What are Underbuilders in Java?

An underbuilder is a class that provides a default implementation of a builder for a specific type. In other words, it's a builder that builds instances of a specific type.

For example, let's say you have a class called `Car` and you want to provide a builder for it. You could define an underbuilder like this:
```
class CarBuilder : Builder {
// Define the default values for the properties of Car
private var color: String = "red"
private var make: String = "toyota"
private var model: String = "corolla"
private var year: Int = 2015

// Define the builder methods that can be called to set the properties
fun withColor(color: String): CarBuilder {
this.color = color
return this
}

fun withMake(make: String): CarBuilder {
this.make = make
return this
}

fun withModel(model: String): CarBuilder {
this.model = model
return this
}

fun withYear(year: Int): CarBuilder {
this.year = year
return this
}

// Define the method to build the Car instance
override fun build(): Car {
return Car(color, make, model, year)
}
}
```
In this example, the `CarBuilder` class is an underbuilder for the `Car` class. It provides a default implementation of the builder that can be used to create instances of `Car`. The `withColor`, `withMake`, `withModel`, and `withYear` methods allow you to set the properties of the `Car` instance, and the `build` method is used to actually create the `Car` instance.

Underbuilders are useful when you want to provide a default implementation of a builder for a specific type, but you also want to allow other builders to extend or modify that implementation. For example, you might have a base builder that provides a default implementation of the properties, and then other builders can extend that base builder to add additional properties or modify the existing ones.