# Can you use Lombok toBuilder on an Abstract Class?

Yes, you can, and you just need an additional line of code in the `Abstract Class`. Let's take a look at how we can do so.

# Context

If you have an abstract class with various subclasses, and you need to define some values through `builder` method, you won't be able to do so by default because Lombok does not know if all the subclasses are annotated with `toBuilder` but we can force all subclasses to implement `toBuilder` method.

# Setup

To make it simple, we only have an abstract class with 2 subclasses, and a couple of fields.

```java
@SuperBuilder(toBuilder = true)
public abstract class AddressDO {
    private String id;
    private String street;
    private String postalCode;
    private String unit;
}

@SuperBuilder(toBuilder = true)
public class HomeAddressDO extends AddressDO {
    private boolean isRental;
}

@SuperBuilder(toBuilder = true)
public class OfficeAddressDO extends AddressDO {
    private String building;
}
```

Now, imagine we have a method that takes in `AddressDO` as the argument

```java
public AddressDO createAddress(AddressDO addressDO) {
    // 1
    return addressDO;
}
```

* We would like to call `toBuilder` on `addressDO` to define some fields
    

# Problem

Take a look at the GIF below

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1672384694804/56ca8276-62e4-4b6b-9d48-954d3d6e781b.gif align="center")

From the GIF, we can see that even though we defined `@SuperBuilder(toBuilder = true)`, we are unable to call `toBuilder` method. As mentioned above, this is done on purpose by [Lombok](https://stackoverflow.com/a/61651054/1778855), since it cannot ensure all subclasses are annotated with `toBuilder`.

# Resolution

So how can we resolve this?

```java
public abstract class AddressDO {
    public abstract AddressDOBuilder<?, ?> toBuilder(); // added this
    // omitted
}
```

We create an abstract `toBuilder` method which will ensure that all subclasses also implement this method.

This is what happens if the subclass did not implement `toBuilder` method

```plaintext
The type HomeAddressDO must implement the inherited abstract method AddressDO.toBuilder()
```

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1672385840262/f89f89c0-a900-4a36-b68e-06decda733b9.png align="center")

And now, if we try again

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1672385983305/b23207e4-e7cf-4dd1-9a0a-e596b5342288.gif align="center")

## Limitation

The limitation is that it only allows for modification to the fields on the abstract class unless we cast it first. However, if we were to cast it, then there is no (less) need for the `abstract method` in the first place.

![](https://cdn.hashnode.com/res/hashnode/image/upload/v1672386197077/90e96406-9c3e-46a2-b9f5-326fbac4d30b.gif align="center")

# Conclusion

We have seen how we can overcome the limitation of calling `toBuilder` in an abstract class, and the limitation of doing so. I wouldn't call this a hack, but do use it only when you need it.

# Source Code

As usual, the full source code is available on [GitHub](https://github.com/bwgjoseph/tutorials/tree/main/spring-boot-lombok-abstract-class-tobuilder)