To register commands with the CommandAPI, there are several methods which can be used, depending on whether you want your command to have aliases or permissions in order to run it.

The following fields are as follows:

• String - The command name

  The first argument represents the command name which will be registered. For instance, to register the command /god, you would use the following:

  CommandAPI.getInstance().register("god", ...);
  

• LinkedHashMap<String, Argument> - The list of arguments

  The CommandAPI requires a list of arguments which are used for the command. The argument map consists of a key which is the tooltip that is displayed as a prompt to users entering commands, and a value which is an instance of an argument (See the section on arguments). This list of arguments is interpreted in the order that arguments are added to the LinkedHashMap.

• String[] - An array of aliases that the command can be run via

• CommandPermission - The required permission to execute a command. (See the section on permissions).

In order to register commands properly, commands must be registered before the server finishes loading. The CommandAPI will prevent command registration after the server has loaded. This basically means that all command registration must occur during a plugin's onLoad() or onEnable() method. With the CommandAPI, depending on which of these functions you load your commands is crutial if your plugin is used with Minecraft's functions.

The CommandAPI has support to unregister commands completely from Minecraft's command list. This includes Minecraft built in commands!

To replace a command, we can first unregister it and then register our implementation of that command.

//Unregister the gamemode command from the server (by force)
CommandAPI.getInstance().unregister("gamemode", true);

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

/* Arguments for the gamemode command. In this sample, I'm just 
 * using a simple literal argument which allows for /gamemode survival */
arguments.put("gamemode", new LiteralArgument("survival"));

CommandAPI.getInstance().register("gamemode", arguments, (sender, args) -> {
    //Implementation of our /gamemode command
});

Developer's Note:

Command unregistration, although powerful, is highly unrecommended. It is the CommandAPI's most "dangerous" feature as it can cause unexpected sideffects, such as command blocks executing commands you wouldn't expect them to. In almost every case, I'd recommend just creating a new command instead of unregistering one to replace it.

For instance, instead of unregistering /gamemode, you could register a command /gm or /customgamemode.