Custom arguments

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

The CustomArgument<T> has two constructors, declared as follows:

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

The first argument is the CustomArgumentFunction, which is a lambda that takes in a String and returns some custom object of type T. The first constructor will construct a CustomArgument which uses the StringArgument as a base (thus, only simple strings). The second argument has the field keyed. When this field is set to true, the CustomArgument will use a Minecraft key as a base, allowing you to use Minecraft keys as input.

Developer's Note:

I may have complicated this too much, so let me clarify what I mean. The CustomArgument constructor is of the following forms:

CustomArgument(nodeName, (String) -> { ... return T; });
CustomArgument(nodeName, (String) -> { ... return T; }, boolean keyed);

Both constructors take in a String as input and return T. When enabling keyed, it allows the input to be of the form of a Minecraft key, but doesn't change the input type.

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.

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 String 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) {

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

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 structure:

/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 overriding 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();