Custom arguments

Custom arguments are an experimental feature which the CommandAPI offers, which allows you to represent any String, or Minecraft key (Something of the form String:String, such as minecraft:diamond) with a custom parser. They basically represent StringArgument with replaced suggestions and a built-in parser for any object of your choice. They are designed to be used for multiple commands - you can define the argument once and can use it wherever you want when declaring commands.

The CustomArgument<T> has the following two constructors:

public CustomArgument(String nodeName, CustomArgumentInfoParser<T> parser);
public CustomArgument(String nodeName, CustomArgumentInfoParser<T> parser, boolean keyed);

There are effectively two forms that this can take:

A custom argument with a string-based parser

The simplest form requires the node name as per any other argument, and a parser which takes in as input a record of info and returns a custom object of your choice. For example, if you wanted to create a custom argument that represents a World, you can use this to return a Bukkit World object.

new CustomArgument(nodeName, inputInfo -> { 
    // code here
    return T; 

The CommandAPI will use an underlying StringArgument to parse this custom argument, so the limitations of string arguments will apply to this argument (it can only contain alphanumeric characters (A-Z, a-z and 0-9), and the underscore character (_)).

A custom argument with a parser that takes in a Minecraft Key

With the second constructor, if you provide true to the keyed field, the input can be of the form of a Minecraft key (so it can have : in the name).

Type params

The custom argument requires the type of the target object that the custom argument will return when parsing the arguments for a command. For instance, if you have a CustomArgument<Player>, then when parsing the arguments for the command, you would cast it to a Player object.

The CustomArgumentInfoParser class

To create a parser for a CustomArgument, you need to provide a CustomArgumentInfoParser function to the constructor. The CustomArgumentInfoParser class is a functional interface which accepts CustomArgumentInfo and returns T, an object of your choosing:

public static interface CustomArgumentInfoParser<T> {

    public T apply(CustomArgumentInfo info) throws CustomArgumentException;


The CustomArgumentInfo record is very similar to the SuggestionInfo record for declaring argument suggestions. This record contains the following methods:

public record CustomArgumentInfo {
    CommandSender sender();
    Object[] previousArgs(); 
    String input();

These fields are as follows:

CommandSender sender();

sender() represents the command sender that is typing the command. This is normally a Player, but can also be a console command sender if using a Paper server.

Object[] previousArgs();

previousArgs() represents a list of previously declared arguments, which are parsed and interpreted as if they were being used to execute the command.

String input();

input() represents the current input for the custom argument that the user has typed. For example, if a user is typing /mycommand hello and the first argument is a CustomArgument, the input() would return "hello".

Example - World argument

Say we want to create an argument to represents the list of available worlds on the server. We basically want to have an argument which always returns a Bukkit World object as the result. Here, we create a method worldArgument() that returns our custom argument that returns a World. First, we retrieve our String[] of world names to be used for our suggestions. We then write our custom argument that creates a World object from the input (in this case, we simply convert the input to a World using Bukkit.getWorld(String)). We perform error handling before returning our result:

// Function that returns our custom argument
public Argument worldArgument(String nodeName) {
    // Construct our CustomArgument that takes in a String input and returns a World object
    return new CustomArgument<World>(nodeName, info -> {
        // Parse the world from our input
        World world = Bukkit.getWorld(info.input());
        if(world == null) {
            throw new CustomArgumentException(new MessageBuilder("Unknown world: ").appendArgInput());
        } else {
            return world;
    }).replaceSuggestions(sender -> {
        // List of world names on the server
        return Bukkit.getWorlds().stream().map(World::getName).toArray(String[]::new);

In our error handling step, we check if the world is equal to null (since the Bukkit.getWorld(String) is @Nullable). To handle this case, we throw a CustomArgumentException with an error from a MessageBuilder. The CustomArgumentException has two constructors, so a message builder isn't required each time:

new CustomArgumentException(String message);
new CustomArgumentException(MessageBuilder message);

We can use our custom argument like any other argument. Say we wanted to write a command to teleport to a specific world. We will create a command of the following syntax:

/tpworld <world>

Since we have defined the method worldArgument() which automatically generates our argument, we can use it as follows:

new CommandAPICommand("tpworld")
    .executesPlayer((player, args) -> {
        player.teleport(((World) args[0]).getSpawnLocation());

By using a CustomArgument (as opposed to a simple StringArgument and replacing its suggestions), we are able to provide a much more powerful form of error handling (automatically handled inside the argument), and we can reuse this argument for other commands.

Message Builders

The MessageBuilder class is a class to easily create messages to describe errors when a sender sends a command which does not meet the expected syntax for an argument. It acts in a similar way to a StringBuilder, where you can append content to the end of a String.

The following methods are as follows:

appendArgInput()Appends the argument that failed that the sender submitted to the end of the builder. E.g. /foo bar will append bar
appendFullInput()Appends the full command that a sender submitted to the end of the builder. E.g. /foo bar will append foo bar
appendHere()Appends the text <--[HERE] to the end of the builder
append(Object)Appends an object to the end of the builder

Example - Message builder for invalid objective argument

To create a MessageBuilder, simply call its constructor and use whatever methods as you see fit. Unlike a StringBuilder, you don't have to "build" it when you're done - the CommandAPI does that automatically:

new MessageBuilder("Unknown world: /").appendFullInput().appendHere();