Current Builders allow setting properties only as an entire field. This is ends up being clunky, as we really want them to be built in a fluent way.

This is especially problematic with Map-based properties, as populating the map requires the product to be present to extract the key:

Map<Bar> bars = ...;
Bar tmp = new BarBuilder().setName("one").build();
bars.put(tmp.key(), tmp);
tmp = new BarBuilder().setName("two").build();
bars.put(tmp.key(), tmp);
Foo foo = new FooBuilder().setBar(bars).build();

with the empty/null equivalence, we can improve this to something like:

Foo foo = new FooBuilder()
    .addBar(new BarBuilder().setName("one").build())
    .addBar(new BarBuilder().setName("two").build())
    .build();

Since these methods are shifting backing structure maintenance to the builder, we also need to ensure proper encapsulation of copy mechanics.