Comment by mananaysiempre

The difference between Go and Java is that in Go a type need not declare its adherence to an interface up front—any type that has methods of appropriate names and signatures is considered to implement the interface, even if its designers were not aware of the interface’s existence. (This involves a small bit of dynamism in the runtime; easily cached, though, as the set of methods of a given type and the set of all interfaces are both fixed by the time the program runs.) Whether that’s a good thing depends on your design sensibilities (ETA: nominal vs structural).