Covariance and contravariance might seem esoteric, but you'll need these concepts more than ever as the Java language evolves. Get started with type dependency in array types, generic types, and ...

Pattern matching simplifies your Java code by allowing you to check an object’s type and use it in one clean step. No more messy type checking and casting; Java does it for you automatically.

Okay, I've got two objects. There's the Hand object, and the Finger object. These are fake. The mappings are correct, I believe, everything works fine usually, I think I'm just misunderstanding ...