Using generated builders of inherited classes when the builders themselves don't inherit

Question

The automapper I'm using, AutoMatter, allows inheritance, e.g.

@AutoMatter
public interface BaseClass {
  Foo foo();
  Bar bar();
}

@AutoMatter
public interface DerivedClass extends BaseClass {
  Baz baz();
}

but its generated builders are not polymorphic, i.e. they are

@Generated("io.norberg.automatter.processor.AutoMatterProcessor")
public final class DerivedClassBuilder {

  private Foo foo;
  private Bar bar;
  private Baz baz;

  public DerivedClassBuilder() {}

  private DerivedClassBuilder(DerivedClass v) {
    this.foo = v.foo();
    this.bar = v.bar();
    this.baz = v.baz();
  }

  private DerivedClassBuilder(DerivedClassBuilder v) {
    this.foo = v.foo;
    this.bar = v.bar;
    this.baz = v.baz;
  }

  // (getters and setters for the builder)

  public DerivedClass build() {
      // ...
  }

  public DerivedClassBuilder from(DerivedClass v) {
    return new DerivedClassBuilder(v);
  }

  public DerivedClassBuilder from(DerivedClassBuilder v) {
    return new DerivedClassBuilder(v);
  }

  // a few more things like hash(), equals(), etc.
}

The thing I need but isn't there is

public DerivedClassBuilder from(BaseClass v) {
  // ...
}

This means, for example, if I have many common fields in the base class, but several different derived classes, I have to build all the common fields manually at some point, e.g.

DerivedClassBuilder builder = new DerivedClassBuilder();
builder.foo(foo);
builder.bar(bar);
builder.baz(baz);

instead of being able to do

DerivedClassBuilder builder = new DerivedClassBuilder(baseClassBuilder);
builder.baz(baz);

...which somewhat destroys the purpose of inheritance in the first place.

If I'm constrained to using this library, is there a pattern that can help me genericize this bit?

I've played around with adding a static helper method in the base interface, using generics to try to represent any derived class, but I seem to get stuck due to type erasure. And there is nothing like derivedClass.BuilderType to help me even if I try to use a copy-intersect library like Dozer.

Admittedly I'm new to Java, so I think either I'm missing something obvious, or it's simply impossible to do what I want to do, given builders that have no relation whatsoever to one another.

Using generated builders of inherited classes when the builders themselves don't inherit

0 Answers0