rubukkit/t/190921

package direct.derekunavailable.untitled;

import org.bukkit.ChatColor;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.EventPriority;
import org.bukkit.event.Listener;
import org.bukkit.event.entity.EntityDamageByEntityEvent;
import org.bukkit.plugin.java.JavaPlugin;

import java.util.HashSet;
import java.util.UUID;

public final class Untitled extends JavaPlugin implements Listener {

    private final HashSet<UUID> rangeListeners = new HashSet<>();

    @Override
    public void onEnable() {
        this.getServer().getPluginManager().registerEvents(this, this);
    }

    @Override
    public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
        String lcCommand = label.toLowerCase();
        // Технически, теперь можешь добавить ещё какой нибудь /remove, чтобы убирать слушателей из списка
        boolean isPutCommand = lcCommand.equals("put");
        Player target;
        if (args.length < 1) {
            if (sender instanceof Player) {
                target = (Player) sender;
            } else {
                sender.sendMessage(ChatColor.RED + "Usage: /" + lcCommand + " <playername>");
                return true;
            }
        } else {
            target = this.getServer().getPlayerExact(args[0]);
        }

        if (target == null) {
            sender.sendMessage(ChatColor.RED + "Player not found.");
            return true;
        }

        if (isPutCommand) {
            this.rangeListeners.add(target.getUniqueId());
            sender.sendMessage(ChatColor.GOLD + "Player " + target.getName() + " added to listeners");
        } else {
            this.rangeListeners.remove(target.getUniqueId());
            sender.sendMessage(ChatColor.GOLD + "Player " + target.getName() + " removed from listeners");
        }
        return true;
    }

    @EventHandler(ignoreCancelled = true, priority = EventPriority.MONITOR)
    public void onEntityDamageByEntity(EntityDamageByEntityEvent event) {
        if (!(event.getDamager() instanceof Player)) {
            return;
        }

        Player damager = (Player) event.getDamager();
        double distance = damager.getLocation().distance(event.getEntity().getLocation());
        final String message = ChatColor.AQUA + "[" + damager.getName() + "] " + ChatColor.RESET + "range: " + String.format("%.2f", distance);

        for (UUID listenerUUID : this.rangeListeners) {
            Player listener = this.getServer().getPlayer(listenerUUID);
            if (listener != null) {
                listener.sendMessage(message);
            }
        }
    }
}