Punishes players who stand in liquid for too long.

AFKPoolListener.java

import org.bukkit.Location;
import org.bukkit.Material;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerJoinEvent;
import org.bukkit.event.player.PlayerQuitEvent;
import org.bukkit.plugin.Plugin;
import org.bukkit.scheduler.BukkitRunnable;

import java.util.HashMap;
import java.util.Map;

public class AFKPoolListener implements Listener {

    // How often to check each player
    private static final long AFK_POOL_TASK_TICKS = 20L;
    // How long until punish in milliseconds
    private static final long TIME_TO_PUNISH = 300000L;

    // Do what you want to the player here.
    private static void punish(Player player) {
        
    }

    // You can add any other liquids here...
    private static boolean isLiquid(Material material) {
        return material == Material.WATER
                || material == Material.LAVA
                || material == Material.STATIONARY_WATER
                || material == Material.STATIONARY_LAVA;
    }

    // Call this in plugin onEnable to register the listener
    public static void enableAFKPoolListener(Plugin plugin) {
        AFKPoolListener listener = new AFKPoolListener(plugin);
        plugin.getServer().getPluginManager().registerEvents(listener, plugin);
    }

    private final Plugin plugin;
    private final Map<Player, AFKPoolTask> playerTasks = new HashMap<>();

    public AFKPoolListener(Plugin plugin) {
        this.plugin = plugin;
    }

    @EventHandler
    public void onJoin(PlayerJoinEvent event) {
        AFKPoolTask task = new AFKPoolTask(event.getPlayer());
        playerTasks.put(event.getPlayer(), task);
        task.runTaskTimer(plugin, AFK_POOL_TASK_TICKS, AFK_POOL_TASK_TICKS);
    }

    @EventHandler
    public void onQuit(PlayerQuitEvent event) {
        AFKPoolTask task = playerTasks.get(event.getPlayer());
        task.cancel();
        playerTasks.remove(event.getPlayer());
    }

    public static class AFKPoolTask extends BukkitRunnable {

        private final Player player;

        private Location lastLocation;
        private Long timeStartedInLocation;

        public AFKPoolTask(Player player) {
            this.player = player;
        }

        @Override
        public void run() {
            if (isInLocationTooLong()) {
                punish(player);
                timeStartedInLocation = null;
            } else {
                updatePlayerStatus();
            }
        }

        private boolean isInLocationTooLong() {
            if (timeStartedInLocation != null) {
                long now = System.currentTimeMillis();
                if (now - timeStartedInLocation >= TIME_TO_PUNISH) {
                    return true;
                }
            }
            return false;
        }

        private void updatePlayerStatus() {
            Location loc = player.getLocation();
            if (loc.equals(lastLocation)) {
                if (timeStartedInLocation == null && isLiquid(loc.getBlock().getType())) {
                    timeStartedInLocation = System.currentTimeMillis();
                }
            } else {
                timeStartedInLocation = null;
            }
            lastLocation = loc;
        }
    }
}