Argument overrideSuggestions(String... suggestions);
Argument overrideSuggestions(Function<CommandSender, String[]> suggestions);
Argument overrideSuggestions(BiFunction<CommandSender, Object[], String[]> suggestions);

Example - Teleport to worlds by overriding suggestions

Say we're creating a plugin with the ability to teleport to different worlds on the server. If we were to retrieve a list of worlds, we would be able to override the suggestions of a typical StringArgument to teleport to that world. Let's create a command with the following structure:

/tpworld <world>

We then implement our world teleporting command using overrideSuggestions() on the StringArgument to provide a list of worlds to teleport to:

//Populate a String[] with the names of worlds on the server
String[] worlds = Bukkit.getWorlds().stream().map(World::getName).toArray(String[]::new);

//Override the suggestions of the StringArgument with the aforementioned String[]
LinkedHashMap<String, ArgumentType> arguments = new LinkedHashMap<>();
arguments.put("world", new StringArgument().overrideSuggestions(worlds));

new CommandAPICommand("tpworld")
    .withArguments(arguments)
    .executesPlayer((player, args) -> {
       	String world = (String) args[0];
		player.teleport(Bukkit.getWorld(world).getSpawnLocation());
    })
    .register();

Example - Friend list by overriding suggestions

Say you have a plugin which has a "friend list" for players. If you want to teleport to a friend in that list, you could use a PlayerArgument, which has the list of suggestions overridden with the list of friends that that player has. Since the list of friends depends on the sender, we can use the function to determine what our suggestions should be. Let's use the following command to teleport to a friend from our friend list:

/friendtp <friend>

Let's say we have a simple class to get the friends of a command sender:

public class Friends {
    public static String[] getFriends(CommandSender sender) {
        if(sender instanceof Player) {
            return //Look up friends in a database or file
        } else {
            return new String[0];
        }
    }
}

We can then use this to generate our suggested list of friends:

LinkedHashMap<String, ArgumentType> arguments = new LinkedHashMap<>();
arguments.put("friend", new PlayerArgument().overrideSuggestions((sender) -> {
    Friends.getFriends(sender);
}));

new CommandAPICommand("friendtp")
    .withArguments(arguments)
    .executesPlayer((player, args) -> {
       	Player target = (Player) args[0];
		player.teleport(target);
    })
    .register();

Developer's Note:

The syntax of inlining the .overrideSuggestions() method has been designed to work well with Java's lambdas. For example, we could write the above code more consisely, such as:

LinkedHashMap<String, ArgumentType> arguments = new LinkedHashMap<>();
arguments.put("friend", new PlayerArgument().overrideSuggestions(Friends::getFriends));

/mycommand arg1 arg2 arg3

/execute run mycommand <suggestions>

Example - Sending a message to a nearby player

Say we wanted to create a command that lets you send a message to a specific player in a given radius. (This is a bit of a contrived example, but let's roll with it). To do this, we'll use the following command structure:

/localmsg <radius> <target> <message>

When run, this command will send a message to a target player within the provided radius. To help identify which players are within a radius, we can override the suggestions on the <target> argument to include a list of players within the provided radius. We do this with the following code:

// Declare our arguments as normal
LinkedHashMap<String, Argument> arguments = new LinkedHashMap<>();
arguments.put("radius", new IntegerArgument());

// Override the suggestions for the PlayerArgument, using (sender, args) as the parameters
// sender refers to the command sender that is running this command
// args refers to the Object[] of PREVIOUSLY DECLARED arguments (in this case, the IntegerArgument radius)
arguments.put("target", new PlayerArgument().overrideSuggestions((sender, args) -> {

    // Cast the first argument (radius, which is an IntegerArgument) to get its value
	int radius = (int) args[0];
	
    // Get nearby entities within the provided radius
	Player player = (Player) sender;
	Collection<Entity> entities = player.getWorld().getNearbyEntities(player.getLocation(), radius, radius, radius);
	
    // Get player names within that radius
	return entities.stream()
		.filter(e -> e.getType() == EntityType.PLAYER)
		.map(Entity::getName)
		.toArray(String[]::new);
}));
arguments.put("message", new GreedyStringArgument());

// Declare our command as normal
new CommandAPICommand("localmsg")
	.withArguments(arguments)
	.executesPlayer((player, args) -> {
		Player target = (Player) args[1];
		String message = (String) args[2];
		target.sendMessage(message);
	})
	.register();

As shown in this code, we use the (sender, args) -> ... lambda to override suggestions with previously declared arguments. In this example, our variable args will be { int }, where this int refers to the radius. Note how this object array only has the previously declared arguments (and not for example { int, Player, String }).