@ -41,7 +41,7 @@ public class CommandSetTitle implements TabExecutor {
             return false;
         }
 
-        String title = String.join(" ", args);
+        String title = String.join(" ", arguments);
         title = ColorHelper.translateColorCodes(title, ColorConversion.RGB);
 
         ItemMeta itemMetadata = heldItem.getItemMeta();
@@ -69,7 +69,7 @@ public class CommandSetTitle implements TabExecutor {
 
     @Override
     public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-                                      @NotNull String[] args) {
+                                      @NotNull String[] arguments) {
         List<String> options = new ArrayList<>();
         options.add("<new title>");
         return options;

src/main/java/net/knarcraft/bookswithoutborders/command/CommandUnSign.java

@@ -90,8 +90,9 @@ public class CommandUnSign implements TabExecutor {
     }
 
     @Override
+    @NotNull
     public List<String> onTabComplete(@NotNull CommandSender sender, @NotNull Command command, @NotNull String alias,
-                                      @NotNull String[] args) {
+                                      @NotNull String[] arguments) {
         return new ArrayList<>();
     }
 

src/main/java/net/knarcraft/bookswithoutborders/config/BooksWithoutBordersConfig.java

@@ -6,6 +6,7 @@ import net.md_5.bungee.api.ChatColor;
 import org.bukkit.Material;
 import org.bukkit.command.ConsoleCommandSender;
 import org.bukkit.configuration.Configuration;
+import org.jetbrains.annotations.NotNull;
 
 import java.nio.file.FileSystems;
 import java.util.ArrayList;
@@ -48,7 +49,7 @@ public class BooksWithoutBordersConfig {
      *
      * @param booksWithoutBorders <p>The books without borders object used for getting required data</p>
      */
-    public static void initialize(BooksWithoutBorders booksWithoutBorders) {
+    public static void initialize(@NotNull BooksWithoutBorders booksWithoutBorders) {
         if (isInitialized) {
             throw new IllegalArgumentException("Settings class initialized twice. This should not happen!");
         }
@@ -388,7 +389,7 @@ public class BooksWithoutBordersConfig {
      * @param configOption <p>The configuration option to get the value for</p>
      * @return <p>The value of the option</p>
      */
-    private static String getString(Configuration config, ConfigOption configOption) {
+    private static String getString(@NotNull Configuration config, @NotNull ConfigOption configOption) {
         return config.getString(configOption.getConfigNode(), (String) configOption.getDefaultValue());
     }
 
@@ -399,7 +400,7 @@ public class BooksWithoutBordersConfig {
      * @param configOption <p>The configuration option to get the value for</p>
      * @return <p>The value of the option</p>
      */
-    private static boolean getBoolean(Configuration config, ConfigOption configOption) {
+    private static boolean getBoolean(@NotNull Configuration config, @NotNull ConfigOption configOption) {
         return config.getBoolean(configOption.getConfigNode(), (Boolean) configOption.getDefaultValue());
     }
 

src/main/java/net/knarcraft/bookswithoutborders/config/ConfigOption.java

@@ -1,5 +1,7 @@
 package net.knarcraft.bookswithoutborders.config;
 
+import org.jetbrains.annotations.NotNull;
+
 /**
  * A representation of the different available config options
  */
@@ -90,7 +92,7 @@ public enum ConfigOption {
      * @param configNode   <p>The config node in the config file this option represents</p>
      * @param defaultValue <p>The default value for this config option</p>
      */
-    ConfigOption(String configNode, Object defaultValue) {
+    ConfigOption(@NotNull String configNode, @NotNull Object defaultValue) {
         this.configNode = configNode;
         this.defaultValue = defaultValue;
     }

src/main/java/net/knarcraft/bookswithoutborders/encryption/AES.java

@@ -1,6 +1,8 @@
 package net.knarcraft.bookswithoutborders.encryption;
 
 import net.knarcraft.bookswithoutborders.BooksWithoutBorders;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import javax.crypto.BadPaddingException;
 import javax.crypto.Cipher;
@@ -52,7 +54,8 @@ public class AES {
      * @param encrypt  <p>Whether to encrypt or decrypt the input</p>
      * @return <p>The encrypted/decrypted input, or null if anything went wrong</p>
      */
-    public String encryptDecryptText(String input, String password, boolean encrypt) {
+    @Nullable
+    public String encryptDecryptText(@NotNull String input, @NotNull String password, boolean encrypt) {
         //Make a key from the password
         SecretKeySpec secretKeySpec = getKeyFromPassword(password);
         //Get cipher instance
@@ -102,7 +105,7 @@ public class AES {
      * @param encryption <p>Whether the input should be encrypted or decrypted</p>
      * @return <p>The input in byte format</p>
      */
-    private byte[] getInputBytes(String input, boolean encryption) {
+    private byte[] getInputBytes(@NotNull String input, boolean encryption) {
         if (encryption) {
             return input.getBytes();
         } else {
@@ -117,6 +120,7 @@ public class AES {
      * @param encryption <p>Whether the output came from encryption or decryption</p>
      * @return <p>The output as a string</p>
      */
+    @NotNull
     private String createResult(byte[] output, boolean encryption) {
         if (encryption) {
             return Base64.getEncoder().encodeToString(output);
@@ -130,6 +134,7 @@ public class AES {
      *
      * @return <p>An AES cipher instance, or null if something went wrong</p>
      */
+    @Nullable
     private Cipher getAESCipher() {
         Cipher aes;
         try {
@@ -147,7 +152,8 @@ public class AES {
      * @param password <p>A user supplied password</p>
      * @return <p>A secret key spec or null if something went wrong</p>
      */
-    private SecretKeySpec getKeyFromPassword(String password) {
+    @Nullable
+    private SecretKeySpec getKeyFromPassword(@NotNull String password) {
         PBEKeySpec spec = new PBEKeySpec(password.toCharArray(), this.passwordSalt, 1000, 128);
         SecretKeyFactory keyFactory;
         try {

src/main/java/net/knarcraft/bookswithoutborders/encryption/GenenCrypt.java

@@ -1,5 +1,7 @@
 package net.knarcraft.bookswithoutborders.encryption;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Random;
@@ -24,7 +26,7 @@ public class GenenCrypt {
      *
      * @param key <p>The key used to generate the codon table</p>
      */
-    public GenenCrypt(String key) {
+    public GenenCrypt(@NotNull String key) {
 
         // define the initial, unshuffled codon list of 4 base codons
         ArrayList<String> originalCodonList = new ArrayList<>();
@@ -114,7 +116,8 @@ public class GenenCrypt {
      * @param input <p>The input to encrypt</p>
      * @return <p>The encrypted input</p>
      */
-    public String encrypt(String input) {
+    @NotNull
+    public String encrypt(@NotNull String input) {
         StringBuilder output = new StringBuilder();
         for (int i = 0; i < input.length(); i++) {
             // insert junk bases
@@ -147,7 +150,8 @@ public class GenenCrypt {
      * @param input <p>The input to decrypt</p>
      * @return <p>The decrypted input</p>
      */
-    public String decrypt(String input) {
+    @NotNull
+    public String decrypt(@NotNull String input) {
         StringBuilder output = new StringBuilder();
         int keyCount = 0;
         int junk = key.charAt(0);

src/main/java/net/knarcraft/bookswithoutborders/encryption/SubstitutionCipher.java

@@ -1,5 +1,7 @@
 package net.knarcraft.bookswithoutborders.encryption;
 
+import org.jetbrains.annotations.NotNull;
+
 import java.math.BigInteger;
 import java.util.StringTokenizer;
 
@@ -17,9 +19,10 @@ public class SubstitutionCipher {
     // using a string for the key, it is converted
     // a series of offsets that each character in the
     // original message is offset by
-    public String encrypt(String in, String key) {
+    @NotNull
+    public String encrypt(@NotNull String in, @NotNull String key) {
         StringBuilder output = new StringBuilder();
-        if (key != null && !key.isEmpty()) {
+        if (!key.isEmpty()) {
             StringTokenizer tokenizer = new StringTokenizer(key, ", "); // tokenizes the key
             // converts each number in the key to an integer and adds to an array
             int[] offsetArray = new int[tokenizer.countTokens()];
@@ -53,9 +56,10 @@ public class SubstitutionCipher {
     // method with a flag for encryption / decryption, but
     // I'm lazy.
     @SuppressWarnings("unused")
-    public String decrypt(String in, String key) {
+    @NotNull
+    public String decrypt(@NotNull String in, @NotNull String key) {
         StringBuilder output = new StringBuilder();
-        if (key != null && !key.isEmpty()) {
+        if (!key.isEmpty()) {
             StringTokenizer tokenizer = new StringTokenizer(key, ", "); // tokenizes the key
             // converts each number in the key to an integer and adds to an array
             int[] offsetArray = new int[tokenizer.countTokens()];
@@ -81,7 +85,8 @@ public class SubstitutionCipher {
     // encryption works just like decryption, but is
     // vulnerable if the same key is used more than once.
     @SuppressWarnings("unused")
-    public String oneTimePad(String in, String key) {
+    @NotNull
+    public String oneTimePad(@NotNull String in, @NotNull String key) {
         StringBuilder output = new StringBuilder();
         for (int i = 0; i < in.length(); i++) {
             output.append((char) (in.charAt(i) ^ key.charAt(i % key.length())));

src/main/java/net/knarcraft/bookswithoutborders/gui/AuthorBookIndex.java

@@ -53,6 +53,7 @@ public class AuthorBookIndex extends BookIndex {
 
         // Display the list of books, with the next and previous buttons
         displayPreviousButton(componentBuilder, command + " author" + authorName, page);
+        componentBuilder.append("\n");
         displayBookList(componentBuilder, command, page, availableBooks);
         displayNextButton(componentBuilder, command + " author" + authorName, page, totalPages);
 

src/main/java/net/knarcraft/bookswithoutborders/gui/BookIndex.java

@@ -83,6 +83,7 @@ public abstract class BookIndex {
      */
     protected static void displayAlphabetIndex(@NotNull ComponentBuilder componentBuilder,
                                                @NotNull String command, @NotNull Map<Character, Integer> firstInstances) {
+        componentBuilder.append("[index] <", ComponentBuilder.FormatRetention.NONE);
         for (int characterIndex = 0; characterIndex <= 25; characterIndex++) {
             char character = (char) ('a' + characterIndex);
             if (firstInstances.containsKey(character)) {
@@ -95,7 +96,7 @@ public abstract class BookIndex {
                 componentBuilder.append(character + "", ComponentBuilder.FormatRetention.NONE).color(ChatColor.GRAY);
             }
         }
-        componentBuilder.append("\n");
+        componentBuilder.append(">", ComponentBuilder.FormatRetention.NONE);
     }
 
     /**
@@ -115,7 +116,6 @@ public abstract class BookIndex {
         } else {
             componentBuilder.append("Previous [<]", ComponentBuilder.FormatRetention.NONE).color(ChatColor.GRAY);
         }
-        componentBuilder.append("\n");
     }
 
     /**

src/main/java/net/knarcraft/bookswithoutborders/gui/PagedBookIndex.java

@@ -63,11 +63,9 @@ public class PagedBookIndex extends BookIndex {
                                      @NotNull Map<Character, Integer> firstInstances) {
         ComponentBuilder componentBuilder = new ComponentBuilder();
 
-        // Display the alphabet index as the header
-        displayAlphabetIndex(componentBuilder, command, firstInstances);
-
         // Display the list of books, with the next and previous buttons
         displayPreviousButton(componentBuilder, command, page);
+        componentBuilder.append("\n");
         displayBookList(componentBuilder, command, page, availableBooks);
         displayNextButton(componentBuilder, command, page, totalPages);
 
@@ -76,6 +74,10 @@ public class PagedBookIndex extends BookIndex {
         displayTotalPages(componentBuilder, page, totalPages);
         componentBuilder.append("   ", ComponentBuilder.FormatRetention.NONE);
         displayPageCommand(componentBuilder, command, page);
+        componentBuilder.append("\n");
+
+        // Display the alphabet index as the header
+        displayAlphabetIndex(componentBuilder, command, firstInstances);
         sender.spigot().sendMessage(componentBuilder.create());
     }
 

src/main/java/net/knarcraft/bookswithoutborders/listener/BookEventListener.java

@@ -5,6 +5,7 @@ import net.knarcraft.bookswithoutborders.utility.BookFormatter;
 import org.bukkit.event.EventHandler;
 import org.bukkit.event.Listener;
 import org.bukkit.event.player.PlayerEditBookEvent;
+import org.jetbrains.annotations.NotNull;
 
 /**
  * A listener for listening to book events
@@ -14,7 +15,7 @@ import org.bukkit.event.player.PlayerEditBookEvent;
 public class BookEventListener implements Listener {
 
     @EventHandler
-    public void onBookSign(PlayerEditBookEvent event) {
+    public void onBookSign(@NotNull PlayerEditBookEvent event) {
         if (event.isCancelled() || !event.isSigning() || !BooksWithoutBordersConfig.formatBooks()) {
             return;
         }

src/main/java/net/knarcraft/bookswithoutborders/listener/PlayerEventListener.java

@@ -17,6 +17,8 @@ import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.PlayerInventory;
 import org.bukkit.inventory.meta.BookMeta;
 import org.bukkit.inventory.meta.ItemMeta;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.io.File;
 import java.util.logging.Level;
@@ -29,7 +31,7 @@ public class PlayerEventListener implements Listener {
     private final BooksWithoutBorders booksWithoutBorders = BooksWithoutBorders.getInstance();
 
     @EventHandler
-    public void onHold(PlayerItemHeldEvent event) {
+    public void onHold(@NotNull PlayerItemHeldEvent event) {
         if (event.isCancelled()) {
             return;
         }
@@ -54,7 +56,7 @@ public class PlayerEventListener implements Listener {
     }
 
     @EventHandler
-    public void onPlayerJoin(PlayerJoinEvent event) {
+    public void onPlayerJoin(@NotNull PlayerJoinEvent event) {
         Player player = event.getPlayer();
 
         //If a book directory exists with this player's name, move it to this player's UUID
@@ -82,13 +84,17 @@ public class PlayerEventListener implements Listener {
         ItemStack offHandItem = InventoryHelper.getHeldItem(player, false);
         if (mainHandItem.getType() == Material.WRITTEN_BOOK) {
             ItemMeta itemMetadata = mainHandItem.getItemMeta();
+            if (itemMetadata != null) {
                 updateBookInHand(player, itemMetadata, true);
             }
+        }
         if (offHandItem.getType() == Material.WRITTEN_BOOK) {
             ItemMeta itemMetadata = offHandItem.getItemMeta();
+            if (itemMetadata != null) {
                 updateBookInHand(player, itemMetadata, false);
             }
         }
+    }
 
     /**
      * Gives a book to a player joining for the first time
@@ -98,7 +104,7 @@ public class PlayerEventListener implements Listener {
      * @param sendMessage <p>Whether to send a message to the joining player</p>
      * @return <p>True if a message has yet to be sent</p>
      */
-    private boolean giveBookToNewPlayer(String bookName, Player player, boolean sendMessage) {
+    private boolean giveBookToNewPlayer(@NotNull String bookName, @NotNull Player player, boolean sendMessage) {
         if (!bookName.trim().isEmpty()) {
 
             //Give the book to the player if it exists
@@ -125,7 +131,7 @@ public class PlayerEventListener implements Listener {
      * @param itemMetadata <p>Information about the held book</p>
      * @param mainHand     <p>Whether to update the book in the player's main hand</p>
      */
-    private void updateBookInHand(Player player, ItemMeta itemMetadata, boolean mainHand) {
+    private void updateBookInHand(@NotNull Player player, @NotNull ItemMeta itemMetadata, boolean mainHand) {
         PlayerInventory playerInventory = player.getInventory();
         ItemStack updatedBook = updateBook(player, (BookMeta) itemMetadata);
         if (updatedBook != null) {
@@ -144,7 +150,8 @@ public class PlayerEventListener implements Listener {
      * @param oldBook <p>Metadata about the held book</p>
      * @return <p>An updated book</p>
      */
-    public ItemStack updateBook(Player player, BookMeta oldBook) {
+    @Nullable
+    public ItemStack updateBook(@NotNull Player player, @NotNull BookMeta oldBook) {
         //handles hacked title-less books
         if (oldBook.getTitle() == null || oldBook.getTitle().length() < 3) {
             return null;

src/main/java/net/knarcraft/bookswithoutborders/listener/SignEventListener.java

@@ -25,6 +25,8 @@ import org.bukkit.inventory.EquipmentSlot;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.PlayerInventory;
 import org.bukkit.inventory.meta.BookMeta;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 import java.io.File;
 
@@ -40,7 +42,7 @@ public class SignEventListener implements Listener {
     private final String slash = getSlash();
 
     @EventHandler
-    public void onSignChange(SignChangeEvent event) {
+    public void onSignChange(@NotNull SignChangeEvent event) {
         if (event.isCancelled()) {
             return;
         }
@@ -80,7 +82,7 @@ public class SignEventListener implements Listener {
     }
 
     @EventHandler
-    public void onClick(PlayerInteractEvent event) {
+    public void onClick(@NotNull PlayerInteractEvent event) {
         if (event.getClickedBlock() == null) {
             return;
         }
@@ -139,7 +141,8 @@ public class SignEventListener implements Listener {
      * @param player       <p>The player which clicked the sign</p>
      * @param hand         <p>The EquipmentSlot of the used hand</p>
      */
-    private void decryptHeldBookUsingSign(Sign sign, Material heldItemType, Player player, EquipmentSlot hand) {
+    private void decryptHeldBookUsingSign(@NotNull Sign sign, @NotNull Material heldItemType, @NotNull Player player,
+                                          @NotNull EquipmentSlot hand) {
         //Decrypt the held book and replace it
         if (heldItemType == Material.WRITTEN_BOOK) {
             player.closeInventory();
@@ -162,7 +165,8 @@ public class SignEventListener implements Listener {
      * @param sign <p>The sign to check</p>
      * @return <p>The color of the sign</p>
      */
-    private ChatColor getSignLine2Color(Sign sign) {
+    @Nullable
+    private ChatColor getSignLine2Color(@NotNull Sign sign) {
         String line = sign.getSide(Side.FRONT).getLine(2);
         if (!BookFormatter.stripColor(line).equals(line)) {
             return ChatColor.getByChar(sign.getSide(Side.FRONT).getLine(2).substring(1, 2).charAt(0));
@@ -180,7 +184,8 @@ public class SignEventListener implements Listener {
      * @param color      <p>The color to match</p>
      * @return <p>True if the given string is what's on the sign</p>
      */
-    private boolean signLineEquals(Sign sign, int lineNumber, String compareTo, ChatColor color) {
+    private boolean signLineEquals(@NotNull Sign sign, int lineNumber, @NotNull String compareTo,
+                                   @NotNull ChatColor color) {
         String line = sign.getSide(Side.FRONT).getLine(lineNumber);
         return line.equalsIgnoreCase(color + compareTo);
     }
@@ -192,7 +197,8 @@ public class SignEventListener implements Listener {
      * @param lines  <p>The lines on the sign</p>
      * @param player <p>The player which edited the sign</p>
      */
-    private void generateGiveSign(SignChangeEvent event, String[] lines, Player player) {
+    private void generateGiveSign(@NotNull SignChangeEvent event, @NotNull String[] lines,
+                                  @NotNull Player player) {
         if (lines[2].length() > 13 || lines[3].length() > 13) {
             BooksWithoutBorders.sendErrorMessage(player,
                     "[Give] signs' 3rd and 4th lines must be 13 characters or less!");
@@ -221,7 +227,7 @@ public class SignEventListener implements Listener {
      * @param event   <p>The event causing the creation of the give sign</p>
      * @param isValid <p>Whether the created sign is valid</p>
      */
-    private void markGiveSignValidity(SignChangeEvent event, boolean isValid) {
+    private void markGiveSignValidity(@NotNull SignChangeEvent event, boolean isValid) {
         String[] lines = event.getLines();
         if (isValid) {
             event.setLine(2, ChatColor.DARK_GREEN + lines[2]);
@@ -240,7 +246,8 @@ public class SignEventListener implements Listener {
      * @param heldItem <p>The type of the held book</p>
      * @param hand     <p>The hand the player is using to hold the book</p>
      */
-    private void decryptBook(BookMeta oldBook, Player player, ItemStack heldItem, EquipmentSlot hand) {
+    private void decryptBook(@NotNull BookMeta oldBook, @NotNull Player player, @NotNull ItemStack heldItem,
+                             @NotNull EquipmentSlot hand) {
         ItemStack newBook;
 
         //Check if the book is encrypted by Books Without Borders
@@ -286,7 +293,8 @@ public class SignEventListener implements Listener {
      * @param player       <p>The player which clicked the sign</p>
      * @param hand         <p>The EquipmentSlot of the used hand</p>
      */
-    private void encryptHeldBookUsingSign(Sign sign, Material heldItemType, Player player, EquipmentSlot hand) {
+    private void encryptHeldBookUsingSign(@NotNull Sign sign, @NotNull Material heldItemType, @NotNull Player player,
+                                          @NotNull EquipmentSlot hand) {
         ItemStack eBook;
         String[] lines = sign.getSide(Side.FRONT).getLines();
         boolean mainHand = hand == EquipmentSlot.HAND;
@@ -306,7 +314,7 @@ public class SignEventListener implements Listener {
      * @param sign   <p>The sign the user clicked</p>
      * @param player <p>The player which clicked the sign</p>
      */
-    private void giveBook(Sign sign, Player player) {
+    private void giveBook(@NotNull Sign sign, @NotNull Player player) {
         String fileName = BookFormatter.stripColor(sign.getSide(Side.FRONT).getLine(2));
         boolean isLoadListNumber = false;
 

src/main/java/net/knarcraft/bookswithoutborders/state/BookDirectory.java

@@ -1,5 +1,8 @@
 package net.knarcraft.bookswithoutborders.state;
 
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
 /**
  * This enum represents the different directories books can be saved in
  */
@@ -26,7 +29,8 @@ public enum BookDirectory {
      * @param directory <p>The directory to transform</p>
      * @return <p>A book directory, or null if the given directory is empty</p>
      */
-    public static BookDirectory getFromString(String directory) {
+    @Nullable
+    public static BookDirectory getFromString(@NotNull String directory) {
         if (directory.equalsIgnoreCase("public")) {
             return BookDirectory.PUBLIC;
         } else if (directory.equalsIgnoreCase("player")) {

src/main/java/net/knarcraft/bookswithoutborders/state/EncryptionStyle.java

@@ -1,5 +1,7 @@
 package net.knarcraft.bookswithoutborders.state;
 
+import org.jetbrains.annotations.NotNull;
+
 /**
  * This enum represents the different available encryption styles
  */
@@ -10,7 +12,7 @@ public enum EncryptionStyle {
 
     private final String name;
 
-    EncryptionStyle(String name) {
+    EncryptionStyle(@NotNull String name) {
         this.name = name;
     }
 
@@ -20,7 +22,8 @@ public enum EncryptionStyle {
      * @param name <p>The name of the encryption style</p>
      * @return <p>An encryption style or null if no match is found</p>
      */
-    public static EncryptionStyle getFromString(String name) {
+    @NotNull
+    public static EncryptionStyle getFromString(@NotNull String name) {
         for (EncryptionStyle style : EncryptionStyle.values()) {
             if (style.name.equalsIgnoreCase(name)) {
                 return style;

src/main/java/net/knarcraft/bookswithoutborders/utility/BookFormatter.java

@@ -25,7 +25,7 @@ public final class BookFormatter {
      *
      * @param rawPages <p>A list of pages</p>
      */
-    public static void formatLastPage(List<String> rawPages) {
+    public static void formatLastPage(@NotNull List<String> rawPages) {
         int maxPageText = 256;
         int fitsNewline = maxPageText - 2;
 
@@ -50,7 +50,7 @@ public final class BookFormatter {
      * @param maxPageText <p>The max number of characters which fit on a page</p>
      * @param fitsNewline <p>The max number of characters on a page which still fits a newline character</p>
      */
-    public static void formatLastPageSplitOverflow(List<String> rawPages, int maxPageText, int fitsNewline) {
+    public static void formatLastPageSplitOverflow(@NotNull List<String> rawPages, int maxPageText, int fitsNewline) {
         while (rawPages.get(rawPages.size() - 1).length() > maxPageText) {
             int splitPosition;
             String fittingText = rawPages.get(rawPages.size() - 1).substring(0, maxPageText);
@@ -78,7 +78,7 @@ public final class BookFormatter {
      * @param rawPages    <p>The raw pages to format</p>
      * @param maxPageText <p>The max number of characters which fit on a page</p>
      */
-    public static void formatLastPageCombinePages(List<String> rawPages, int maxPageText) {
+    public static void formatLastPageCombinePages(@NotNull List<String> rawPages, int maxPageText) {
         int lastPageIndex = rawPages.size() - 1;
         int nextToLastIndex = rawPages.size() - 2;
         if (rawPages.get(nextToLastIndex).length() + rawPages.get(lastPageIndex).length() <= maxPageText) {
@@ -93,7 +93,7 @@ public final class BookFormatter {
      * @param rawPages    <p>The raw pages to format</p>
      * @param fitsNewline <p>The max number of characters on a page which still fits a newline character</p>
      */
-    public static void formatLastPageAddNewline(List<String> rawPages, int fitsNewline) {
+    public static void formatLastPageAddNewline(@NotNull List<String> rawPages, int fitsNewline) {
         int pageIndex = rawPages.size() - 1;
         if (rawPages.get(pageIndex).length() <= fitsNewline && !rawPages.get(pageIndex).isEmpty()) {
             rawPages.set(pageIndex, (rawPages.get(pageIndex)) + "\n");
@@ -106,7 +106,8 @@ public final class BookFormatter {
      * @param bookMeta <p>The book meta to change</p>
      * @return <p>The changed book meta</p>
      */
-    public static BookMeta formatPages(BookMeta bookMeta) {
+    @NotNull
+    public static BookMeta formatPages(@NotNull BookMeta bookMeta) {
         List<String> formattedPages = new ArrayList<>(Objects.requireNonNull(bookMeta).getPageCount());
         for (String page : bookMeta.getPages()) {
             formattedPages.add(ColorHelper.translateColorCodes(page, ColorConversion.RGB));

src/main/java/net/knarcraft/bookswithoutborders/utility/BookHelper.java

@@ -53,6 +53,7 @@ public final class BookHelper {
      * @param sender        <p>The command sender trying to get the directory</p>
      * @return <p>The path of the directory, or null if not possible to get</p>
      */
+    @Nullable
     public static File getBookDirectoryPath(@NotNull BookDirectory bookDirectory, @NotNull CommandSender sender) {
         String bookFolderString = getBookDirectoryPathString(bookDirectory, sender);
         if (bookFolderString == null) {

src/main/java/net/knarcraft/bookswithoutborders/utility/BookLoader.java

@@ -53,6 +53,10 @@ public final class BookLoader {
     public static ItemStack loadBook(@NotNull CommandSender sender, @NotNull String fileName, @NotNull String isSigned,
                                      @NotNull String directory, int numCopies) {
         BookDirectory bookDirectory = BookDirectory.getFromString(directory);
+        if (bookDirectory == null) {
+            BooksWithoutBorders.sendErrorMessage(sender, "Unrecognized book directory!");
+            return null;
+        }
 
         //Find the filename if a book index is given
         try {
@@ -142,7 +146,9 @@ public final class BookLoader {
      * @param directory     <p>The relative directory given</p>
      * @return <p>A file or null if it does not exist</p>
      */
-    private static File getFullPath(CommandSender sender, String fileName, BookDirectory bookDirectory, String directory) {
+    @Nullable
+    private static File getFullPath(@NotNull CommandSender sender, @NotNull String fileName,
+                                    @NotNull BookDirectory bookDirectory, @NotNull String directory) {
         File file;
         String slash = BooksWithoutBordersConfig.getSlash();
         String bookFolder = BooksWithoutBordersConfig.getBookFolder();

src/main/java/net/knarcraft/bookswithoutborders/utility/IntegerToRomanConverter.java

@@ -86,6 +86,7 @@ public final class IntegerToRomanConverter {
      * @param times     <p>The number of times to repeat the character</p>
      * @return <p>The repeated string</p>
      */
+    @NotNull
     private static String repeat(char character, int times) {
         return String.valueOf(character).repeat(Math.max(0, times));
     }

src/test/java/net/knarcraft/bookswithoutborders/encryption/AESTest.java

@@ -3,6 +3,7 @@ package net.knarcraft.bookswithoutborders.encryption;
 import org.junit.Test;
 
 import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNotSame;
 
 public class AESTest {
@@ -16,6 +17,7 @@ public class AESTest {
 
         String encrypted = aes.encryptDecryptText(plainText, password, true);
         assertNotSame(encrypted, plainText);
+        assertNotNull(encrypted);
         String decrypted = aes.encryptDecryptText(encrypted, password, false);
         assertEquals(plainText, decrypted);
     }