EntityRegistry.java

@SuppressWarnings("rawtypes")
public class EntityRegistry extends RegistryMaterials
{

    private static EntityRegistry instance = null;

    private final BiMap<MinecraftKey, Class<? extends Entity>> customEntities = HashBiMap.create();
    private final BiMap<Class<? extends Entity>, MinecraftKey> customEntityClasses = this.customEntities.inverse();
    private final Map<Class<? extends Entity>, Integer> customEntityIds = new HashMap<>();

    private final RegistryMaterials wrapped;

    private EntityRegistry(RegistryMaterials original)
    {
        this.wrapped = original;
    }

    private static EntityRegistry getInstance()
    {
        if (instance != null)
            return instance;

        instance = new EntityRegistry(EntityTypes.b);

        try
        {
            Field registryMaterialsField = EntityTypes.class.getDeclaredField("b");
            registryMaterialsField.setAccessible(true);

            Field modifiersField = Field.class.getDeclaredField("modifiers");
            modifiersField.setAccessible(true);
            modifiersField.setInt(registryMaterialsField, registryMaterialsField.getModifiers() & ~Modifier.FINAL);

            registryMaterialsField.set(null, instance);
        }
        catch (Exception e)
        {
            instance = null;

            throw new RuntimeException("Unable to override the old entity RegistryMaterials", e);
        }

        return instance;
    }

    static void registerCustomEntity(int entityId, String entityName, Class<? extends Entity> entityClass)
    {
        getInstance().putCustomEntity(entityId, entityName, entityClass);
    }

    private void putCustomEntity(int entityId, String entityName, Class<? extends Entity> entityClass)
    {
        MinecraftKey minecraftKey = new MinecraftKey(entityName);

        this.customEntities.put(minecraftKey, entityClass);
        this.customEntityIds.put(entityClass, entityId);
    }

    @SuppressWarnings("unchecked")
    @Override
    public Class<? extends Entity> get(Object key)
    {
        if (this.customEntities.containsKey(key))
            return this.customEntities.get(key);

        return (Class<? extends Entity>) wrapped.get(key);
    }

    @SuppressWarnings("unchecked")
    @Override
    public int a(Object key)
    {
        if (this.customEntityIds.containsKey(key))
            return this.customEntityIds.get(key);

        return this.wrapped.a(key);
    }

    @SuppressWarnings("unchecked")
    @Override
    public MinecraftKey b(Object value)
    {
        if (this.customEntityClasses.containsKey(value))
            return this.customEntityClasses.get(value);

        return (MinecraftKey) wrapped.b(value);
    }
}