Even for flattened types, the "abstraction penalty", or, more precisely, its converse, the "concreteness penalty", in Java will be low, as you don't directly pick when an object is flattened. Instead, you declare whether a class cares about identity or not, and if not, the compiler will transparently choose whether and when to flatten the object, depending on how it's used.
Even for flattened types, the "abstraction penalty", or, more precisely, its converse, the "concreteness penalty", in Java will be low, as you don't directly pick when an object is flattened. Instead, you declare whether a class cares about identity or not, and if not, the compiler will transparently choose whether and when to flatten the object, depending on how it's used.