[abecedarius]

Where it matters 'who' gets what powers or responsibilities, sending messages is a powerful way to think about it, not an implementation detail.

In Java it is pretty much just a function call, because the code in the callee has the ability to wreak all kinds of havoc in real Java programs, so the pattern of "messages" is quite imperfectly related to the authorities you're bounding.

More at http://habitatchronicles.com/2017/05/what-are-capabilities/

[pjmlp]

It would be if virtual method tables weren't a thing, and Java didn't do dynamic dispatch by default.

Then there are Proxy objects, reflection, and invokedynamic as well, for those "doesNotUnderstand:" scenarios.