public static boolean chkChestState(Block block){
        BlockState state = block.getState();

        if(state instanceof org.bukkit.block.Chest) {
            Chest chest = (Chest) state;
            if (chest.getInventory().getHolder() instanceof DoubleChest) {
                Bukkit.getConsoleSender().sendMessage("DoubleChest");
                DoubleChest doubleChest =(DoubleChest) chest.getInventory().getHolder();
                InventoryHolder leftChestHolder = doubleChest.getLeftSide();
                InventoryHolder rightChestHolder = doubleChest.getRightSide();
                if(leftChestHolder instanceof Chest){
                    Chest leftChest = (Chest) leftChestHolder;
                    if(leftChest.getCustomName() != null && leftChest.getCustomName().equals("カスタムネーム")){
                        Bukkit.getConsoleSender().sendMessage("Custom_DoubleChest");
                        return true;
                    }
                }
                if(rightChestHolder instanceof Chest){
                    Chest rightChest = (Chest) rightChestHolder;
                    if (rightChest.getCustomName() != null && rightChest.getCustomName().equals(PUBLIC)) {
                        Bukkit.getConsoleSender().sendMessage("Custom_DoubleChest");
                        return true;
                    }
                }
            } else {
                Bukkit.getConsoleSender().sendMessage("SingleChest");
                if (chest.getCustomName() != null && chest.getCustomName().equals("カスタムネーム")) {
                    Bukkit.getConsoleSender().sendMessage("Custom_SingleChest");
                    return true;
                }
            }
            return true;
        }
        return false;
    }

これを使って、チェストとラージチェストを判断する。

if分でor(||)を使えば、もっといい感じになりそう。