LongCommandExecutor

import io.github.chi2l3s.nextlib.api.command.CommandRegistry;
import io.github.chi2l3s.nextlib.api.command.LongCommandExecutor;
import io.github.chi2l3s.nextlib.api.command.SubCommand;
import org.bukkit.command.CommandSender;
import org.bukkit.permissions.Permission;

public class ExampleCommand extends LongCommandExecutor {
  public ExampleCommand() {
    addSubCommand(new SubCommand() {
      @Override public void onExecute(CommandSender sender, String[] args) {
        sender.sendMessage("Sub executed");
      }
      @Override public java.util.List<String> onTabComplete(CommandSender s, String[] a) {
        return java.util.List.of("sub");
      }
    }, new String[]{"sub", "s"}, new Permission("example.use"));
  }
}

// Registration (plugin.yml must declare command 'example')
new CommandRegistry(this).registerCommand("example", new ExampleCommand());