Excerpt of MineSwine Vehicles Code.

Vehicles.java

//© MineSwine 2019.
//Written by Caden Kriese (flogic)
public class Vehicle {

    private World world;
    private ItemStack[] inventoryContents;
    private ItemStack[] inventoryArmorContents;
    private ItemStack weapon;
    private ArmorStand stand;
    private VehicleType type;
    private BossBar speedBar;
    private BossBar healthBar;
    private Player lastDamager;
    private Player rider;
    private double speed;
    private double acceleration;
    private int health;
    private long lastDamageTime;
    private boolean flyable = true;
    private boolean modelVisible = true;

    public Vehicle(VehicleType type) {
        this.type = type;
        lastDamageTime = 0;
        health = type.getMaxHealth();
        acceleration = (double) type.getAcceleration() / 50;
        speed = type.getMaxSpeed();

        speedBar =
                Bukkit.createBossBar(
                        ChatColor.translateAlternateColorCodes(
                                '&', type.getColor() + "&o&l" + type.getName() + " &8&l- &eSPEED"),
                        BarColor.YELLOW,
                        BarStyle.SOLID);

        healthBar =
                Bukkit.createBossBar(
                        ChatColor.translateAlternateColorCodes(
                                '&', type.getColor() + "&o&l" + type.getName() + " &8&l- &4HEALTH"),
                        BarColor.RED,
                        BarStyle.SOLID);

        GameDefinition gameDefinition = JediCraft.getInstance().getJediConfig().getGameDefinition();
        world = gameDefinition.getGameWorld();
    }

    private static void killRider(Player rider, Player lastDamager) {
        if (rider != null) {
            if (lastDamager != null) {
                rider.eject();
                rider.setLastDamageCause(
                        new EntityDamageByEntityEvent(
                                lastDamager, rider, EntityDamageEvent.DamageCause.ENTITY_ATTACK, 300));
                rider.setHealth(0);
            } else {
                rider.eject();
                rider.setHealth(0);
            }
        }
    }

    public void spawn(Location loc) {
        loc.setYaw(0);
        stand = world.spawn(loc, ArmorStand.class);
        speed = 0;
        stand.setGravity(true);
        stand.setItemInHand(type.getItem());
        stand.setVisible(false);
        stand.setArms(true);
        stand.setBasePlate(false);
        stand.setInvulnerable(true);
        stand.setMaxHealth(type.getMaxHealth());
        stand.setHealth(type.getMaxHealth());
        health = type.getMaxHealth();
        flyable = true;
        stand.setCustomName(
                ChatColor.translateAlternateColorCodes(
                        '&', type.getColor() + "&o&l" + type.getName() + " &8[" + getHealthBarText() + "&8]"));
        stand.setCustomNameVisible(true);
        weapon = WeaponUtils.spawnItemStack(type.getWeapon());
        stand.setRightArmPose(EulerAngleUtils.getInstance().getDefaultAngle());
        Entity standEntity = stand;
        // HITBOXES
        // Equivalent of doing CraftEntity.getHandle().setSize(float, float);
        try {
            Class<?> craftEntityClazz = NMSUtils.getOBCClass("entity.CraftEntity");
            Class<?> nmsEntityClazz = NMSUtils.getNMSClass("Entity");

            if (craftEntityClazz == null || nmsEntityClazz == null)
                return;

            Method getHandle = NMSUtils.getMethod(craftEntityClazz, "getHandle");
            Object nmsEntity = getHandle.invoke(standEntity);
            Method setSize = NMSUtils.getMethod(nmsEntityClazz, "setSize", Float.class, Float.class);
            setSize.invoke(nmsEntity, type.getXzSize(), type.getySize());
        } catch (Exception e) {
            StringUtils.printException(e);
        }
        VehicleManager.getInstance().addVehicle(this);
    }

    public void setRider(Player rider) {
        if (this.getRider() != null) {
            playerExit();
        }
        if (PlayerManager.getInstance().getGamePlayer(rider).getVehicle() != null) {
            PlayerManager.getInstance().getGamePlayer(rider).getVehicle().playerExit();
        }
        inventoryContents = rider.getInventory().getContents();
        inventoryArmorContents = rider.getInventory().getArmorContents();
        rider.getInventory().clear();
        rider.getInventory().setArmorContents(new ItemStack[0]);
        this.rider = rider;
        rider
                .getInventory()
                .setItem(
                        8,
                        new ItemStackBuilder(Material.STAINED_CLAY)
                                .setDurability((short) 5)
                                .setName(ChatColor.GREEN + "MODEL ENABLED")
                                .build());
        rider.setCanPickupItems(false);
        rider.addPotionEffect(new PotionEffect(PotionEffectType.INVISIBILITY, 99999, 2, false, false));
        GamePlayer gp = PlayerManager.getInstance().getGamePlayer(rider);
        gp.setVehicle(this);
        gp.setAntiCheatLenient(true);
        short durability = type.getItem().getDurability();
        if (type.hasSkins()) {
            durability += PlayerManager.getInstance().getGamePlayer(rider).getVehicleSkin();
        }
        stand.setItemInHand(
                new ItemStackBuilder(Material.FLINT_AND_STEEL)
                        .setDurability(durability)
                        .build());
        stand.setGravity(true);
        stand.setRightArmPose(
                stand
                        .getRightArmPose()
                        .setX(Math.toRadians(rider.getLocation().getPitch()))
                        .setY(Math.toRadians(rider.getLocation().getYaw())));
        stand.setPassenger(rider);
        speedBar.addPlayer(rider);
        healthBar.addPlayer(rider);
        updateUI();
        LocaleManager.ENTER_VEHICLE.prefixedMessage().send(rider);
        flyable = true;
    }

    private String getHealthBarText() {
        //10
        String bar = "::::::::::";
        int healthSub = (health / (type.getMaxHealth() / bar.length()));
        bar = "&a" + bar.substring(0, healthSub) + "&c" + bar.substring(healthSub);
        return ChatColor.translateAlternateColorCodes('&', bar);
    }

    private Vector getDirection() {
        return ArmorStandUtils.getUtil()
                       .getVector(
                               stand.getRightArmPose().getY() - Math.toRadians(180),
                               (stand.getRightArmPose().getX() - Math.toRadians(90)) * -1)
                       .normalize();
    }

    public void update(KeyType currentKey) {
        if (speed > 10
                    && (rider.getInventory().getItem(4) == null
                                || rider.getInventory().getItem(4).getType() != Material.BOWL)) {
            if (weapon.getAmount() > 1) {
                weapon.setAmount(1);
            }
            rider.getInventory().setItem(4, weapon);
        } else if (speed < 10) {
            stand.setFallDistance(0);
        }
        // Model Visibility
        if (rider.getInventory().getHeldItemSlot() == 8) {
            ProtocolManager manager = JediCraft.getInstance().getProtocol();
            PacketContainer container = new PacketContainer(PacketType.Play.Server.ENTITY_EQUIPMENT);
            modelVisible = !modelVisible;
            String message =
                    ChatColor.DARK_GRAY
                            + "Model Visibility: "
                            + (modelVisible ? ChatColor.GREEN + "TRUE" : ChatColor.RED + "FALSE");
            new Message(message).sendAsActionBarMessage(rider);
            container.getItemSlots().write(0, EnumWrappers.ItemSlot.MAINHAND);
            container.getIntegers().write(0, stand.getEntityId());
            if (modelVisible) {
                container.getItemModifier().write(0, new ItemStack(stand.getItemInHand()));
                rider
                        .getInventory()
                        .setItem(
                                8,
                                new ItemStackBuilder(Material.STAINED_CLAY)
                                        .setDurability((short) 5)
                                        .setName(ChatColor.GREEN + "MODEL ENABLED")
                                        .build());
            } else {
                container.getItemModifier().write(0, new ItemStack(Material.AIR));
                rider
                        .getInventory()
                        .setItem(
                                8,
                                new ItemStackBuilder(Material.STAINED_CLAY)
                                        .setDurability((short) 14)
                                        .setName(ChatColor.RED + "MODEL DISABLED")
                                        .build());
            }
            try {
                manager.sendServerPacket(rider, container);
            } catch (InvocationTargetException exception) {
                StringUtils.printException(exception);
            }
        }
        if (rider.getInventory().getHeldItemSlot() != 4) {
            rider.getInventory().setHeldItemSlot(4);
        }
        if (rider.getVehicle() == null || rider.getVehicle() != stand) {
            playerExit();
        }
        if (!flyable) {
            updateUI();
            return;
        }
        if (currentKey == KeyType.FORWARDS) {
            if (speed < type.getMaxSpeed()) {
                if (speed + acceleration > type.getMaxSpeed()) {
                    speed = type.getMaxSpeed();
                } else {
                    speed += acceleration;
                }
                updateUI();
            }
        } else if (currentKey == KeyType.BACKWARDS) {
            if (speed > 0) {
                if (speed - acceleration < 0) {
                    speed = 0;
                } else {
                    speed -= acceleration;
                }
                updateUI();
            }
        } else if (currentKey == KeyType.SNEAK) {
            playerExit();
        }
        updateVelocity();
    }

    private void updateVelocity() {
        if (rider != null) {
            //CALCULATE YAW
            double standYaw = Math.toDegrees(stand.getRightArmPose().getY());
            double playerYaw = rider.getLocation().getYaw();
            double difference = ArmorStandUtils.getUtil().angleDifference(standYaw, playerYaw);
            double roll = 0;
            if (Math.round(standYaw) != Math.round(playerYaw)) {
                if (difference > -3 && difference < 3) {
                    // Not much difference to spline left just set it to players yaw
                    standYaw = playerYaw;
                } else {
                    //Calculate movement spline based on handling.
                    double degrees = (difference / (13 - type.getHandling())) + standYaw;
                    //reduce difference to a reasonable degree for roll.
                    roll = difference * 0.015;
                    double maxRoll = Math.toRadians(25);
                    if (type.isLandVehicle())
                        maxRoll = Math.toRadians(15);
                    // dont roll more than 25 degrees or 15 for land vehicles
                    if (roll < -maxRoll) {
                        roll = -maxRoll;
                    } else if (roll > maxRoll) {
                        roll = maxRoll;
                    }
                    standYaw = degrees;
                }
            }

            //Calculate pitch based on incline
            double playerPitch = rider.getLocation().getPitch() - 90;
            double standPitch = Math.toDegrees(stand.getRightArmPose().getX());
            if (type.isLandVehicle()) {
                standPitch = -90;

                Vector direction = getDirection();
                direction.setY(0);
                BlockIterator blockIterator = new BlockIterator(stand.getLocation().getWorld(), stand.getLocation().toVector(), direction, 0, 7);

                if (!blockIterator.hasNext())
                    return;

                Block block = null;
                while (blockIterator.hasNext()) {
                    block = blockIterator.next();
                    if (block.getType() != null && block.getType() != Material.AIR) {
                        break;
                    }
                }
                Location loc = new Location(block.getWorld(), block.getX(), block.getY(), block.getZ());

                boolean decline = loc.getBlock().getType() == Material.AIR;
                boolean tooHigh = false;

                if (loc.getBlock().getType() == Material.AIR) {
                    while (loc.getBlock().getType() == Material.AIR) {
                        loc.setY(loc.getBlockY() - 1);
                    }
                    //make it relative to the vehicle which is 1 block above the ground.
                    loc.add(0, 1, 0);
                } else {
                    while (loc.getBlock().getType() != Material.AIR) {
                        loc.setY(loc.getBlockY() + 1);

                        if (loc.getBlockY() - block.getY() >= 3) {
                            tooHigh = true;
                            break;
                        }
                    }

                    if (loc != block.getLocation())
                        loc.add(0, type.getHoverDistance(), 0);
                }

                double opposite = loc.distance(block.getLocation());
                double adjacent = block.getLocation().distance(stand.getLocation());
                //tan^-1(opp/adj)=𝛳
                double inclineAngle = Math.toDegrees(Math.atan2(opposite, adjacent));

                if (inclineAngle > 30)
                    inclineAngle = 30;
                if (!decline)
                    inclineAngle *= -1;
                if (tooHigh)
                    inclineAngle = 0;

                standPitch += inclineAngle;
            } else {
                if (Math.round(standPitch) != Math.round(playerPitch)) {
                    // Not much difference to spline left just set it to players pitch
                    if (standPitch - playerPitch > -3 && standPitch - rider.getLocation().getPitch() < 3) {
                        standPitch = playerPitch;
                    } else {
                        standPitch = ((playerPitch - standPitch) / 12) + standPitch;
                    }
                }
            }

            //TODO implement this method of rotating instead of the arm yaw.
//      net.minecraft.server.v1_12_R1.Entity nmsEntity = ((CraftEntity) stand).getHandle();
//
//      NBTTagCompound tag = new NBTTagCompound();
//
//      nmsEntity.c(tag);
//
//      tag.setFloat("Rotation", (float) standYaw);
//
//      EntityLiving el = (EntityLiving) nmsEntity;
//      el.a(tag);

//      try {
//        Class<?> craftEntityClazz = NMSUtils.getOBCClass("entity.CraftEntity");
//        Class<?> nmsEntityClazz = NMSUtils.getNMSClass("Entity");
//        Class<?> nmsEntityLivingClazz = NMSUtils.getNMSClass("EntityLiving");
//        Class<?> nmsNBTTagClazz = NMSUtils.getNMSClass("NBTTagCompound");
//
//        if (craftEntityClazz == null || nmsEntityClazz == null || nmsNBTTagClazz == null) {
//          System.out.println("CLAZZ NULL");
//          return;
//        }
//
//        Method getHandle = NMSUtils.getMethod(craftEntityClazz, "getHandle");
//        Object nmsEntity = getHandle.invoke((stand));
//        Object entityLiving = nmsEntityLivingClazz.cast(nmsEntity);
//        Object nbtTagCompound = nmsNBTTagClazz.newInstance();
//
//        Method c = NMSUtils.getMethod(nmsEntityClazz, "c", nmsNBTTagClazz);
//        c.invoke(nbtTagCompound);
//
//        Method setDouble = NMSUtils.getMethod(nmsNBTTagClazz, "setDouble", String.class, Double.class);
//        setDouble.invoke(nbtTagCompound, "Rotation", standYaw);
//
//        Method saveTagData = NMSUtils.getMethod(nmsEntityClazz, "a", nmsNBTTagClazz);
//        saveTagData.invoke(entityLiving, nbtTagCompound);
//      } catch (Exception e) {
//        StringUtils.printException(e);
//      }

            stand.setRightArmPose(new EulerAngle(Math.toRadians(standPitch), Math.toRadians(standYaw), roll));

            // Equation to convert Blocks Per Second to vector.
            Vector speedVec = getDirection().multiply(speed / 20);
            if (type.isLandVehicle())
                speedVec = ArmorStandUtils.getUtil().getVector(Math.toRadians(standYaw - 180), -1 * Math.toRadians(standPitch - 90)).multiply(speed / 20);

            stand.setVelocity(speedVec);
        }
    }

    private void updateUI() {
        speedBar.setProgress(speed / type.getMaxSpeed());
        if (health > 0)
            healthBar.setProgress((double) health / type.getMaxHealth());
        else
            healthBar.setProgress(0.0d);

        if (speed > 10
                    && (rider.getInventory().getItem(4) == null || rider.getInventory().getItem(4).getType() == Material.AIR)) {
            rider.getInventory().setItem(4, weapon);
        } else if (speed < 10
                           && rider.getInventory().getItem(4) != null
                           && rider.getInventory().getItem(4).getType() != Material.AIR) {
            weapon = rider.getInventory().getItem(4);
            rider.getInventory().setItem(4, new ItemStack(Material.AIR));
        }
        String nameSuffix = rider.getName().endsWith("s") ? "'" : "'s";
        stand.setCustomName(
                ChatColor.translateAlternateColorCodes(
                        '&',
                        type.getColor()
                                + "&l"
                                + rider.getName()
                                + nameSuffix
                                + " &o"
                                + type.getName()
                                + "&8["
                                + getHealthBarText()
                                + "&8]"));
    }

    public void damage(double amount, EntityDamageEvent.DamageCause cause) {
        if (health <= 0 && stand.isOnGround()) {
            killRider(rider, lastDamager);
            world.createExplosion(
                    stand.getLocation().getX(),
                    stand.getLocation().getY(),
                    stand.getLocation().getZ(),
                    1.6f,
                    false,
                    false);
            destroy(true);
            return;
        }

        if (cause == EntityDamageEvent.DamageCause.FALL && type.isLandVehicle())
            return;

        // DEATH MECHANICS
        if (cause == EntityDamageEvent.DamageCause.FALL && health <= 0) {
            killRider(rider, lastDamager);
            world.createExplosion(
                    stand.getLocation().getX(),
                    stand.getLocation().getY(),
                    stand.getLocation().getZ(),
                    1.6f,
                    false,
                    false);
            destroy(true);
            return;
        } else if (health == -1) {
            return;
        }

        amount *= type.getArmorDamageReduction();
        if (amount >= health) {
            vehicleDeath();
            return;
        }

        if (amount > health) {
            amount = health;
        }
        health -= amount;

        // Don't let the armor stand die.
        stand.setHealth(stand.getMaxHealth());
        if (rider != null) {
            rider.playSound(rider.getLocation(), "vehicle.damage", 1, 1);
            VignetteUtils.getInstance()
                    .fadeBorder(rider, (int) (100 - Math.round((double) ((health * 100) / type.getMaxHealth()))), 5);
            new Message(ChatColor.DARK_RED + "- " + Math.round(amount)).sendAsActionBarMessage(rider);
            updateUI();
        } else {
            stand.setCustomName(
                    ChatColor.translateAlternateColorCodes(
                            '&',
                            type.getColor() + "&l&o" + type.getName() + " &8[" + getHealthBarText() + "&8]"));
        }
    }

    private void vehicleDeath() {
        health = -1;
        if (rider != null && !stand.isOnGround() && !type.isLandVehicle()) {
            LocaleManager.VEHICLE_KILLED.prefixedMessage().send(rider);
            updateUI();
        }
        Vector speedVec = stand.getVelocity().multiply(2);
        stand.setVelocity(speedVec);
        world.createExplosion(
                stand.getLocation().getX(),
                stand.getLocation().getY(),
                stand.getLocation().getZ(),
                1f,
                false,
                false);
        flyable = false;
        if (stand.isOnGround() || type.isLandVehicle()) {
            if (rider != null) {
                rider.eject();
                rider.setHealth(0);
            }
            world.createExplosion(
                    stand.getLocation().getX(),
                    stand.getLocation().getY(),
                    stand.getLocation().getZ(),
                    1.6f,
                    false,
                    false);
            destroy(true);
            health = 0;
            return;
        }
        //Create a 'Falling out of the sky' effect,
        //giving the player some time to escape their vehicle.
        new BukkitRunnable() {
            @Override
            public void run() {
                if (stand.getPassenger() != null) {
                    killRider(rider, lastDamager);
                    world.createExplosion(
                            stand.getLocation().getX(),
                            stand.getLocation().getY(),
                            stand.getLocation().getZ(),
                            1.6f,
                            false,
                            false);
                    destroy(true);
                }
            }
        }.runTaskLater(JediCraft.getInstance(), 5 * 20L);
    }

    public void destroy(boolean remove) {
        speedBar.removeAll();
        healthBar.removeAll();
        if (rider != null)
            playerExit();
        rider = null;
        stand.remove();
        if (remove)
            VehicleManager.getInstance().removeVehicle(this);
    }

    public void playerExit() {
        if (rider != null) {
            ProtocolManager manager = JediCraft.getInstance().getProtocol();
            PacketContainer container = new PacketContainer(PacketType.Play.Server.ENTITY_EQUIPMENT);
            container.getItemSlots().write(0, EnumWrappers.ItemSlot.MAINHAND);
            container.getIntegers().write(0, stand.getEntityId());
            container.getItemModifier().write(0, new ItemStack(stand.getItemInHand()));
            try {
                manager.sendServerPacket(rider, container);
            } catch (InvocationTargetException exception) {
                StringUtils.printException(exception);
            }
            short durability = type.getItem().getDurability();
            if (type.hasSkins())
                durability += PlayerManager.getInstance().getGamePlayer(rider).getVehicleSkin();

            stand.setItemInHand(
                    new ItemStackBuilder(Material.FLINT_AND_STEEL)
                            .setDurability(durability)
                            .build());
            healthBar.removePlayer(rider);
            speedBar.removePlayer(rider);
            speed = 0;
            rider.removePotionEffect(PotionEffectType.INVISIBILITY);
            rider.setCanPickupItems(true);
            rider.getInventory().setContents(inventoryContents);
            rider.getInventory().setArmorContents(inventoryArmorContents);
            stand.setRightArmPose(
                    new EulerAngle(
                            EulerAngleUtils.getInstance().getDefaultAngle().getX(),
                            stand.getRightArmPose().getY(),
                            EulerAngleUtils.getInstance().getDefaultAngle().getZ()));

            stand.setCustomName(
                    ChatColor.translateAlternateColorCodes(
                            '&',
                            type.getColor() + "&l&o" + type.getName() + " &8[" + getHealthBarText() + "&8]"));

            GamePlayer gp = PlayerManager.getInstance().getGamePlayer(rider);
            gp.setVehicle(null);
            JediCraft.getInstance()
                    .getServer()
                    .getScheduler()
                    .runTaskLater(
                            JediCraft.getInstance(),
                            () -> {
                                if (rider == null) {
                                    gp.setAntiCheatLenient(false);
                                }
                            },
                            20L * 5L);
            rider = null;
        }
    }
}