//Create LinkedHashMap
LinkedHashMap<String, Argument> arguments = new LinkedHashMap<>();

//Add an argument called "target", which is a PlayerArgument
arguments.put("target", new PlayerArgument());

LinkedHashMap<String, ArgumentType> arguments = new LinkedHashMap<>();
arguments.put("arg0", new StringArgument());
arguments.put("arg1", new PotionEffectArgument());
arguments.put("arg2", new LocationArgument());

new CommandAPICommand("cmd")
    .withArguments(arguments)
    .executes((sender, args) -> {
        String stringArg = (String) args[0];
        PotionEffectType potionArg = (PotionEffectType) args[1];
        Location locationArg = (Location) args[2];
    })
	.register();

Argument overrideSuggestions(String... suggestions);
Argument overrideSuggestions(Function<CommandSender, 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));