import org.bukkit.Material;
import org.bukkit.block.Block;
import org.bukkit.block.Chest;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerInteractEvent;
import org.bukkit.inventory.DoubleChestInventory;
import org.bukkit.inventory.Inventory;
import org.bukkit.plugin.java.JavaPlugin;

public class ExamplePlugin extends JavaPlugin implements Listener {
    @Override
    public void onEnable() {
        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void onPlayerInteract(PlayerInteractEvent e) {
        Block b = e.getClickedBlock();
        if (b.getType() == Material.CHEST || b.getType() == Material.TRAPPED_CHEST) {
            Chest chest = (Chest) b.getState();
            Inventory inv = chest.getInventory();
            if (inv instanceof DoubleChestInventory) {
                e.getPlayer().sendMessage("This is a double chest");
            } else {
                e.getPlayer().sendMessage("This is not a double chest: " + inv.getClass().getName());
            }
        }
    }
}