Dont forgot that the java mocking you are proposing requires JMockit, which in turn requires Reflection, and requires you to understand how JMockIt actually works... There is a lot of bookkeeping.

In the go example, its just pure core go language, nothing more is needed todo the mocking. In my opinion the go example is a lot simpler than the java example that requires a big mocking framework.

Not to forget, that in go, you can implement interfaces around a simple type such as a int, yes a simple int can implement a whole interface. And there is no explicit bookkeeping needed