Create a command

public class SimplePluginCommand extends PluginCommand<ExamplePlugin> {
 
    public SimplePluginCommand(ExamplePlugin owner) {
        super("simplecommand", owner);
        this.setDescription("Simple Command");
        this.setPermission("example.simplecommand");
    }
 
    @Override
    public boolean execute(CommandSender sender, String commandLabel, String[] args) {
        if (!this.testPermission(sender)) {
            return false;
        }
 
        if (sender instanceof Player player) {
            player.giveItem(ItemTypes.DIAMOND.createItem());
            player.sendMessage(TextFormat.AQUA + "You have been given an item created from item type!");
        }
 
        sender.sendMessage(TextFormat.GREEN + "Command successfully executed!");
        return true;
    }
}

public class RandomEffectsCommand extends PluginCommand<ExamplePlugin> {
 
    private final Random random = new Random();
 
    public RandomEffectsCommand(ExamplePlugin owner) {
        super("randomeffects", owner);
        this.setDescription("Give to player random effects");
        // Adding command parameters
        this.getCommandParameters().clear();
        this.addCommandParameters("default", new CommandParameter[]{
            CommandParameter.newType("player", CommandParamType.TARGET, new PlayersNode())
        });
        // Enable param tree parsing (if used new command api)
        this.enableParamTree();
    }
 
    @Override
    public int execute(CommandSender sender, String commandLabel, Map.Entry<String, ParamList> result, CommandLogger log) {
        ParamList list = result.getValue();
 
        List<Player> players = list.getResult(0);
        if (players.isEmpty()) {
            log.addNoTargetMatch().output();
            return 0;
        }
 
        // Get random effect type and create effect instance
        EffectType effectType = EffectType.get(random.nextInt(1, 31));
        Effect effect = Effect.get(effectType)
            .setAmplifier(random.nextInt(1, 3))
            .setDuration(random.nextInt(20, 1200));
 
        for (Player player : players) {
            player.addEffect(effect);
        }
 
        log.addSuccess(TextFormat.GREEN + "Random effect has been given to " + players.size() + " players!").output();
        return 1;
    }
}

@Override
public void onEnable() {
    this.getServer().getCommandMap().register("ExamplePlugin", new SimplePluginCommand(this));
    this.getServer().getCommandMap().register("ExamplePlugin", new RandomEffectsCommand(this));
}